'pants

I've been living with these two warning-lights, for years, but recently noticed the rear wheels locking up quite easily - something I'm pretty sure is new - so I got back into trying to diagnose/fix the problem.

Following FSM, I switched on the ign and shorted pin 13 of the DTC to ground. ABS warning light began to pulse. Fault code was 7, assuming I'm understanding the interpretation correctly, and 8 if not...but either way, I followed the diagnostics.

For "7", it's: "Electronic control module [CAB] fuse pellet open, isolation output missing, or valve wiring shorted to ground." (For "8", it's identical, but also casts suspicion on the other of the two RWAL solenoids, which tested OK, too.)

More detailed FSM troubleshooting instructions note the following possibilities:
"Short to ground from terminal 1 in the 14-way connector to the RWAL valve." (by which they've GOTTA mean the CAB connector, since the RWAL connector is only a 4-pin deal). Tested - found no short present.
"Short in the RWAL valve's isolation solenoid." (SB 3-6 ohms). It tested within range, and solenoid will audibly click when hot-wired.
"CAB." OK, I will deal with that, then.

So I found a "guaranteed" used CAB on ebay, ordered and installed it, and...you guessed it, lights stay on. The DTC is supposed to reset when the CAB is disconnected, which of course I'd had to do to replace the original anyway, but the same or similar (7 or 8) fault-code continues to display.

(I found no "fuse pellet" during a close inspection of my old CAB PC board - it's easy to open up the plastic housing - and the rebuilders indicated they were not aware of such an animal either...only wish I HAD, since it would at least be something to point at, if not replace.)

I've tried other stuff that isn't really indicated in the DTC to fix this, eg replaced the speed-sensor, replaced the MC (pedal-sinking was alarming - I guess I'd lost one brake circuit) but like I say, if I'm going to believe anything in the FSM, I should have faith in the accuracy of the codes...right?

All I can think to do at this point is to see if I can find an actual rebuilt CAB that's at least less likely to be internally flawed than my ebay purchase, but it now seems to me like that's inevitably going to be just an expensive disappointment.

Anyone?

Mahalo - Dave

VADSLRAM

That was my symptoms when the brake light switch went on me.
The switch has about 6 sets of contacts and if they don't make contact the brain thinks the pedal is stuck or some other problem and lights go on, It also disables the ABS system so when the back tires want to break loose there is nothing to prevent it.

'pants

AYFKM??

I will definitely check that. It's hard to believe that the FSM would skip that as a possible cause.

I just looked at the FSM "stop lamp switch" schematics (appears in two or more locations depending on coverage), and yes, there are three sets of contacts, so six terminals, with just one set of contacts devoted to the CAB input...sounds just goofball enough to be the perfect fix. If I get continuity between terminals 1/2 while switch is in place & there's no pedal action, that's a problem - hopefully THE problem.

Future ref - dug from schematics:
terminals 1/2 (N.C.) are CAB-sensing pair, normally grounded via 2, and isolated when pedal down
terminals 3/4 (N.C.) form one set, open on braking, to interrupt cruise-control
terminals 5/6 (N.O.) are the stop-lamp-switching pair

I'll post back after weather improves and I've had a chance to check this...

Busboy

I forget exactly how the 3 sets of contacts work but basically there is one set for the brake lights, one set to disengage the cruise and the last set to ensure that the brake pedal is NOT depressed when the cruise is engaged, so I don't remember if you get the same reaction from all three sets when the pedal is operated. I must be sleeping or something... I see you have already found the contacts and which are Normally open and which are normally closed, should be able to pin point from that info. The brakes switches are very poorly made and have been a known problem for many years.

'pants

Well, my hopes were dashed; the switch is out on my desk now and tested fine, though yeah, I suppose it could be an intermittent problem, like oxide-crap in the switch that... what, somehow keeps contacts closed when the pedal is down...but...I kinda think replacing it would be good money after bad. (I could see dirt keeping contacts from CLOSING dependably, but not so much from opening that circuit.)

