Understanding the function of the AMP indicator in your vintage Volvo, and how to proceed when it comes ON.  Including Original Equipment Charging System Checks, Troubleshooting and Repair
Copyright © 2005
*  Aug 2005  Revised Oct 2010 , Apr 2012, R. Kwas, Updates Ongoing

Regulator Bypass Test
Generator checks
Brushes

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The info presented here applies to unmodified Volvo (*) 544, 122 and 1800 (Bosch generator) 12V electrical systems, and is intended to help in the understanding of these systems for the purpose of trouble locating and repair after the AMP indicator comes ON.  I have long ago upgraded just about all of my vintage vehicles to modern, high output alternators with fully electronic internal regulators, and I’m a big advocate of this, but I thought I would compile this article for those who prefer to remain original, as locating the troubles once the AMP indicator comes ON can be tricky for qualified individuals and totally baffling for the inexperienced.  The article is not yet complete and neat, but there is enough info here to get one well started...and now that I've added a detailed FIGURE 1 plus explanation, it is really getting in need of a rewrite/cleanup... 

Notice:  I stand behind all the information presented here, having put it together with the utmost care, but in the end, the reader is responsible for its use and the consequences of their own actions.  The information presented should be used in conjunction with normal, cautious shop practice.  I will not be responsible for your actions and knuckles.  Work safely!

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Since the deceivingly simple AMP indicator is actually employed in a slightly complicated manner (not to mention that is not even indicating AMPs - that was just a handy - and short, electrical sounding word that would fit on the tiny indicator), a complete understanding of this indicator’s function is necessary.  It may, in some circles, be called an “Idiot Light”, but the manner in which it was designed into the electrical system is anything but idiotic…its actually rather ingenious…in a do-the-maximum-with-a-minimum kind-of way! 

The AMP light itself is a dirt-simple (3 Watt) incandescent lamp, but the manner in which is cleverly worked into the electrical system allows it to light up and indicate two very different failure conditions.  Herein lies the complication.  In the first place, in the normal condition (IGNition ON, engine running / all-things-fine), the lamp actually has voltage applied to both sides, and so does not light!  Understanding this simple fact is essential to appreciating what is happening here, and unraveling the complication.  Reminder:  For any electrical device to function it requires voltage across its terminals.  This voltage serves to encourage the flow of electrical current through the load, powering it up…and indeed, the AMP indicator as well, must have voltage across it to light! 

Reviewing, a complete 122S wiring diagram, or FIGURE 1. below, it can be seen that, one side of the AMP indicator is connected to Fuse 1 (IGNition power).  The other side is connected to, and monitors the charging system output at, the Voltage Regulator (VR) D+ terminal.  The reader will note and should understand, that in this situation, with voltage on both sides of the lamp, no current flows, and the indicator remains unlit!  


FIGURE 1.  AMP Indicator and related circuitry for a 122.  Numbers associated with wiring are wire cross-sectional area in mm2.  O.C. denotes “Other Connections”. 

Again, in order to light the lamp, the voltage-across condition must be satisfied.  Only this will allow current to flow.  The really observant or electrically enlightened reader will also notice, that this condition may be met by tying either side to ground.  [See:  Idea 1 below]

Condition 1:  Normal, non-failure mode.

The two conditions, which will cause the AMP indicator to light while the engine is running, are therefore:

Condition 2:  Failure of charging system (…the armature of generator does not produce voltage, but instead, completes circuit to chassis allowing lamp current to flow…for example…when IGN key is turned ON, but before starting the engine).

Condition 3:  Fuse 1 open (…at which time, the other Fuse 1 loads complete the circuit to chassis allowing lamp current to flow). 

The “Other” Condition:  AMP indicator glows slightly while engine is running.  This is a frequently occurring condition known to just about all owners of these vehicles, discussed separately below.

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Condition 1  is the normal non-failure condition of Ignition ON and Charging system output normal. 

