The pedal is firm and the brakes function perfectly. It is definitely an electrical issue. If I only knew where that diode was I could probably fix it!

 

And a quick search says...

https://www.ford-trucks.com/forums/1...ock-light.html

Post #9 seems the most informative, post #6 just says the general location.

 

For 1990 the Diode/Resistor Assembly is located in the main harness, near the Anti-Lock brake module.

For 1988 from the diagram post above for 1988 it shows it near S520. S520 is located near warning buzzer T/O.

T/O = a Take Out in a wiring harness (where wires split off a wiring harness).

 

Note that post #9 does not say what year the information is from and the Thread is about a 1992 truck that everything was different on than the 1991. The 1992 manual says "LH rear of engine compartment, taped in harness near C202 [6.7in from C202] (B9)". So it looks like it was from the 1992 manual.

So i would go by post #6 in that thread as it was from a 1989 manual.

 

OK I found it in the 1989 F-series shop manual:


Both of these seem to say the same place:
For 1990 the Diode/Resistor Assembly is located in the main harness, near the Anti-Lock brake module.
/

 

Thank you Dixie!
Why is it when the person doing the search because they have a problem and desparetly need help can't find squat but everyone else can! I should have looked at more posts that included the antilock light but I didn't since mine only has the brake light on ant the rabs is working properly.

 

Note that the Thread Dixie460 pointed to would be of no help to you as post #9 says it is under the hood and yours is inside the cab behind the dash.

 

Thank you Subford! That is great info to have. My truck has the rabs valve down on the frame but does not have a control module like a "real" rabs system. Is that wiring and diode still there or is it somewhere else?
Sorry to be such a pain to everybody. I know most people would just remove the bulb and forget it, but I like to try to keep my stuff right. I am a very capable mechanic and can do most everything, but electrical is not my strong point. Thanks again.

 

[QUOTE=eakermeld;14891934]Thank you Subford! That is great info to have. My truck has the rabs valve down on the frame but does not have a control module like a "real" rabs system. Is that wiring and diode still there or is it somewhere else?
Sorry to be such a pain to everybody. I know most people would just remove the bulb and forget it, but I like to try to keep my stuff right. I am a very capable mechanic and can do most everything, but electrical is not my strong point. Thanks again.[/QUOTE

I take the part about not having a module back, I see it now! But I have run out of time today to work on it. I will do some more testing and report back.

 

Here is an update. Checked the diode in the harness and it's doing what it's supposed to. So the next step was to go back to whatever I did before I saw the warning light, which was the instrument cluster. So pulled it out of the dash and went over every connection on it looking for burnt of bad spots in the circuitry. Nothing. So I went and got my old cluster w/o tach and put it in. No light. Ok lets try the other one again. No light. WOO HOO! It's a miracle! Put the dash panels back and fired it up to move it out of the shop........there's that friggin' light again. WTH! So I guess there must be something bad in the printed circuit of the cluster that I cannot see that touches something when it's fastened down. Next time I get a chance to work on it, I'm going to take things apart one screw at a time and watch that light like a hawk and see what makes it go out.

If anybody has any insight on this, fire away!

 

I may have missed something on this problem.
You are changing clusters, why?
Is the replacement cluster from the same year as your truck?
If it was the same year was it from a diesel and yours is a gas truck?
Dose one cluster have a tachometer in it and the other does not?
Did you get the replacement cluster from ebay, salvage yard or one of your other trucks?

 

I'm changing clusters because the little stud that holds the speedo needle broke off. Extremely rare failure, but pretty much normal for my luck.

Cluster came from a running 1988 truck out of a salvage yard. Everything I read said what I already knew, that anything 87 thru 91 is simply plug-n-play. Light wasn't on in that truck and wasn't on in mine for the first day.

From gas to gas.

Yes, the replacement has a tach, the old one did not. That was another reason I wanted it.

I know this is my fault because I did the oil pressure gauge mod but didn't change the sender the same day I changed the cluster and drove it for a day with the gauged pegged past H which burned something up. If you unhook the sender wire the light just barely glows. Hook it up and start the truck or ground it out and as soon as the gauge moves the light comes on bright. Now it's just a matter of figuring out what got cooked.

Thanks again for all the help you have given me so far.

 

To update the thread. It was my fault. I burned up something in either the printed circuit or the indicator warning control assembly by doing the oil pressure gauge mod without changing the sender. So the fix was......

Installed Autometer sender part#2241. This sender has an OHM range from 70 to 10. Works perfect for the OEM gauge after the mod is done.

Got ANOTHER gauge assembly, did the oil pressure gauge mod, cleaned it up and installed it.

Now everything works perfectly. Thanks to everybody, especially Subford, for all your help.