I ran across yet another reference in the FSM to a fuse within the CAB - not that it'd normally be user-serviceable - but I'd like to know if it's there, because if I could find it, it would at least be a smoking-gun, and I'd either ghetto-repair it or feel better about shelling out the $$ for a replacement. No PCB components jump out as being different and therefore likely to be a fuse, "pellet fuse," etc. Anyone?

"A fuse internal to the CAB, provides a fail-safe
device which prevents unwanted control over the isolation
and dump solenoids. The fuse is in series with
the isolation and dump solenoids output circuits. If
the internal fuse is open, the CAB cannot provide
voltage to energize either solenoid and antilock stops
are prevented. I f the fuse is open, the braking system
will operate normally but without antilock control
over rear brake pressure."

Busboy

Due to the long history of these switches I would change it out on spec, they used to be less than $20.

bandit408

I used to work on these at the dealer. Have you verified the fuses including those for trailer wiring are good? I always verified with a DMM (or Power Probe) as visual would not always show a bad fuse.

patdaly

FWIW, the brake switch can be misadjusted and still function for brake lights but still turn on the ABS light.............. Ask me how I know.

Readjust it, it is free and will confirm that it is not the issue.

'pants

Pat, I hope you're my savior yet again.

I was thinking along just those lines when I reinstalled the stop-lamp switch, hurriedly, to head out last evening. I didn't have time then to go re-read or print what's in the FSM on 'adjustment,' but now that I've re-read it, almost none of the key parts make sense:

"STOP LAMP SWITCH
(1) Push and hold brake pedal down
(2) Pull switch plunger all the way out to fully
extended position.
(3) Push switch plunger Inward 4 detent positions
(or clicks). This is required preset position. Plunger
will extend approximately 14 mm (0.55 in.) out of
housing at this setting.
(4) Release brake pedal. Then lightly pull pedal
fully rearward. Pedal will adjust switch plunger to
correct position as pedal is moved to rear.
CAUTION: Do not use excessive force to move the
pedal rearward for switch adjustment. Excessive
force will damage the switch."

but..."Pulling the plunger out" is...what the switch's internal spring does, no?
um, "detent positions?" and four of them? that are intended to be 'felt' when one presses the plunger? I've got no clue what they're on about with that - mine surely doesn't work that way. If that sentence were not so clearly written, I'd think they were talking about a whole-switch-mounting setup that has four detent positions (again, not what *I* have, as far as I could tell)

I did notice that there's an extremely slight "unused pedal return" range, so to speak - I can pull it up towards driver very slightly. Whether that's enough to make the difference in the 1/2 contacts being kept open is another question. Anyway, ignoring the FSM instructions above about adjustment, I noticed there are a couple of maybe-5/16"-hex-head screws that looked like they clamp down through a slotted bracket on which the whole switch rides -- now THAT would have been my guess as to the real means of switch adjustment. Is that it?

But before I go stabbing off into the darkness again, I was thinking I'd test the contacts in situ to see if that's really keeping the ABS contacts open, even with the pedal relaxed/up...but then realized by the time I got done with that I'd be in a cervical collar. But happily, the FSM shows that the ABS contact in that switch leads up to pin 7 in the CAB connector, which would be much easier to test from. So if pin 7 is grounded while the pedal is relaxed, well...it's my huckleberry...

'pants

I tested as I'd described above - for continuity to ground from pin 7 of the CAB connector (white/pink). And dammit, it performed exactly as it should (no pedal = switch closed/ground connected to CAB), testing consistently for ten or twenty cycles, so I will leave the mystery of proper stop-lamp-switch adjustment to some future date.

Also tested all fuses in the compartment, verifying that 12V is getting through them. One ckt failed (a 5A fuse for "illum") in that there appears to be no feed to it - but FSM shows it having to do with panel-illumination, which isn't even a problem AFAIK, so that's nothing.

If it matters, the ABS/brake warnings come on immediately on key-on, only going out during key-over to start, then immediately on again.