Condition 3 Checkout of open Fuse 1 causing AMP indicator to be ON. 

An open Fuse 1 is quickly verified, as it is accompanied by the loss of function of other Fuse 1 loads…specifically, these are:  Directional indicators, Fuel gauge, Blower, and Wipers.  If the AMP light ever comes on full, while the engine is running, or does not extinguish shortly after starting, this is the first thing, which should be checked for, as it is the simplest.  The driver, even a technical layman can, and should do this…as soon as practical. 

Functional test when AMP indicator comes ON with engine running:  Turn any of the other Fuse 1 loads ON.  If loads do not function, Fuse 1 should be suspect.  Open hood at the next opportunity, and with IGN ON and engine running, activate left directional indicators (these are easily seen while standing next to the vehicle on the drivers side).  Remove the fuseblock cover (you should know where it is!), and visually check the state of Fuse 1.  If the fuse is visually OK (Suggestion:  Always install Euro-fuses with element visible to allow for an easy inspection!), spin it in its holder…if this restores the connections so that the directional indicators start blinking (and causes the AMP light to extinguish), corrosion may have been present and breaking the connection.  Spinning the fuse has cut through this corrosion at the conical fuse ends, and reestablished a connection…  This is actually a rather common failure…on the injected 140s and 1800s, this failure occurring on the fuel-pump relay fuse is also well-known…

…if, on the other hand, the wiper worked when the switch was tried, Condition 2 may exist, or the classic “Other” Condition caused by long-term corrosion.  Checkout and correction of Condition 2 will require a more careful investigation of voltages under the hood using a multimeter.  This is down-and-dirty technical stuff, and should be performed by someone who is at least familiar with the operation of a multimeter.  

Verification of open Fuse 1 (or associated connections) condition:  Shut  IGNition OFF, allowing motor to come to a stop.  Turn IGN to the ON position, but do not START.  If AMP indicator does not light, Fuse 1 should be suspect.  Explanation:  When IGN power is applied, but before starting, the AMP indicator is supplied on one side with IGN power and on the other with a connection to chassis through the non-electric producing armature of the generator.  The AMP indicator should light…if it does not, it must be assumed that it is not being supplied with power, and therefore Fuse 1, or associated connections are suspect. 

If replacing a (blown) Fuse 1 remedies the condition …it’s a mixed blessing.  This is the simplest failure to fix, but one has to wonder why the fuse actually blew in the first place…because actually blowing the 25A rated Fuse 1 is fairly unusual. 

If on the other hand, just the action of moving the fuse in its holder (or spinning it) is enough to restore function, that suggests the more common poor connection due to corrosion condition. 

Consider yourself lucky for not having a dreaded charging system failure , but deduct two points for neglect, and, at earliest possible opportunity, clean fuse-ends and the circular connection holes in the clips properly with abrasive, apply Zinc Anti-Corrosive Electrical Paste, and reinstall fuse.  Suggested readingGas-Tight-Joint Tech article. 

Having checked Fuse 1 and found it to be intact, and spinning it to try to cut through any corrosion did not fix the symptoms, another check, requiring a multimeter should be performed before moving on to checks of the charging system.  With the IGN ON and engine running, the IGN power should be measured at the Fuse 1 and Fuse 2 (input) terminals on left (Black wires), as well as (output) terminals (Red wires), on right side.  If voltage is measured at the input but not the output, an open connection at the fuseblock exists…this may be caused by a poor connection due to corrosion at the push-on terminals, corrosion between the riveted contact plates or conical fuse-ends (as both discussed in the Gas-Tight-Joint Tech article), or a blown or fatigued fuse which was previously missed. 

If checks around the fuseblock reveal no problems areas, a failed charging system must be considered to be causing the AMP light to be ON.. 

Condition 2  Checkout of Charging System causing AMP light to be ON.  This is the next thing, which should be checked.  It is down and dirty technical stuff, requiring tools, a multimeter and at least a basic understanding of how to correctly put these to use without hurting oneself (knuckles not withstanding). 

