Cut-Out Contact Notes

Feb 2023 R. Kwas  (Comments Added)

These notes are applicable to vintage Volvo cars with Generator based Charging Systems.  Alternator based Chg Sys do not need a Cut-Out Contact, since the rectifying diodes in an Alt block and prevent unwanted current in the Battery Discharge direction.  In the Gen based Sys, when the engine turns the pulley, it also generates electrical current...but conversely, when the engine is not turning the pulley, and it does not only not generate power, the system must add a provision for stopping the Gen from becoming a power user.  This is where the COC comes in, to prevent the Battery Discharge current.

COC and Associated Circuitry
Function of the COC
Operating Conditions and state of COC Considered
    COC with Low Gen Output
    COC with High Gen Output
    COC with Decreasing Gen Output

    COC with No Gen Output (Normal, Non-Failure Conditions)
    COC remains closed with Ign OFF (Failure Condition!)

Reference Information
    Bosch VRegulator Views
    Adjusting COC


The Cut-Out Contact viewed as a part of the entire Bosch Generator based Charging System.


COC and Internal VReg Detail: 

Cut-Out Contact is visible (open) in Yellow and Spring with adjustment in Blue, which magnetic force
must overcome to close this Contact, is also apparent.  The spring force should not be
adjusted by mere mortals without precision test equipment and techniques.


The Cut-Out Contact, located within the Voltage Regulator of a Generator based Charging System, is required, and a very necessary part of such a Chg Sys!  The Generator Output is routed through the COC.  It connects, and equally importantly, disconnects the (output) B+ or Armature Terminal of Generator, to/from the Battery.  This connection must be CLOSED when engine is running, Gen is turning and putting out current, and able to contribute to the electrical Buss, powering Loads, and charging the Batt, but it must be OPENED, after engine is shut-off, and Gen is no longer turning and producing an output.  If the connection from Batt to Gen output terminal were to remain intact when the Gen was not producing an output (or even producing an output below the level of the Bat at that time), it would actually present a discharging load on the Batt, as the non-producing Armature would now represent a Resistance to Ground. 

To get an idea of how much of a load on the Batt this would be, some measurements were made.  Actual Resistance measurements on this sample Gen, removed loooong ago for an Alt conversion, were found to be highly variable, from as high as hundreds of Ohms, to as low as in the single digits, likely due to the carbon film which developes, between Brushes and Commutator intermittently interfering with the contact...the point is that if the engine and Gen were stopped in this low Resistance location, AND the COC were not opening for some reason (a Fault/Failure Condition!), the Batt charge would surely not last very long...

An occasional failure mode is for the COC to remain closed or "welded shut", this may (but not under all conditions) be accompanied by the AMP Indicator remaining fully lit after Ignition is shut OFF, but it would surely be accompanied by the Batt being drained of charge!  This Failure Condition is considered here


When measured from Gen Case to D+ Terminal, and turning, Resistance was quite variable, but settling out at as low
as in the single digits, as seen here.  With a non-opening COC, that would represent a serious load of possibly more
than one amp which would drain much if not all Batt charge overnight!


...and since the charging current path is not fused, pulling the fuses as a part of troubleshooting to locate where an "overnight-discharge current" might be going, would not stop the Phantom Load!  Link to Swedespeed Thread:  Battery Draw, fuses pulled, ign off- What could it be?  ...note that the thread is for a 6V 544, but the info presented here is for a 12V sys and generally applicable.

COC and Associated Circuitry and Function:


  It should be clear and apparent here, that without the COC(1), Armature would be connected directly to Batt,
so when NOT producing an output with which to contribute to the elec buss, it would actually present a load on the Batt! 
Note:  Only a single Generator Armature(4) coil of many is shown for simplification.   


Function of the COC during Different Generator Operating Conditions:  When it is being turned fast enough to produce an output Voltage higher than the Battery(3), this Voltage difference will result in (charging) Current flowing into the Batt and also out to the Loads.  This Current flowing in the Voltage (Sensing) Coil(5) produces a magnetic force on the COC(1) linkage strong enough to overcome the Tension Spring(T) which allows the COC(1) to close.  Gen output (D+) is thus connected to vehicle power buss, and supplies all Loads plus charges the Batt, but because of the Voltage sensing, only when it is advantageous to do so!  The current routed through the COC also flows through the Current Coil(2), which reinforces the magnetic field produced in its soft-iron core, adding hysterisis into the system to prevent the contact from chattering.  When engine is not running and turning Gen so that it is not putting out, the electromagnetism drops to zero and Tension Spring forces COC to open. 

