123GT Charging System Notes

First Published:  Jan2018, R. Kwas, updates on-going (Additional comments)

Notes and Functional Description
Reference Information
Additional Information
Bosch / Motorola Similarities and Differences
Troubleshooting Notes

Much of the info here is applicable to the other Volvo models of the same era (Injected 1800s, and 140s) which had Alternator based Charging Systems as original equipment:

1800E/ES Charging System Notes


The author's posting in response to a 123GT (Alternator equipped) apparent Charging System problem.
"The alternator in my 123 gt appears not to be working properly."  by "Noddy 123GT"

Link to Thread:  https://www.volvoforums.org.uk/showthread.php?t=276399  (Corrections/Additional not part of the original posting.)

123GT Wiring Diagram excerpt with (Alternator based) Charging System, relevant components and connections shown. 
AMP Indicator and Charging System initial Field Current are supplied by Fuse1,of the Primary Fuseblock, as is typical.
The Secondary FB also present supplies some lighting Loads, and this area can stand some consideration. 
See:  https://www.sw-em.com/123GT_Reliability_Weakness_and_Simple_Improvement.htm  

Note:  The above Wiring Diagram applies to both SEV/Motorola and Bosch based Charging Systems, because no detail is shown for Alt and VReg.  Once the interaction between Alt and VReg and internals of these components is studied in more detail as shown below in Additional Information: Bosch / Motorola Similarities and Differences, it becomes clear that they are different, confirming that Regulators are unique to each of the two Alts, and may not be mixed and matched.  When replacing either Alt or VReg, this factor must be observed! 


Notes and Functional Description: 

Ign Sw supplies Ign Pwr to Term 61 (and VReg monitors this voltage) of Alt by way AMP Ind, daisy-chain connection to other Ign Pwr Loads, and of Fuse 1 (Important: F1 and associated connections must be intact for Chg Sys to function!).  [UPDATE:  I overstated this...if F1 is open, Alternator based Chg Sys of the GT will function by self-excitation.  See:  Reference Information below!]
When VReg senses a low Sys V, it enables Alt output by enabling Alt Field Current (this is where I'm not absolutely certain if it is by "supplying power side" or "sinking low side" which is more typical...and this may differ from Motorola to Bosch units [that's why it also important not to mix and match VRegs!]...maybe someone who is intimately familiar with internals of Mot and Bosch Alts can clarify/verify here!).[UPDATE:  High-Side is supplied to both Bosch and SEV/Motorola systems, see below!]
Output of Alt at B+ is routed to Bat by way of Sec FB. [It is important never to intentionally disconnect Alt output from the rest of System, even for test purposes...as voltage can soar to damaging levels! Do Not run Alt with No Load!  See also: http://www.sw-em.com/123GT_Reliabili...mprovement.htm

AMP Indicator Function: Before motor is started and Chg Sys has output, one side of AMP Ind gets Ign Pwr and Term 61 sinks power to light AMP Ind.  Once motor is started and VReg enables Alt output (due to voltage drop caused by Starting energy having been taken out of Bat), Term 61 stops sinking Indicator power and Indicator extinguishes (has 12V on both sides at this time!).
As with Generator based Chg Systems, two very different conditions can cause Voltage across AMP Ind and cause it to light up:
Condition 1. No Chg Sys output.
Condition 2. Open F1 (but open F1 will result in no Chg Sys output!...and with the well-known FB and corrosion issues, this would be particularly important place [as all the other connections on Wiring Diag ] to assure good connections! Ref: http://www.sw-em.com/gastight.htm ) [...on second thought...Alternator would probably self-excite from residual magnetism, above some minimum RPM.  See Reference Info below.  Verify with system voltage measurements!]

I suggest you connect a voltmeter to Bat to monitor Bat/Sys V, start engine, raise idle to get Chg Sys output. Before starting, meter will show Bat V, after starting, Bat V should rise as Chg Sys contributes its output to Sys and charges Bat. If it does not rise, Chg Sys has no output.  [Then its time for troubleshooting, and an effective technique is to separate function of the VReg and Alt as detailed in Troubleshooting Notes below.]  "


Reference Information: 




Additional Information:  

Bosch / Motorola Similarities and Differences:  Bosch and SEV/Motorola Alternator based charging systems are substantially similar (external to the Alt and VReg components), but with important differences (when considering the internal details).  Both systems supply the "high side" of Alt Field at DF, and "low side" of Field is directly connected to Chassis (through its respective Slip-Rings and Brushes).  Systems differ in that the voltage at terminal 61 during operation, is different by the voltage drop (about 1Volt) across series D2 diode present only in the SEV Alt (this means that combining a Bosch VReg with a SEV Alt would likely result in Battery being undercharged because of the reduced voltage, and vice-versa, when combining the SEV VReg with a Bosch Alt, would result in Battery being overcharged by this increased voltage)...not a damaging (or worse) condition of operation, probably not even immediately noticeable without precise Voltage/Current tests, but certainly not desirable long-term, as in either case, the Battery will be either undercharged and Starting and reserve power will suffer, or overcharged and Battery-life may be affected!  