With engine running, measure voltage from D+ terminal (Generator output) of VR to chassis.  If this voltage is less than 2 Volts, charging system (combo of the generator and regulator) has no output. 

Regulator Bypass Test:  [As the generator is not regulated during this test it should only be performed momentarily...that is, just long enough to perform the test].  While continuing to measure the output voltage at D+, short the DF terminal of the VR to chassis (all wiring normally in place), using a screwdriver or similar at the regulator DF terminal.  This takes the place of the VR and completes the path to ground, allowing field current to flow (see:   Simplified Charging System Wiring below).  An output in excess of 12V should now be read on the D+ terminal, and this voltage should also be measured, without a drop, on the battery plus terminal.  Raising the RPMs should raise this voltage, as the generator output comes up.  This suggests a Voltage Regulator problem.  Replace VR.  Note:  Successful repair of the VR is beyond the scope of this article and without calibrated test equipment and setup, likely beyond the abilities of even the most experienced backyard wrencher!  There is a lot happening in the Voltage Regulator!  (Link:  Bosch Generator based Charging System in Detail)

If bypassing the VR does not result in an output, the generator itself must be suspect. 

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From e-mail exchange with Taylor F.:  8/26/2013  Subject:  Re: ..... and alternator conversion kit?

> Are you certain the Generator is the problem?
>
> If you are certain the problem lies with the Generator and have spares and or a competent shop which can repair/rebuild it, or can do it yourself, that certainly is one alternative (check sw-em site for hints and troubleshooting info) but you or the shop you entrust the project to would have to be one familiar with the Bosch Generator based charging system, or the project has only a small chance of having a successful outcome.
>
> Often however, if the problem lies with the Charging Control...better known as the Voltage Regulator, which has mechanical contacts and which is therefore subject to wear...repair could be even trickier...(Link: 
Bosch Generator based Charging System in Detail)
>
> The Alt upgrade on the other hand has a lot going for it, which you are undoubtedly aware of...and reliability is definitely one of them...

---

A simple test to bypass the VReg and manually call for Generator output while a Voltmeter is connected to battery.  Raise engine RPMs to about 2K, then for purposes of test, use a test wire to connect Df terminal of VReg to chassis (this bypasses VReg and applies Field current manually, by completing connection to chassis, which Vreg would normally)... monitor system voltage.  If it noticeably rises, and system/battery voltage starts climbing, indicating Battery is charging, it suggests Gen is OK and putting out, and VReg is not working as expected.  Testing VReg is beyond the casual mechanic as it requires special equipment, techniques and experience. 
 
Of course all terminals and connections involved should be clean and tight before undertaking any of this...I highly recommend loosening, cleaning with emery cloth, and reconnecting with a film of ACZP (send me a SASE, and I'll send a freebie sample!), and tightening all electrical connections having anything to do with voltage monitoring and charging system....those are:   All VReg (including chassis mounting), Generator, Battery, Ignition Switch, Engine to Chassis braid, Fuse 1 connections. (including fuse-ends and FB terminals).

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Generator checks.

In-Situ Reciprocity Test.  This is a test, helpful for determining the state of health of the generator.  It is performed which the generator installed and wired normally, but with the fanbelt removed.  A side-effect here is that the wiring from the VR is checked at the same time.  A successful test, gives a certain level of confidence in the generator, harness and connections.  Performing the Reciprocity Test is simple…after loosening and removing the fanbelt, use two temporary test jumper wires to 1. Connect the DF terminal to chassis, and 2. Connect the B+ to the D+ terminals at the VR.  This applies battery power and field current to the generator, and by applying a marvelous principle of science, which states that many processes run backwards as well as forwards, the generator should spin as a motor.  If it does not, Generator is suspect.  Remove test wires and proceed to…

