ABS Light On

[JohnShead]

I have a 1995 Suburban that is supposed to have 4 wheel ABS. My ABS light comes on shortly after I start moving. I replaced my front brake pads today so while I was at it I took the rotors off to see what the wheel speed sensors look like. Of course they were filthy with lots of tiny metal filings stuck to them. I cleaned them up to see if that might fix it. I wanted to check the rears but I can't find them. There is nothing electrical in the vicinity of the rotating parts of the rear wheels. The light still comes on so the cleaning had no effect. How can I have 4 wheel ABS with no speed sensors on the rear wheels? Any ideas why the light comes on?

 

[WarWagon]

They use the output speed sensor of the transmission and pulse the rears as a pair. You need to pull codes out of the ABS unit and it will tell you why it's unhappy.

 

[JohnShead]

Thanks, I'll check the codes when I get a chance. The brakes are fine so it's a low priorty. I guess I just don't have antilock, kind of like the time before Electronic Control Modules etc. when everybody had to know how to brake properly. Good thing I learned to drive in the 60's.

 

[ak diesel driver]

if you have lots of metal on the speed senor your hub is probably on it's last leg

 

[SS FORCE]

if you have lots of metal on the speed senor your hub is probably on it's last leg

I suggest lifting that side and spinning the tire to check/listen for bearing wear. Or while driving take a corner a little faster than normal putting the questioning side on the outside of the turn (turn left if its the right side) and see if you get any noise. I lost mine on a trip and every time I turned left the right side sounded as if it was about to fold under the truck.

WarWagon, I have been to several places to see if they can pull the ABS codes and all say they don't have the scanners to do so. Whats so different about pulling ABS codes apposed to others?

 

[WarWagon]

Scanners for OBD I?

A paperclip does the job.

OBD I ABS codes are not stored in the ECM so code some code readers can't read them. It is a separate data pin on the ALDL for ABS.

On the ALDL connector (Assembly Line Diagnostic Link) under the dash, Take a small jumper wire and place one end into the A port, which is located on the top row at the far right. Place the other end into the H port, which is located on the bottom row, second from the left. Then turn the ignition key to "On". The ABS light will come on during the lamp check, then go out. It will flash any stored codes and repeat this sequence three times, then will flash once, pause, and flash twice (code 12). This just indicates that it is in diagnostic mode (code 12).

Some ALDL plugs have raised letters on the bottom to indicate A H etc. pins.

 

[SS FORCE]

Scanners for OBD I?

A paperclip does the job.

OBD I ABS codes are not stored in the ECM so code some code readers can't read them. It is a separate data pin on the ALDL for ABS.

On the ALDL connector (Assembly Line Diagnostic Link) under the dash, Take a small jumper wire and place one end into the A port, which is located on the top row at the far right. Place the other end into the H port, which is located on the bottom row, second from the left. Then turn the ignition key to "On". The ABS light will come on during the lamp check, then go out. It will flash any stored codes and repeat this sequence three times, then will flash once, pause, and flash twice (code 12). This just indicates that it is in diagnostic mode (code 12).

Some ALDL plugs have raised letters on the bottom to indicate A H etc. pins.

Sorry I should have verified. OBDII

 

[Big T]

I have the same problem on my 1999. Replaced sensor 2x both sides. Replaced both bearing hubs with sensors. Replaced bearing hub that went bad on side the ABS code said was bad. Swapped a good ABS pump head unit in. Tried a completely different wiring harness. I finally just gave up and ignore it. I have 206K miles on the truck and it should go 300K, but I've resolved to the fact that the ABS will NEVER WORK!

I do plan to do the jumbo disc brake conversion and the rear disc brake conversion. The brakes on the Suburban are just woefully substandard.

 

[FellowTraveler]

I have the same problem on my 1999. Replaced sensor 2x both sides. Replaced both bearing hubs with sensors. Replaced bearing hub that went bad on side the ABS code said was bad. Swapped a good ABS pump head unit in. Tried a completely different wiring harness. I finally just gave up and ignore it. I have 206K miles on the truck and it should go 300K, but I've resolved to the fact that the ABS will NEVER WORK!

