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Brake warning light question/problem...

Keitha

Keitha

1/2 ton status
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Location
Costa Mesa, CA
1991 3/4 ton 4x4
The brake warning dash light comes on while driving and I can not figure out why? It functions fine when I put the parking brake on and turns off when I realease the parking brake. The problem is that as soon as I hit 35 MPH it comes back on again (and stays on) but a bit dimmer. The 35 MPH speed is pretty consistant too as the light will not come on until I hit that speed. Occasionally at highway speeds it will flicker a bit but typically will stay on until I shut the motor off. This is not an intermitant issue, it happens every time I drive.

Any advise? Could it be the primitive ABS system on these trucks?
thanks,
 
What is your speedo doing? If its jumping around at all, your Vehicle speed sensor VSS might be going bad, or ahve a bad connection.

Had a lose wire on my 91 burb (old body style, vs your 90's body style), and the speedo woudl jump all over the place. Once that happened, and i hit the brakes a time or 2, they felt funny and the brake light came on. THe ABS was getting a bad speed signal.

If it wasn't just at 35, i'd say it was low system pressure or something, as that light functions both as a parking brake light and a warning idiot light, but the speed thing makes me think its abs related.

Wait, your 91 is a 90's style, right? That shoudl have 4 wheel abs, and its own ABS light. Confirm what era truck we are working on, and we can help you trouble shoot.
 
1991 is the last of the old, pre-90's era (same as you have apperantly). It does not have an independant ABS light and I believe that it only has rear ABS. Speedo is perfect and right on the money accuracy wise (verified by road-side radar speed signs). I am pretty sure that the vss is working fine.
 
I had some problems with my brake warning light as well but had nothing to do with speed. My ebrake would release but if it didnt come up all the way the warning light would come on. Learned to just nudge it up with foot! probly useless info here but it happened to me so who knows?
 
Erik's K5 said:
I had some problems with my brake warning light as well but had nothing to do with speed. My ebrake would release but if it didnt come up all the way the warning light would come on. Learned to just nudge it up with foot! probly useless info here but it happened to me so who knows?
Click to expand...

Thats what i do, but don't think that is the issue here.

I am still betting for some sort of ABS signal setting the light off. May be VSS related, and not affect teh speedo. There is at least 2 vss sensors on that 4l80/xfer case setup, if not 3. I forget.

Wouldnt' hurt to crawl under there when its cold and unplug/replug all the harnesses/sensors around the trans.
 
I will try to track down the ABS wires - Does anyone know which they are?
I can not figure the ABS out on this Sub: there are no speed sensors on the wheels that I can find. How does the system know when to engage ABS? I am not even convinced that it has ABS. It does have a black "computer" box located adjacent to the brake master cylinder.
 
Yah, i don't know where the abs setup lives. BUT, im guessing it only works in 2wd, in which case, the speed sensor on the tcase output would be all it needs
 
Their is what seems to be a sensor on the prop valve. Check wiring there. Also check the switch to pedal and make sure nothing lose or broke. I woulda said bleed your brakes anyway but that part is upto you.
 
Solved: It was simple as adjusting the brake light switch. On my Suburban there are 2 of them, one controlls the cruise controll and brake lights and the other is for the RWAL and TCC.

thanks for the help,
 
Keitha said:
Solved: It was simple as adjusting the brake light switch. On my Suburban there are 2 of them, one controlls the cruise controll and brake lights and the other is for the RWAL and TCC.

thanks for the help,
Click to expand...

Ok, so after i fixed my VSS wiring to fix my speedo, and put new front pads on, my brake light still stays on.

What is the proper way to adjust this switch you speak of? Mine is a 91 too
 
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