Passive Resistance Tests.  With IGNition OFF, battery plus disconnected, and loose fanbelt, disconnect the D+ and DF wires from the generator.  With the multimeter in the Ohms function, verify the internal connections as shown in the graphic following.  The D+ terminal (Armature output, which runs through the brushes) to chassis reading should read about::  50 Ohms, remaining fairly steady and without a lot of fluctuation when spinning the generator pulley in the normal running direction.  The DF to D+ terminal should read about 4.8Ohms, unaffected by a spinning armature.  This is a basic check on internals of the generator as shown below.  Note that the power connection for the field is permanently made, within to the generator, but requires an external connection to chassis in order to allow field current to flow.  This in fact is, what the VR does as it monitors the system voltage against an internal reference, and what the troubleshooter has done, manually, when performing the previous Regulator Bypass Test

                          D+              DF

Source:  Volvo Factory Service Manual

Poor Connections!  Don’t discount these, since poor connections are a definite possible cause of electrical problems, especially on a 40 year old vehicle, it wouldn’t be such a bad idea to assure all connections are clean and tight before starting to replace expensive electrical components.  One would feel pretty silly if hours of troubleshooting and expensive replacement work were all for naught, if a problem turns out to really have been caused by a corroded or loose connection! 

Zinc anti-corrosive electrical paste is the Anti-Seize of the electrical world!  Use this paste after disconnecting and cleaning electrical connections of the vehicle.  The connections critical to the charging system are, voltage regulator (including its mounting bolts which make the connection to the vehicle chassis!), generator, battery, starter, IGN switch, fuseblock, groundstrap (connecting chassis and engine). 

The “Other” Condition:  AMP indicator glows slightly.  The well-known condition of a dimly lit AMP light is not a separate condition, but a variation of Condition 1.  It is caused by the slight voltage drop, which occurs, by design, across the Fuse 1**, as the indicator lamp is a small, sensitive lamp, even a slight voltage across it will satisfy the voltage across rule, and cause a slight glow of the lamp.  This is absolutely normal, and even occurred when the car was new!  If however, additional voltage drop occurs due to poor connections, the voltage across the lamp increases, and the lamp glows more brightly. 

The effect of a dimly lit AMP Indicator (particularly evident when turning ON Fan Switch* in dark conditions) is a direct result of voltage drops due to current passed (IR Drop **).  Specifically, as can be seen in FIGURE 1. , Voltage Drop across Fuse1 (which is inherent for any fuse and cannot be avoided) plus associated connections in Fuseblock (which can and should be avoided), is caused by passing the currents of the downstream loads. 

*  The observant reader may have noticed that the dimly glowing AMP Indicator effect is definately more pronounced when turning ON Fan Switch than when turning ON Wipers, although both are Fuse1 loads and the Wiper current is higher in magnitude.  This seemingly inconsistent action can be explained by the fact that the Voltage Drop resulting from Fan current is actual higher than the Voltage Drop resulting from Wiper Motor, and this occurs because of the wire gauges involved…a heavy wire gauge (4mm2) runs between Fuse1 and Wiper Switch and a much lighter gauge wire (1.5mm2) connects Wiper Switch to the Fuel Gauge...and since voltage drop on a wire for a given current is less on a heavy wire than a thin wire, it should be clear why the AMP Indicator typically gets brighter when setting the Fan Switch to the High position while being largely unaffected by the Wiper Switch

**  IR Drop Explained:  For the non-engineer reader, the IR Drop is simply the Voltage which develops across any resistance in the path of current.  The term comes from Ohm’s Law.  Normally, the IR Drop is ignored and not spoken of because components and conductors in a circuit are intentionally sized when designing the circuit, so that these Voltages are miniscule, because any voltage dropped along the way is not available at the load to drive current through it and do the intended work.  In other words, when things were new, things were simple:  Voltage at the Source was controlled by some sort of Switch and it was either ALL across the Switch (when Switch is open) OR ALL across the Load (when Switch is closed).  In the vintage vehicle world however, corrosion and loose or otherwise poor connections can, and often do, become significant contributors to circuit action.  This must be carefully considered and taken into account when troubleshooting issues that arise! 

