Got a '97 F350 diesel with the brake light and ABS lights on. I know that the ABS monitor the brake circuit and will throw the ABS light. The following is what I've done to troubleshoot, any help much appreciated. I do have plenty of brake.

Replaced Master with new one, it was "weeping" at the seals. Bench bled it till no bubbles showing. Bled all lines, popped rear drums to look for leaks, none found. Shoes are at around 80% left. Manually adjusted shoes.

Checked vacuum on the pump. First tested line to booster. Running 25KG. Tested all lines off of manifold, running 25KG. Pulled heater line under hood on passenger side (black line), running 25KG.

Pumped pressure to booster with Mitey Vac. Held for 10 minutes or so. Went to zero after hitting break peddle (not sure if I'm doing this part right). Left booster line unhooked, checked peddle, it was hard.

Red light doesn't come on unless I pump peddle rapidly, usually comes on after 6 or so tries. Goes out in a minute or so, unless it trips ABS, then won't go out till I power off and turn back on, then neither light on. Sometimes trips both while driving down the road and hit brakes.

Heater works on defrost, and vents, but does not blow warm air to floor. Actuator motor is working under dash when switching from all three positions (defrost, vent, floor). It does sound like a leak under the dash when powering off but not sure.

Don't think it's the VSS as I can get it to work (brake light coming on) while in park in the yard.

Checked ABS code by grounding b/o wire. Got a code of 4.

Any ideas?

 

Do the brakes work OK?
If the light is only coming on when you pump the pedal you are using up the vacuum faster than the pump can keep up...especially at idle. Maybe the pump is getting weak??
Favorite Quote: If it ain't broke...fix it till it is.. LOL!

 

Hey there CJ.

Yes brakes are firm. Light will sometimes come on while driving when I push and hold brakes down, and that usually trips the ABS light as well. Personally, I'm leaning towards the pump or RABS. Don't think it's the VSS as I can get it to trip while in the yard in park. Speedo is also fine at all speeds from 0 up to 100 KM.

 

It could be the vacuum pump can't keep up with the added demand (constant brake pressure). Check the vacuum pump for recovery rate. Drain the vacuum system, start the engine with a gauge hooked up,...time it to get to 18-20 in hg. Not sure about your KG measurements. With the gauge still hooked up see if the vacuum drops way down after a couple brake applications.

Don't rule out The VSS (aka RABS) it can be intermittent. Have you pulled it and looked for metal shavings on the end...check the wire harness too. The pins and connectors can get corroded. If you decide to get a new one be sure to ask for a RABS not a VSS, you will get the wrong part.

 

Thanks again. Meant hg, not kg (us Canucks ). I'll try draining the system and hooking up the Mitey Vac. It is running around 25 on the scale, at least it was yesterday, and I did pump the brakes a couple of times. It would drop to about 20 or so then come right back up. I've also got a '97 factory manual and will run the checks for "constant on" light to see what I can find.

 

I fully understand the Canuck thing, I married one LOL!

Here's more things to check:

Yellow Rear ABS Warning Light Flashes Four Times When Retrieving Diagnostic Trouble Codes (Code 4)
Terminal backout in the anti-lock electronic control module harness connector at Pin 6.

Terminal backout in the RABS valve harness connector at Pins 2 or 3.
Bulkhead connector (between engine compartment and instrument panel) terminal backout or loose connection.

Anti-lock electronic control module or RABS valve connectors not fully mated with component.

Intermittent open or open circuit in Circuits 535 (LB/R) or 274 (BK/W).

Intermittent short or short circuit in Circuit 535 (LB/R).

Defective RABS valve.

GO to Pinpoint Test H .
This is the pdf file I posted earlier. It has the connector views too.

 

Hope she ain't reading what you post, us Canucks can get pretty wound up

Well, it just keeps getting more interesting. Did the following as per manual:

Disconnect RABS module (behind glove box) to see if red light comes on, if not replace RABS module. Mine didn't come on, but the ABS & Red light came back on when started. For giggles I checked the bl/orange diagnostic by shorting again. This time I got 16 blinks indicating all is well in the ABS circuit (did it twice). WTF!, yesterday it was definitly 4, and I tested it twice.

Might be an electrical gremlin, or the pump/booster is going out. I'll drain the air out tommorrow and hook up the mitey vac to see what I get for readings. Re-bled the brakes today too, checked the whole system for leaks (including pulling the drums again) and no leaks anywhere. I think I'll pull the VSS/RABS on the rear differential and check for filings as per your advice, and look at the connections too.

 

Think I found it, and shoulda tried the easy things first. Went ahead and tested vacuum at the pump, and on the booster this morning. Everything in spec.

Then it dawned on me. The ABS monitors the brake light circuit and trips the ABS if it detects something, that is the brake light being on. At least that's my take on it from reading some posts here and at dieselstop.

Checked the connector at the switch on the Emegency brake and it was very loose, and corroded. Cleaned it up and reconnected, took it for a drive, ran heat on all three settings (still not very strong on floor setting but a bunch of crap blows out every once in a while, so it might be plugged), hit brakes numerous times and neither light came on. Got my fingers crossed, but I think I nailed it this time....