Bosch and SEV based Charging System Circuits showing similarities and differences.  Initial Source of Field Current path through Charge Indicator (aka:  AMP Indicator  See also:  AMP Indicator in the 1800E/ES  ) is shown in Orange.  Once Alternator produces output, Field Current path is shown in Green. Alt Output is shown in Red.  Contacts of VRegs in both circuits are shown in the position enabling Field Current (and that's where they always start and park [Note:  Alt based Ch Systems do not need Cut-Out Relays like Generator based systems because the rectifier diodes of Alt Sys are unidirectional, allowing the power being generated into the vehicle elec sys, and blocking power from going into Alt and discharging Battery after engine is shut off.  Ref:  Cut-Out Relay] ...the contact down is the voltage high/Field current OFF position).  Switching function in later, solid state versions is performed functionally the same and done by transistors, but I would expect solid state VRegs to also be unique to Bosch / SEV Alts (to be confirmed!)

Troubleshooting Notes:

General Information:  As always, it is important for connections of the Charging System to be clean and tight (this refers to all connections shown in the Wiring Diagram Excerpt above...yes there are quite a few of them...!).  Poor connections cause voltage drops which can manifest themselves in many different, sometimes simple, sometimes creative, and in some cases "whacky symptoms", to challenge even the best troubleshooter!  When servicing connections, the author's advice is to clean contacts and current carrying areas to shiny metal, and apply ACZP, when reconnecting.  This will give long-term trouble-free operation! 

AMP Indicator ON...How to proceed:  Check Fanbelt and for any obvious issues under the hood first!  Next visually check Fuse 1 of Primary Fuseblock (by removing Cover of Fuseblock only and without otherwise touching and disturbing Fuse 1!).  If it is intact (not blown or otherwise electrically open **), only then, twist it in its Fuseholder, and recheck AMP Ind...if this simple action has restored normal function (by cutting through corrosion which had finally grown to the point of preventing a good connection), deduct 5 points for neglect and not following my advice and protecting fuse-ends with ACZP!  Clean and protect conical fuse-ends at next opportunity!  (Ref:  Gas-Tight-Joint Tech Article).  If AMP Ind continues to remain lit (AMP Ind ON Condition 2 [as above in Notes and Functional Description:] has been eliminated as the cause), more serious troubleshooting efforts need to be undertaken at the next opportunity as without further checks, it would seem that Condition 1 - No Charging System Output, is the cause.  Car can be driven, bearing in mind that when Charging Sys is off-line...all vehicle power is being supplied by Battery, so this is limited. 

**  A blown Fuse 1 is a pretty rare occurrence!  If it was truly blown by a fault overcurrent, we can likely associate this event with a memorable action...like having dropped an Ignition Hot wire under the dashboard...etc...replace Fuse and work more carefully to prevent recurrence!  If it was weakened by corrosion and this was a "nuisance blow", replace Fuse.  See also:  Reference:  Blowing Fuse 1 on a 122

Regulator Bypass Test:  When Charging System fails to have an output (AMP Indicator remains ON when engine is running), a good troubleshooting strategy is to separate function of the VReg and Alt, by manually controlling the Alt and applying Field Current (test operator overrides VReg function and intentionally commands Field current) while monitoring system voltage.  The way to do this is to use a test wire between D+ and DF terminals of VReg to bypass Regulator function while engine is running....if when doing this, a rise in System Voltage is observed when raising RPMs to around 2000 (as well as AMP Ind going OFF), Alt is functioning and output is pulling Sys Voltage up.  VReg is not providing the Field Current path.  See:  Before Replacing VReg below! 

Alternator Checks:  If AMP Ind stays ON, and no rise in Sys V is observed, Alt is not putting out current even though Field terminal is being manually powered (simply measuring the voltage at this terminal is not enough!).  If a Current meter is available, measure (Field) current into this terminal.  It should be 2-2.5A.  If Low, shut off engine, disconnect wire at F(ield) terminal of Alt and check resistance of Field to chassis...it should be a consistent 4-5 Ohms  (including while turning pulley)...if intermittently or (worse) continuously open, Brushes and their interface with Slip-Rings need to be checked and serviced! 

If Field Resistance and/or current check out normally, Stator and Rectifier Diodes need to be checked.  This requires disassembly of the Alt and measuring the Diodes with a meter like a DVM.   

Before replacing VReg, all of its connections should be verified to be clean and tight!...a simple matter of a loose or corroded chassis connection at D- would cause a VReg malfunction!  BUT...notice also, that VReg chassis connection is not made locally and externally at VReg, but remotely and internally to Alt.  A poor chassis connection would result in a voltage being measurable at the D- terminal of VReg.  Any more than 0.25 to 0.5V measured at D- terminal (to chassis) during operation, will require inspection of the chassis connections inside Alt! 

 PLACEHOLDER FOR MORE [I invite readers to submit test and failure troubleshooting experiences!]


Identifying SEV Marchal / Motorola Alternators: 




1800E/ES Charging System Notes:  Info added as much of the 1800E and ES systems are similar to the 123GT system (maybe the color codes and details change), but otherwise, info is quite applicable as the Charging Sys circuit is actually similar! 


1800E Charge System Excerpt showing critical components.  V Reg can be Relay or Solid State type. 

Note that Initial Alt Field Excitation is by way of AMP Indicator (Orange current path shown above), but Alternator will function by self-excitation without it! 

Link to a Forum Thread where a disconnected Excitation wire caused the AMP Indicator not to function (as expected really, as there was no current path to sink Indicator Current, or provide initial Field excitation current!), but Chg Sys kept on functioning (undoubtedly self-excited, and this is also rather expected!). 
Thread:  Alternator not charging:



External material sources are attributed.  Otherwise, this article is Copyright © 2018-2021.  Ronald Kwas.   The terms Volvo, Bosch, SEV Marchal / Motorola are used for reference only.  I have no affiliation with any 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 its 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! 

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