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Summary

In order to light the AMP indicator, the voltage-across-condition must be satisfied.  The two conditions are:

1.        Fuse 1 open

2.        Failure of charging system.

Given good connections, there are two conditions under which the AMP indicator may light, but add to that, frequently occurring faults due to subtle corrosion and/or poor connections, locating and remedying the trouble can get complicated…fast!

Following an order of Simplest-to-Complicated fault isolation, check Fuse 1 and associated connections first, then Charging system, breaking this into Generator, then Voltage Regulator, components.  

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Voltage Regulator (VR) function for a generator electrical system explained

1.  The VR’s main function is to control the electrical system by monitoring system voltage and activate the generator to contribute electrical power when system voltage (and therefore battery charge) is low. 

3.        A second function is required in generator systems.  That is, to disconnect the generator from the battery when the IGN is OFF and generator is not contributing (in fact, the generator would take (and waste) battery power if left connected under this condition).


Simplified Charging System Wiring Diagram

Link to Sw-Em Tech Article:  Bosch Generator based Charging System in Detail

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**  Review:  How a fuse works

A fuse is an intentional weak electrical link, which has a small but important resistance (R - Ohms) to the flow of electrical current (I - Amps).  As current flows to the downstream load, this resistance causes a small voltage drop (due to Ohms Law V=IR), which in turn causes power dissipation (P - Watts) to occur (also known as I2R heating).  As the current increases, so does this heating (exponentially, in fact).  By designing and constructing fuses carefully, these can be used as simple, effective, circuit protective devices, with a predictable (normal and expected) level of current which will be allowed to pass, and a predictable amount of (fault) current which will, stop the fault current from being allowed to pass.  The fuse “blows” (melts, breaking the connection and thereby power delivery to the load), when excessive current is drawn by the circuit it is in series with. 

Link to additional info:  Fuses, Allocation and Troubleshooting.htm#Understanding_the_information_an_open_fuse_can_give

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Idea 1.  It is easy to see that depending on which side is supplying the power and which side is supplying the ground (return current path to battery negative), determines the direction of current flow through the indicator.  This immediately brings to mind those cute little bi-color LEDs, which have two different color LEDs in one package, back to back, and so light up differently depending on current direction.  These might be nice to use here…Green for Fuse 1 blown, Red for Charging system failure.

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Links:  

Brushes:  

Thread on British Forum: AMP Light on full red on startup:  http://www.volvoforums.org.uk/showthread.php?t=232356


From that thread, Derek was kind enough to post this picture with measured dimension,
of a new Volvo PN 273500 Generator brush. 

One can see on the picture that once Brush is worn past about the 50% point, its' width will be more than its' height.  This is what Leif Anderson's "road-fix" below is based on...

Excerpt from a post of mine in that thread:

"I specifically recall years ago...Leif Anderson, while visiting the US and joining us for the Mt. Snow Volvo Meet...when someone's Ch Sys failed due to short/worn Brushes, he nonchalantly walked by a bunch of guys up to their elbows in underhood Volvo dirt and Generator dust in the parking lot of an event (when we should have been hobnobbing and drinking Chablis with everyone else) and suggested simply putting the same Brushes back into guide with a 90deg twist, as they were worn shorter than in the wider dimension...this made them taller and able to get more preload force from the Spring...and it worked!...to return function of Ch Sys as a temporary fix...long-term, this leaves a step-wear in commutator so requires proper service including removing any step from commutator ASAP, but is a good emergency road fix in a pinch."

...now that's what I call thinking "outside the herring tin" (Swedish equivalent of a popular saying)!  