Anything else? A rebuilt CAB, and... maybe continued problems?

patdaly

LOL, I have never adjusted them like the FSM says. I just reach under, twist it out, pull the plunger out a click or two, and shove it back in and twist to lock.

Never failed me yet.

That being said, you don't have a brake switch issue, because the light does not come on immediately, rather they come on after you have reached around 20 MPH.

I would put a new 5A fuse in that circuit anyway, you never know when one part of a circuit interacts with another........

'pants

Thanks, Pat.

I got all my eggs in the "new CAB" basket.

Failing that, when you see a beaten white '97 dually fishtailing and getting sideways while braking hard, think of me...

'pants

Filling in the one or two heartbreaking chapters I didn't write in the last few months, I've replaced the CAB with a new (rebuilt), and as of today, replaced the hydroboost. Same-same. ABS & brake-lights remain on. I'd suspect broken-wires-in-diff-speed-sensor-plug (and will check this when I get my dump-bed operable again) but the fact that the lights come on before I've even moved the truck don't jibe with it being a speed-sensing fault...right?

I wouldn't be doing all of this just because of the warning lights; for the last 3 days it's felt like no power to brakes (but no problem with steering) and I've had a chronic hydroboost drip that just needed to get fixed before any more rubber beneath it gets dissolved, so the hydroboost seemed like a likely candidate.

Now I've got the power-brakes back, but I'm also back into the "OPPOSITE of RWAL," wherein it's almost impossible to stop, at least from low-speed test-driving, WITHOUT my rear wheels (all four - it's a dually) skidding audibly.

Previously, I changed the MC and diff speed sensor, and as I mentioned above I just replaced the hydroboost today. Now I'm turning my gaze on the combination valve, because, well, the rear wheels are locking up most of the time right now (and this thing is supposed to proportion, right?) and because it's maybe the 2nd-to-last item that I have NOT replaced yet (I've not touched the RWAL valve - I think the last item possible - because last I checked they just weren't available anymore, and because the solenoids work...neither of which means it's any good, I suppose). I couldn't do a worse job of shotgunning if I'd just ordered all new brake components years ago when this problem originally cropped up.

If I take the proportioning/combo valve out, is there anything to "look for" to indicate it's been misbehaving?

Trouble-codes said that it was the CAB - probably still do. (yeah, thanks, trouble-codes...)

Busboy

I did a little reading in the 2000 factory manual and I know yours is a 1997 and may be different. It says that the codes do NOT reset by disconnecting the batteries and must be reset using the DRB scan tool or 50 restarts. It also says that if the CAB is replaced the tire size and revolutions per mile must be programmed in using the method in the chassis diagnostics manual. It also says that the brakes must be bled then an anti skid test done to cycle the valves then the brakes bled again. Also on 2500 and 3500 the brake proportioning valve is eliminated and an "electronic brake distribution" EDB is built into the ABS to control the rear brakes. The EBD will not work if the brake switch is not closed, (brake pedal depressed), the red brake warning light is on, vehicle is in 4X4, an ABS system fault critical to EDB, or more than 2 ABS system faults are present. With all this in mind the red light can also be put on by the parking brake switch either from pedal droop or out of adjustment.

'pants

Jeez, if I had a 2000/ 3rd gen, I'd be asking you to just shoot me now. But yeah, pretty sure those hoops didn't exist for 1997-vintage models, though later today I'll (again) review the FSM to see what I can see. I've been up and down the FSM brake sections repeatedly (though not recently) so I'm pretty sure that stuff would have registered in my ailing memory.

I did make the silly mistake of overlooking the brake-switch height resetting yesterday when I replaced the hydroboost - discovered the glow of always-on brake lights last night - but also not expecting that to change my warning-light situation, since the ABS light comes on before the vehicle is even set in motion (during which time it would be entirely reasonable to have one's foot on the brakes).

From some other threads, I do want to get back into the harness between diff-sensor and the next connector (wherever that is) and verify continuity. Seems at least a few people have reported open circuits there, from wire broken within insulation near the sensor at the diff, I think.