The brake warning light came on this morning when I first started the truck. The rear antilock light is not on. The brake fluid level is almost full. Unhooked the parking brake switch and the light stayed on. Went ahead and drove to town and the brakes seem normal. What should I check next? What might be the problem?

 

A couple of things I didn't mention the first time. When you first start it the light is out, the rear antilock light cycles like is is supposed to, then when it goes off that's when the brake light comes back on. Also I swapped gauge clusters a couple of days ago, but the light didn't come on till 2 days later so I don't think that has anything to do with it. The brake system on this truck really doesn't have very many things on it that can go wrong, so just looking for the things that trigger the light so I can eliminate them one by one to find the problem.

 

What are we working on (year)?

 

Sorry, had a stupid moment there! It's the 89 F250 460 C6 in my sig.

 

Warning Lights
The RABS uses both the BRAKE and REAR ABS instrument panel warning lights to alert the driver to a system malfunction. Both lights must be working properly to assist in problem diagnosis. The red BRAKE warning light is used to indicate a low fluid level condition, parking brake applied condition or, for vehicles equipped with diesel engines, a low vacuum condition. To check this light, insert the key in the ignition lock cylinder and turn it to the start position. The light should glow in this position. If it fails to glow, service of the electrical system is required.

NOTE:
If the red brake light continues to glow after the key is in the run position, repair the brake system as required. If the brake system checks out OK, follow the Troubleshooting Chart to diagnose the problem.

The yellow REAR ABS warning light is used to indicate a malfunction in and a deactivation of the RABS. To check this light, insert the key in the ignition lock cylinder and turn it to the on or start position. The light should perform a self-check, glowing for approximately two seconds. If the light fails to glow or continues to glow after two seconds, refer to the BRAKE and REAR ABS Warning Lights Condition Chart for the proper diagnostic and testing procedure.

 

OK. My yellow light is doing exactly what it is supposed to do so no RABS problem. That's good.

The red BRAKE light does not come on when the key is turned on and engine not running, but when you start the engine in about 5 seconds the light comes on. Parking brake is not on (unhooking the switch made no difference) and brake fluid level is well within acceptable levels. Not a diesel, so no vacuum switch to worry about. Can I jump the wires at the master cylinder reservoir to check and see if the fluid level switch has gone bad? What else is there to look at?

 

There should be 3 wires at the master cylinder reservoir. Yes but if you jump to the wrong one the light just will not come on then jump to the other wire for a test. Use a ground or the black wire at the master cylinder reservoir switch for ground.



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Note I do not have a 1989 diagram only 1990 and 1988. The diagram above is for a 1990 but it seems like most of the wiring is the same for the 1989 and 1990.

 

Thank you very much for the info! I will check things out and report back.

 

Yea, I was telling myself this but knowing that I screwed something up!

Well, here is what's going on. I checked everything I could think of in every way I could think of and nothing changed. I could make the REAR ANTILOCK light glow or not glow anytime I wanted, so nothing wrong there. I could make the BRAKE light glow or not glow with the key ON engine off, but it stayed on any time the engine was running. As I was sitting there looking at the dash I started the truck and noticed the light came on at the exact same time the oil pressure came up. So I unhooked the sender and WHAT DO YOU KNOW, the light went out! The red ENGINE light came on, but that's what its supposed to do. Hooked it back up and light came back on. With the key on and engine off, if you look real close, you can see a faint glow from the light. So I guess it's pull the cluster and see what has happened. I did the oil pressure gauge mod but didn't have the correct sender so I hooked it back to the original sender (which it just an 8 psi on-off switch basically) which made it read pegged past H and that's probably what screwed it up.

Did I fry my new cluster? Did I fry something else? Is my stupidity reversible? If anybody knows, I'm all ears!

 

I think it is a diode causing the problem.

I Subford's wiring diagram above there is an item called a "Dual Brake Warning Diode/Resistor Assembly" part #E9DB-14A601-JA. Where is this located on the truck? Anybody know if it is still available? Any help or opinions on any part of this thread would be very much appreciated!

 

I had the same problem.
Check the master cylinder.

 

See post #10. Believe me, I wish it was that simple. I have checked everything you can think of. Thanks for the response. Sometimes I do overlook the obvious, Can't see the forest cause the trees are in the way kind of thing!

 

I have searched the web to no avail. Anybody have any input on this?

 

Have you had the brake pedal, feeling like there is no pressure in it? Or maybe say go soft on you as you apply it?