Link to good info (VW) on Brushes and changing them:  http://www.reluctantmechanic.com/step-by-step/change_generator_brushes.php

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Notes / References / Additional: 

Link to Zinc anti-corrosive electrical paste

Link to Sw-Em Tech Article:  Bosch Generator based Charging System in Detail

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Additional:  The official green Volvo Service manual calls for a nominal field coil resistance of 4.8 Ohms.  No armature readings are given.  After a call on the Brickboard and Volvoniacs fora, for actual resistance readings on generators made, some helpful owners (Thanks Phil, Joe, McMike ) provided readings on known good units, and also suspected bad units.  The typical good readings are included in the previous text for reference.  The readings from suspected bad units are included below along with comments to help owners troubleshoot these generators.  Finally, service recommendations are made for repairing the non-working units. 

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Suspected Unit 1

D+ to D- (Armature, static):  Open (infinite Ohms)

D+ to D- (Armature, while turning):  Open

DF to D+ (Field only):  Open

Comments:  When making resistance measurements on old, possibly less than clean automotive components, its obvious to assure good connections are made…so the first thing I would ask in response to these readings is:  Are your sure you were making a good connection with the meter probes?  If the answer is: “Yes”, the second thing I’d ask is:  Are there any guts in the thing or is it perhaps just an empty shell? 

Service Recommendation:  Open inspection band, retake readings on wires or connections inside to verify, if no change, disassemble totally, and recheck Armature and Field resistances while apart.

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Suspected Unit 2

D+ to D- (Armature, static):  48Ohms

D+ to D- (Armature, while turning):  Bouncing from 20 to 200 Ohms

DF to D+ (Field only):  4.8 Ohms

Comments:  Possibly worn brushes, possibly a contaminated commutator, or both.  Field resistance is nominal, so this generator has possibilities!

Service Recommendation:  Open inspection band, check height of brushes and color of commutator, using emery board clean commutator to a shiny copper, clean using carb cleaner or brake cleaner solvent to flush (conductive) dust from brushes, and copper particles from commutator clean-up, then compressed air to dry. 

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Notes on Generator Troubleshooting:  

From e-mail exchange with Taylor F.:  8/26/2013  Subject:  Re: ETA on 122 seat suspension kits and alternator conversion kit?

> Are you certain the Generator is the problem?
>
> If you are certain the problem lies with the Generator and have spares and or a competent shop which can repair/rebuild it, or can do it yourself, that certainly is one alternative (check sw-em site for hints and troubleshooting info) but you or the shop you entrust the project to would have to be one familiar with the Bosch Generator based charging system, or the project has only a small chance of having a successful outcome.
>
> Often however, if the problem lies with the Charging Control...better known as the Voltage Regulator, which has mechanical contacts and which is therefore subject to wear...repair could be even trickier...
>
> The Alt upgrade on the other hand has a lot going for it, which you are undoubtedly aware of...and reliability is definitely one of them...

---

A simple test to bypass the VReg and manually call for Generator output while a Voltmeter is connected to battery.  Raise engine RPMs to about 2K, then for purposes of test, use a test wire to connect Df terminal of VReg to chassis (this bypasses VReg and applies Field current manually, by completing connection to chassis, which Vreg would normally)... monitor system voltage.  If it noticeably rises, and system/battery voltage starts climbing, indicating Battery is charging, it suggests Gen is OK and putting out, and VReg is not working as expected.  Testing VReg is beyond the casual mechanic as it requires special equipment, techniques and experience. 
 
Of course all terminals and connections involved should be clean and tight before undertaking any of this...I highly recommend loosening, cleaning with emery cloth, and reconnecting with a film of ACZP (send me a SASE, and I'll send a freebie sample!), and tightening all electrical connections having anything to do with voltage monitoring and charging system....those are:   All VReg (including chassis mounting), Generator, Battery, Ignition Switch, Engine to Chassis braid, Fuse 1 connections. (including fuse-ends and FB terminals).

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This article is Copyright © 2005 - 2015.  Ronald Kwas.  The terms Volvo and Bosch, are used for reference only.  I have no affiliation with either of these companies other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment. 

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