This one is really bugging me.

Everything in the circuit works, just not when its all connected.

When everything is plugged in, the preglow indicator light on the instrument panel does not light when keyed, but the plugs are energized for ~20 - 30 seconds. I read ~ 10.7 volts at the GP, and the car starts fine. I've pulled all the GPs and they glow red hot within 2 seconds when connected across the battery. So the GPs are all good and seem to be working fine when everything is connected.

If I disconnect the wiring harness to the GPs (unplug at the relay), the preglow indicator lamp lights up when keyed and times out in ~ 15 sec. and I read 12.5 volts at the pins to for the GP wiring harness. So the lamp, relay and timer all seem to be working fine when the GP wiring harness is disconnected.

My suspicions are that there is either a short across two or more wires or a weak (broken) wire(s) within the GP wiring harness. My next step will be to disconnect the wiring harness at both the relay and and the plugs and test for continuity.

Any suggestions?

Keep doing what you are doing. The relay could also be getting weak. IIRC, you should be reading more that 10.7 volts at the GP.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Thanks.

The voltage readings at the relay and at the GPs were identical to my "positive control" (85 300TD).

IIRC, the GPs have 11.2V stamped on them.

It turned out that the "stem" on GP #1 (the stud that the 8mm nut threads onto), was loose. It passed the bench test because I was compressing it by pushing it down onto the battery terminal, but was loose when I reinstalled it in the car.

Of course it was #1, otherwise the light would have worked.

Dropped in a "new" one out of the 85 and all is well.