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Help with 1990 Suburban


ButchHandy
11-08-2005, 10:33 AM
Hi everyone,

Looking for some help on where to start looking for a couple of problems on my Suburban. It is a 1990, with a 350, 4x4. Stock.


First, the dash brake light blinks/flashes while driving down the road. Almost like a wire loose somewhere, but I can't find it. If grabbed every wiring harness I can find and moved them around to try to get the light to come on, but have had no luck finding it. I've checked the brake fluid level, and it is full. Brakes work fine. I've checked the e-brake to make sure it wasn't me hitting it a little with my leg, but that doesn't seem to be it either.
Just some hints from anyone that may know of a common place a wire could be shorting to cause this, or what else could cause that light to blink? Kind of distracting in the evening!


Second, when vehicle is running, the RPMs will sometimes just increase about 800-1000 RPMs on their own. Just enough to scare you if you are sitting behind another car, and don't have the brake pedal pushed hard. The increased RPMs will last anywhere from a few seconds, to as much as a minute. Doesn't matter if the A/C is on or not, it will do it either way. Very random though.


Any help would be appreciated!

2000CAYukon
11-08-2005, 02:04 PM
The brake light flashing is telling you something is wrong with the brakes or abs. The flashing will be in a repeating pattern. Something like 4 short flashes, then a long flash which repeats.

Watch the light and post back the pattern that you see.

The RPM change could be a vacuum leak so check all the vacuum hoses for cracks. The TBI gasket is also another source of vacuum leaks.

//2000CAYukon

ButchHandy
11-08-2005, 02:23 PM
It is not a pattern. It is just a random flashing of the light coming on, like a short. Sometimes quick flashes that you can't see unless it is dark outside. Sometimes, it will come on, and stay on for as much as 3-4 seconds while driving. Doesn't really come on, unless you go over a bump, or a dip in the road, or make a sharp turn. Turning to the right seems to make it more likely to come on than a left hand turn. What wire would be able to short out that would cause the dash brake light to come on at all? That is why I only suspected the e-brake switch, since I know it makes the light come on when pressed.

My Suburban doesn't have ABS, so nothing along that way...


The motor is new. About 5000 miles on it now. The idle problem showed up about 1000 miles ago. It has new lines and gaskets when we replaced the motor, but I checked them to make sure everything still was connected, but didn't find anything wrong. May be time to pull out the starter fluid and see if I can find a leak that way....(Oh crap! I'm on FIRE!)


Thanks a lot for your input though!!!!



The brake light flashing is telling you something is wrong with the brakes or abs. The flashing will be in a repeating pattern. Something like 4 short flashes, then a long flash which repeats.

Watch the light and post back the pattern that you see.

The RPM change could be a vacuum leak so check all the vacuum hoses for cracks. The TBI gasket is also another source of vacuum leaks.

//2000CAYukon

2000CAYukon
11-08-2005, 03:09 PM
I have the service manuals for 90 and will look at them tonight to see which wires control the brake light.

The emergency brake when depressed will light up the brake light. It could be a faulty switch. Disconnect the switch at the emergency brake to see if the flashing light stops.

//2000CAYukon

ButchHandy
11-08-2005, 06:17 PM
Ok, thanks. I will disconnect it tomorrow morning to see if that helps. I have wiggled that wire around to see if the light would come on any, but didn't try to disconnect it.

Let me know what else you find that could cause that light to come on, and I will report back my findings to you tomorrow.

later!



I have the service manuals for 90 and will look at them tonight to see which wires control the brake light.

The emergency brake when depressed will light up the brake light. It could be a faulty switch. Disconnect the switch at the emergency brake to see if the flashing light stops.

//2000CAYukon

2000CAYukon
11-09-2005, 12:22 AM
Ok, thanks. I will disconnect it tomorrow morning to see if that helps. I have wiggled that wire around to see if the light would come on any, but didn't try to disconnect it.

Let me know what else you find that could cause that light to come on, and I will report back my findings to you tomorrow.

later!

I think the switch may be bad so you should really wiggle the switch not just the wire.

I took a look at the factory manual. 3 things can set the brake light:

1) Parking brake switch
2) ABS Controller (you should have rear abs - look for a black box on the side of the master cylinder)
3) Brake Pressure Switch (located between abs controller and master)

I would disconnect each switch one at a time to see if the light stops flashing; otherwise, it is the controller or a worn wire shorting out.

//2000CAYukon

ButchHandy
11-09-2005, 06:57 AM
Thanks for the info. I will try the parking brake switch this morning on my way to work and let you know tonight how that works.

How do I disconnect the ABS safely for testing? I'm unaware of mine having ABS. I'm used to seeing an "ABS" light on the dash for vehicles that have it, and it's not labeled anywhere on the vehicle, so I have assumed that it doesn't have it. There is a black box beside of the master cylinder. About the size of a thin book. I thought it was the computer for the whole vehicle, not just for ABS, since it had so many wires going to it. (Where is the vehicle PCM then?) Does it make a difference that my Suburban is a Canadian edition? Everything on the dash is in metric measurements.

As for the Brake Presssure Switch, I guess that is just something that has to be replaced to properly test, or can I just disconnect it and still safely drive it for testing purposes?



I think the switch may be bad so you should really wiggle the switch not just the wire.

I took a look at the factory manual. 3 things can set the brake light:

1) Parking brake switch
2) ABS Controller (you should have rear abs - look for a black box on the side of the master cylinder)
3) Brake Pressure Switch (located between abs controller and master)

I would disconnect each switch one at a time to see if the light stops flashing; otherwise, it is the controller or a worn wire shorting out.