I do plan to do the jumbo disc brake conversion and the rear disc brake conversion. The brakes on the Suburban are just woefully substandard.

My 1999 Burb stops on a dime always. You running a NP246 too, or? If so the ABS is tied into the autotrac controller. If running different sized tires than stock the ABS controller has to be calibrated for the newer tire size "tire size brake calibration for GM C/K truck under 15k GVW" this is perhaps the most ignored calibration that is needed when changing tires size w/ABS.

 

[Big T]

Running 265 75 tires. At highway speed it's scary how little braking I have. With no ABS, can't lock the wheels and I'm two footing the brake pedal. I've got stainless lines, braided stainless hoses. Upgraded proportioning valve and the unsized rear brake cylinders from the 3500 dually. At slower speeds it will stop good, but not highway speed (ie 70 mph)

Once in a blue moon the ABS will come on and work. My code reader handles ABS, but not for these trucks. Last time I took it to a dealer to read the ABS they charged me $100 and it was a fault in reading the RFP sensor. I replaced it and nothing. I have since replaced that bearing which came with a new sensor and nothing. I posted it all here and got no help that was worthwhile, so I gave up.

 

[FellowTraveler]

265 is not far off factory tire diameter so not really a big issue in your case. The following link has device controller for tire size brake calibration c/k http://www.obd-2.com/

Once and a blue moon indicates broken wire or plug connector not making full contact into its socket suggest using non-conductive probe and push all pins into your PCM I'd bet some are not completely seated. There is a known issue with the harness on the 1999 Burbs in passenger footwell it the harness can be pulled by passenger placing feet up in well and stretching out this harness needs to be fastened/secured so it can not be pulled or jarred by passenger's feet.

As hydroboost goes the p/s fluid heats up fast the p/s pump can't keep pressure up so hydroboost output fades "the p/s pump paddles warp too" check psi output, I run two p/s coolers to beat back the heat one in the chassis rail the other is a 4L60e transmission fin/plate cooler using AMSOIL.

Check for gassing between pads and rotors when brakes are applied at speed, NAPA has real great pads that grab rotors but they don't have a long life span but well worth the extra $$ then there is the non-directional slotted rotors that help too.

Another common brake issue is contaminated brake fluid which IMHO needs to be changed out flushed once a year.

 

[Big T]

I suspected broken plug, so I tried an entirely different harness runninig from the ABS pump to the sensor connect on the right side and nothing. I will look at installing a new plug there.

PCM has been in and out a couple times but no correlation there with the ABS fault. ABS code indicated no signal from RF sensor.

Per advice here, I'm using Raybestos Advanced Technology Pads and and slotted rotors. Been happy with that set-up.

 

[JohnShead]

I finally got around to checking my codes, DTC35 "Open or Grounded Rear Speed Signal Circuit", and DTC65 "Open or Shorted Pump Motor Relay". While reading through the replies here I followed links about calibrating the VSSB for different tire sizes. My speedometer reads 3mph slower than my GPS so I did the DIP switch mod on my VSSB so I could easily change the setting to find the closest one. None of the changes have any effect on my speedometer so I assumed my VSSB was bad. I tried driving without it but the truck wouldn't shift out of first gear and there was no speedometer, so the VSSB is doing something. I went to the junkyard and found another VSSB that had obviously different jumpers than mine originally had and the speedometer still shows 3mph slow and the ABS light still comes on shortly after I start moving. The flow chart for DTC35 indicates I have an intermittent speed signal connection and I should check for open/short circuits at DLC connector, bulkhead connector, BPMV connector, and VSSB wiring or connector. I'm getting the codes through the DLC so I assume it's OK. BPMV connector has never been unplugged in 20 years until yesterday when I checked the voltage per the flowchart, the plug appears fine and voltage is within specs. I'm not sure where to look for bulkhead connector problems. VSSB connector seems OK too. The flowchart for DTC65 says it may be set if the brakes are applied during an erratic speed signal, among other reasons. It seems that whatever is setting DTC35 could also be causing DTC65 to be set.

