CUCV - GEN1 and GEN2 Exciter Circuit troubleshooting

[Warthog]

READ the first few pages of the Troubleshooting Section of the TM 9-2320-289-34 Tech Manual. It explains how to read the wiring diagrams.

GEN1

Sometimes the TM wiring diagrams don't post all the circuits. The GEN1 exciter circuit is one of those.

The GEN1 exciter circuit is controlled by the heater fuse. Why GM built it this way is anyone's guess. Check it first before any other troubleshooting.

Follow along as I trace the GEN1 circuit in a stock system:

1. From the Negative terminal of the rear battery (same as the positive terminal of the front battery) to the 12v Engine Wiring Harness Block (12v Bus) on the firewall - wire RED 2A - Diagram E-1

View attachment 448078

2. From the 12v Bus to the firewall wiring block - wire RED 2E - Diagram E-3

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3. Thru the firewall plug to the inside half of the plug and continued to the next diagram - wire RED 2A - Diagram E-3

View attachment 448081View attachment 448079

4. From Diagram 3 thru the wiring harness to the ignition switch - wire RED 2C. Thru the switch and to the heater fuse - wire ORN 300 - Diagram E-4

View attachment 448082

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Now this is where the diagrams are not marked. I have physically traced the wires and have verified the circuit.

The Heater fuse supplies power to both the heater circuit and PIN#5 of the instrument cluster. PIN #5 supplies power to the GEN1 bulb, the 4x4 Indicator Lamp and the Oil Pressure circuit.

I have added the wire to the Diagram

5. From the Heater fuse to PIN #5 of the cluster plug, thru the circuit board and GEN1 bulb back to PIN #8. From PIN #8 of the cluster plug to the interior firewall plug - wire BRN 25 - Diagram E-9

View attachment 448083View attachment 448084

6. Thru the firewall and to the GEN1 excitor plug - wire BRN 25 - Diagram E-9

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The brown wire at the GEN1 exciter circuit plug has to read 12v when ever the key is on. The red wire will read 12v at all times


So to sum up. The Heater fuse controls many things: the heater, the GEN1 excitor circuit, oil preasure circuit and the 4x4 lamp circuit. An issue with any of these will cause a low voltage condition and possibly interfer with the GEN1 operation.

Did you also notice the black "goo" in the firewall plug? GM used some type of grease that transforms into this black goo and DOES cause issues with circuits. Taking the plug apart and cleaning the contacts does wonders.

 

[Warthog]

Here is a Service Bulletin that help explain how an alternator works. It is not for the 'Isolated Ground" units we use but covers how they work.

The "lamp" will be on when no power is being generated and "excites" the windings. The "lamp" will go out indicating that the alternator is producing voltage

 

[Warthog]

reserved

 

[Warthog]

Figured it was time to complete the GEN2 portion

GEN2 is a little more complicated. It has both a 12v circuit and a 24v circuit. The 12v circuit energizes the GEN2 relay when the key is on and the 24v circuit send power to the alternator exciter plug.

Lets work on the 12v side first.
1 – From the Ignition Fuse #15 – wire .8 PNK/BLK 139A thru the splice and out wire 139G (continued to figure E9). Diagram E-4


2 – Wire .8 PNK/BLK 139G to GEN2 relay under the dash. This relay is energized when the key is in the run position. Diagram E-9


3 – Relay is grounded at the 6 legged spider connection next to the parking brake


24V side

4 – From rear battery to the 24v Positive Terminal Block – 32 RED 1 (SGT). Diagram E-1


5 – From 24v Positive Terminal Block to the firewall plug – 1 RED 2Z (fusible link) then 3 RED -2Y(SXL). Diagram E-1

6 – thru the firewall plug

7 – Firewall plug to the fuse box Fuse #12 – 3 RED 2Y and 3 RED 2X (spliced with starter relay). Diagram E-1

8 – Fuse #12 to instrument cluster pin #6 - .8 ORN 922. Diagram E-9

9 – Instrument Cluster Pin #6 thru the GEN2 bulb out to in #7. Diagram E-9

 

[Warthog]

10 – Instrument cluster pin #7 back to GEN2 relay 24v input – 1 BRN/RED 920. Diagram E-9

11 – GEN2 relay to the GEN2 Diode under the dash cover, 1 BRN 25Z. Diagram E-9

12 – GEN2 Diode to the firewall plug 1 BRN 25Z (not labeled in the diagrams). Diagram E-9

13 - Thru the firewall plug

14 – Firewall plug to pin #1 on GEN2 exciter plug – 1 BRN 25Z (SXL). Diagram E-9