//2000CAYukon

2000CAYukon
11-09-2005, 12:20 PM
Thanks for the info. I will try the parking brake switch this morning on my way to work and let you know tonight how that works.

How do I disconnect the ABS safely for testing? I'm unaware of mine having ABS. I'm used to seeing an "ABS" light on the dash for vehicles that have it, and it's not labeled anywhere on the vehicle, so I have assumed that it doesn't have it. There is a black box beside of the master cylinder. About the size of a thin book. I thought it was the computer for the whole vehicle, not just for ABS, since it had so many wires going to it. (Where is the vehicle PCM then?) Does it make a difference that my Suburban is a Canadian edition? Everything on the dash is in metric measurements.

As for the Brake Presssure Switch, I guess that is just something that has to be replaced to properly test, or can I just disconnect it and still safely drive it for testing purposes?

You can disconnect the pressure switch and still drive it to test if that is causing the brake light to flash.

The black box is the ABS controller (also called the RWAL controller - Rear Wheel Anti-Lock). If disconnecting the 2 switches does not stop the flashing light, you should be able to unplug the wiring harness at the ABS controller. This esentially disables your anti-lock brakes. It may, however, throw the brake light. I will try this on my 90 K1500 and let you know if that throws a light or not. However, you can drive without the ABS controller connected but you will not have anti-lock brakes in the rear so be careful!

//2000CAYukon

ButchHandy
11-09-2005, 07:17 PM
I disconnected the e-brake switch this morning, and the problem still happend on my way to work.

Went to go to lunch with a few co-workers, and when they got in the vehicle on the passenger side, I noticed that the light started blinking. I got out, and pushed upwards on the drivers side of the vehicle as if I was going to try to roll the vehicle onto the passenger side, and I was able to make the light come on, and stay on as long as I was pushing on it. Didn't even have to push that hard with both of them in the vehicle.

So....I am very sure that I have a short somewhere now. Next comes the fun part of trying to find it! Don't even have a clue where to start of something like this!

Thanks a lot for your help, and the info about my ABS system. Learn something new everyday.


Now to fix the idle problem....Any more ideas on this before I pull out the ether to try and find a vac leak? I've had one person tell me to check the Idle Air Control valve to see if it was real dirty. Will be looking at that this weekend when I can do without the vehicle for a few hours.


See ya!



You can disconnect the pressure switch and still drive it to test if that is causing the brake light to flash.

The black box is the ABS controller (also called the RWAL controller - Rear Wheel Anti-Lock). If disconnecting the 2 switches does not stop the flashing light, you should be able to unplug the wiring harness at the ABS controller. This esentially disables your anti-lock brakes. It may, however, throw the brake light. I will try this on my 90 K1500 and let you know if that throws a light or not. However, you can drive without the ABS controller connected but you will not have anti-lock brakes in the rear so be careful!

//2000CAYukon

2000CAYukon
11-09-2005, 08:00 PM
I disconnected the e-brake switch this morning, and the problem still happend on my way to work.

Went to go to lunch with a few co-workers, and when they got in the vehicle on the passenger side, I noticed that the light started blinking. I got out, and pushed upwards on the drivers side of the vehicle as if I was going to try to roll the vehicle onto the passenger side, and I was able to make the light come on, and stay on as long as I was pushing on it. Didn't even have to push that hard with both of them in the vehicle.

So....I am very sure that I have a short somewhere now. Next comes the fun part of trying to find it! Don't even have a clue where to start of something like this!

Thanks a lot for your help, and the info about my ABS system. Learn something new everyday.


Now to fix the idle problem....Any more ideas on this before I pull out the ether to try and find a vac leak? I've had one person tell me to check the Idle Air Control valve to see if it was real dirty. Will be looking at that this weekend when I can do without the vehicle for a few hours.


See ya!

Follow the wire from the brake pressure switch and the wires from the ABS controller. They go directly to the light (along with the ebrake switch). Note that you should still disconnect the brake pressure switch to see if it is broken.

On the idle, removing the IAC valve is a good start.

//2000CAYukon

ButchHandy
11-09-2005, 08:27 PM
Ok, I will try that. I will work on it this coming weekend when I have time to screw with it, and can have someone sit there watching the light for me while I mess with the wires again.

I'll let you know what I find on Sunday or Monday.

Thanks!



Follow the wire from the brake pressure switch and the wires from the ABS controller. They go directly to the light (along with the ebrake switch). Note that you should still disconnect the brake pressure switch to see if it is broken.

On the idle, removing the IAC valve is a good start.

//2000CAYukon

Dirtsled
11-10-2005, 11:18 AM
I have had MAF's (Mass Air Flow Sensors) create some weird problems for me on my rigs that I spent $$'s and days trying to track down. You wouldn't think a MAF would cause RPM's to fluctuate especially that much but it wouldn't hurt to either unplug it completely or Bypass it with a simple transistor to see if there is a change.

Maybe the temperature sending unit is going bad and telling the computer that the engine is cold every now then??

You seem like you know what your doing so those are just some easy things I would check.

sub006
11-10-2005, 11:03 PM
I have a '90 Sub with 384,000 original miles that I bought new.

A couple of years ago my brake light started flashing different patterns, almost like Morse code. I looked the brakes over and had a professional check them out, everything was fine.

Although I considered covering it with black friction tape, I got used to the light.

After two or three months it went out on its own and has not returned. I think many electronic devices become possessed and "act up". Just make sure the mechanical problems indicated are not happening and ignore the light like pilots and astronauts sometimes have to do!

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