Does anyone see anything wrong with this 1984 FB CPU circuit board?

I'm having issues with the lighting in my instrument cluster. I've already cleared the bulbs, fuses, connections to the cluster.

All driving and turn signals work on the outside. No background illumination in the cluster or turn signal hazard lighting indicators.

All other cockpit lights and warning buzzers work.

 

Shouldn't I just be able to put probes to the blades in the big and small connectors and if no continuity that would confirm an issue on the board?

If positive continuity as intended then that would infer that it is not in the board?

If yes to the above, I cant find a chart that shows the sequence. Doe anyone have a continuity testing sequence for an 84 CPU.

 

It's probably best to put it back and test with that circuit on/off and measure input/outputs.

When I was in the Navy as an Electronics Technician, we had a six step troubleshooting methodology. You're basically jumping right into step 5 without examining the wiring diagram to give a logical guess as to what can cause this issue. We need to step back and at least do step 4 and take some measurement before jumping into the faulty component. The measurement will confirm if that CPU is the issue. After that then we dig into that CPU to determine which component(s) are the cause.

1. Symptom Recognition.
2. Symptom Elaboration.
3. List Probable Faulty Functions.
4. Localize the Faulty Function.
5. Localize the Faulty Component.
6. Failure Analysis.

Measuring continuity will only get you so far. You can only measure continuity from the plug blade to the first solder joints on that circuit. Basically only testing the foil runs.

Which CPU is this?

 

85 fb gsl

 

But which CPU? The flasher/turn signal, auto trans, cluster, etc... If I know which CPU, I can look at the wiring schematic.

 

I didn't know there were multiple.

I pulled the one from the driver's side firewall behind the fuse block.

The issue is impacting the wipers, instrument cluster illumination, instrument cluster flashers, and the horn.

All instrument cluster (along the bottom) warning light indicators are working.

Also, I tested the system without the CPU connected and the fuse still blew.

...without the flasher, fuse still blew.

...without whatever that third one is, fuse still blew.

Now, I believe what you say; however, wouldn't that cause have been eliminated if the CPU wasn't connected?

 

If the fuse blows without this module connected, we can say for sure there is a direct short and that this module is not the cause. Which fuse is blowing and I'll take a look at that circuit.

 

Thank you!

It's the 15a wiper fuse 85 FB.

No wipers, instrument cluster lights (except warning lights - they all work), horn.

I already tested the entire wiper / turn signal switch for continuity. It passed, but that was last year.

 

Try testing the wiper motor itself for continuity/short. You can also unplugging the wiper motor and see if it still blows the fuse.