I've now added the AMP Indicator, to COC/Charging Circuit Diagram, as it is an important component, giving the driver important information about the status of the Chg Sys, but it can be seen below, that this Indicator, and in-turn the Driver watching it, can be fooled under some conditions!  The CHG Indicator of Alt based Chg Systems functions similarly...for details on this see: 


Operating Conditions and State of COC Considered:

COC with Low Gen Output:  

Gen Output is not enough to generate magnetism necessary to close COC.  Generator contributes no current to elec Buss, Batt is not charged, but discharged, and must supply ALL Loads.  [AMP Indicator is OFF, because there is voltage at D+, but as Gen output is at a low level, and COC is still open, NO charging takes place!  As the simple device it is, the AMP Indicator can be fooled (and thus fool us) under this operating condition!]

Action during low RPM / low Gen output.  (Arrows show Conventional Current.)


COC with High Gen Output: 

Gen Output is enough to generate magnetism necessary to close COC (...and once closed, current flowing through Current Coil(2) and COC additionally reinforces this magnetism).  Current flows to elec Buss, supplies ALL Loads, and Batt also gets charge current.  [AMP Indicator is OFF and Batt is being charged!]

Action during high RPM / high Gen output.


COC with Decreasing Gen Output: 

If Generator Output Voltage drops below (charged) Battery voltage with decreasing engine RPMs, while COC is closed, a current flows from Battery to Generator (reverse direction).  Since direction of current in the Current Coil(2) is now in the opposite direction, the magnetic field which closes the contact, is weakened, eventually to less than the Spring(T) force, opening the contact and disconnecting Generator from the Power Buss.  Batt must again supply all Loads.  [AMP Indicator is OFF, but little to no charging takes place!]

Action during decreasing Gen output.


COC with No Gen Output (Normal, Non-Failure Conditions)

No Generator Output.  Magnetic field acting on COC drops to zero, opening COC, and disconnecting Generator from the Power Buss.  This is perfectly normal for instance, when Ign has been turned ON with Ign Key, but engine has not yet been started, since Gen is not yet turning and producing an output.  Batt must again supply all Loads.  [AMP Indicator is ON, Powered by Ign+ and connection to chassis by way of non-producing Gen Armature!  Batt is being Discharged!]

No Gen output (Ign ON).  COC is open and the Ign+ voltage


COC remains closed with Ign OFF (Failure Condition!)

If COC remains closed even after Ign is OFF, and Mag force has dropped, a welded Contact has occurred (this can occur with a worn contact).  The COC is not able to provide its primary function, and a Discharge Current will be allowed flow from Batt to Chassis.  [AMP Ind will be ON when Ign has been turned OFF.  This unusual condition should NOT be overlooked, as it brings with it a discharged Batt!] 

Failed Condition of remains closed with Ign OFF, and allows Discharging of Batt!



Reference Information:  

Bosch VRegulator Views:

VReg viewed from above, with cover removed. 

The COC function is handled by the left Coil assembly in this view of the VReg, and this is only one part of the multiple functions of the entire VReg.  The right coil assembly is responsible for controlling the Charging System, through Voltage and Current regulation, and outside the scope of this article.  See:  Voltage Regulation   

VReg internals. 


VReg viewed from below:

Note B+ terminal is connected to VReg internals by way of a redundant, dual rivet connection.  This is a quality construction redundancy feature on the part of Bosch, to assure low Resistance and long, trouble-free service life!


Adjusting COC

Careful dressing/abrading of the COC, as the owner did to clear the welded contact condition reported in the linked thread above is about the most invasive action I recommend.  I recommend against adjustment of the tension of spring which controls the action of the COC, by non electrically qualified individuals, but I thought I'd show the procedure in the factory manual here.  

Source:  Volvo Factory Manual ELECTRICAL SYSTEM 120 (12Volts) TP10566/1 1970



External sources attributed.  Otherwise this information is Copyright © 2023.  Ronald Kwas.   The terms Volvo and Bosch are used for reference only.  I have no affiliation with either of these companies 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, or worshipped, 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, and future!

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 donít, youíre 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, or analogy (see:  ABS!) and probably wise-a** comment. 


B A C K ! . . .to Tech Articles Index Page