More problems

[Beedee]

I seem to be having my fair share of problems with the truck lately.
First of 2
My truck is a 98 with auto, I remember reading in one of the posts that the reason that the tach drops when stepping on the brakes is because the ECM cuts the idle down when braking, I have noticed lately that it isn't dropping the RPM's down when I brake anymore.
2nd. problem, It seems like the ABS is trying to do its thing for no reason, the roads can be just fine, but when i'm just coming to a stop the truck will shudder and you can feel it in the pedal, just like the ABS is activated. I have had a couple of occasions when the brakes have released and I have went further into the intersection then i wanted to, can cause a little panic when it happens.
Are the two problems interrelated? There are no lights coming on. I flushed the brake fluid just because it was time again, P/S is up to proper level.
I'm taking the truck with me this time when I go to work, so will have some time to look at it, then off to Alberta at the end of the month for new job, so would like to have it fixed up for the trip.
How hard is it to disable the ABS, can you just pull a fuse and it will default back to the old way?
Once again, thanks for the help
Brian

[gmctd]

Your TCC is stuck on - the brake switch is failing\failed, causing both problems

[a5150nut]

IIRC, there has been problems with dirt, mud, ice, and rust collecting in the wheel sensors causing your sort of braking problems. I'd start by checking behind the wheels on all four corners. Also look for disconected wires there to. Or do a search of the forums under ABS or brake problems.

[redbird2]

abs can be disabled by un pluging the 2 wire connector going to the abs control valve mounted on the left inner fender, the wires should be large wire 12 ga possible one is red other black or brown don't reminber off top of head I did this to my 98 K2500 when the right wheel speed sensor failed so not to get ABS application when stopping. however mines turned on the ABS light on the dash. your 98 only has 3 speed sensor , one each left and right front wheels, and one on output of transfer case ( well that how the 3 98 I have are set up)

as for your first question not sure does your cruise control work if not I think GMCTD is on to some thing about the brake switch not working all tied togeather as the ECM doesn't know you stepped on the brake I hope this helps a little good luck keep us post what you find

[Beedee]

gmctd, Don't know what a TCC is, what is that short for.
A5150nut, I ran a search but couldn't really find what I was looking for, but I will have a look at the sensors and see if there is something there.
Redbird2, Thanks for the info. I didn't want to disconnect with out knowing if if would affect the braking, as for your other question, the cruise works fine.
Where is the TCC located?
Thanks for the help guys.
Will be away from the computer for the next several days while I get back to my place of employment.

[gmctd]

The brake switch controls the brake lites, cruise control, and the Torque Convertor Clutch lockup.

Usual failure symptom is the TCC won't lockup, or stays locked up when the brakes are applied - symptoms of that are tach indication doesn't drop, and the shudder when coming to a stop.

The dual function brake switch is on the brake pedal - more accurately, the brake booster rod attached to the brake pedal - not adjustable, only replaceable

Could also indicate the TCC solenoid in the trans is sticking\stuck, or worst-case, the TCC piston is sticking

I'd try the brake switch, first..............

[Beedee]

So is there a way to test the switch, or is it just as simple to get a new one and change it out? Should the brake lights work properly, or are they on a different circuit?
The cruise works fine, I'm pretty sure that it is going into lockup.

[gmctd]

Seperate circuits for brake lites and PCM\trans - you should get 4 distinct 'bumps' from dead stop - 2nd, 3rd, 4th, and TCC lock.

After lock TCC should unlock each time you ease off the APP - tach drops - then lock again as you resume speed.

After lock TCC should unlock each time the brake pedal is pushed -tach drops - complete stop should keep TCC unlocked.

Release the brake pedal and TCC should lock appropriately

[Beedee]

okay, to narrow it down a bit more, as I was driving down town tonight, I noticed that the tach would drop when I applied my brakes, but only when i was moving, if I am at a dead stop and release and apply the brakes there is no difference in the tach, so it would be from say 1000 rpm it would drop about 400 when moving and I think its holding steady at 600 when stopped.
Am I making any sense

[JohnC]

if I am at a dead stop and release and apply the brakes there is no difference in the tach, so it would be from say 1000 rpm it would drop about 400 when moving and I think its holding steady at 600 when stopped.
Am I making any sense
\

Yes, and that's normal. Once you're stopped, it just idles.