 

[NVW]

I finally got around to checking my codes, DTC35 "Open or Grounded Rear Speed Signal Circuit", and DTC65 "Open or Shorted Pump Motor Relay". While reading through the replies here I followed links about calibrating the VSSB for different tire sizes. My speedometer reads 3mph slower than my GPS so I did the DIP switch mod on my VSSB so I could easily change the setting to find the closest one. None of the changes have any effect on my speedometer so I assumed my VSSB was bad. I tried driving without it but the truck wouldn't shift out of first gear and there was no speedometer, so the VSSB is doing something. I went to the junkyard and found another VSSB that had obviously different jumpers than mine originally had and the speedometer still shows 3mph slow and the ABS light still comes on shortly after I start moving. The flow chart for DTC35 indicates I have an intermittent speed signal connection and I should check for open/short circuits at DLC connector, bulkhead connector, BPMV connector, and VSSB wiring or connector. I'm getting the codes through the DLC so I assume it's OK. BPMV connector has never been unplugged in 20 years until yesterday when I checked the voltage per the flowchart, the plug appears fine and voltage is within specs. I'm not sure where to look for bulkhead connector problems. VSSB connector seems OK too. The flowchart for DTC65 says it may be set if the brakes are applied during an erratic speed signal, among other reasons. It seems that whatever is setting DTC35 could also be causing DTC65 to be set.

Your code readings seem to be off if you are speaking OBD1 in a 6.5. http://www.thetruckstop.us/forum/threads/a-little-help-here.2657/#post-50700

 

[THEFERMANATOR]

Your code readings seem to be off if you are speaking OBD1 in a 6.5. http://www.thetruckstop.us/forum/threads/a-little-help-here.2657/#post-50700

He's refferring to ABS codes, not ECM codes. GM used the same 2 digit codes for both, but depending on which module the codes were read from determines which code it is. And code 35 is the one you need to focus on as it says an erratic speed sensor signal can trigger the code 65, and 35 is for erratic speed sensor signal. If the ECM is not giving any trouble codes for a speed sensor problem, then I would say your VSSB is working. The VSSB sends out 2 different speed sensor signals, a 4K PPM signal for the ECM and radio, and then a 128K PPM signal for the ABS. I would be checking the wiring between the VSSB and ABS module if it was me.

 

[great white]

Sorry I should have verified. OBDII

Doesn't matter. The RABS/4WAL is independent from the rest of the electronics on both obdi and ii trucks.

They give out flashout codes via the abs light (like obdi does with the ecm and cel).

As ferm mentions, they are 2 digit fault codes.

A paper clip works on all the trucks. Its pins A and H in the ALDL if I recall. Might want to confirm that with a manual first though. Its been a while since I worked on one.

Another interesting fact a lot of guys don't know about these trucks and self diagnostics: the pushbutton 4x4 also has flash out fault codes. I just can't remember at the moment how to pull them out. Cant remeber if it was in the ALDL or if it was key on and holding buttons. Shop manual will tell you how.

 

[great white]

Doesn't matter. The RABS/4WAL is independent from the rest of the electronics on both obdi and ii trucks.

They give out flashout codes via the abs light (like obdi does with the ecm and cel).

As ferm mentions, they are 2 digit fault codes.

A paper clip works on all the trucks. Its pins A and H in the ALDL if I recall. Might want to confirm that with a manual first though. Its been a while since I worked on one.

Another interesting fact a lot of guys don't know about these trucks and self diagnostics: the pushbutton 4x4 also has flash out fault codes. I just can't remember at the moment how to pull them out. Cant remeber if it was in the ALDL or if it was key on and holding buttons. Shop manual will tell you how.

I should qualify that a little bit more I guess. You can query the codes independent of obdii.

But, you can also query the abs module through obdii if your scanner has the enhanced gm modules since the 4WAL is on the bus. Car code has them already built in, but everyone here is pretty aware car code is not exactly "user friendly". Other scanner software (that usually has a simpler user interface) usually requires you to pay extra for "enhanced" gm code reading.

So, long story short: works both ways on obdii truck for querying ABS codes.

 

[JohnShead]

I have no ECM codes all are from the 4WAL system read with my Tech2. I found the Anti-Lock Brake System manual for 1995 Suburban online. I'll be working my way through the Functional Test and troubleshooting flowcharts, but probably not until next week. I'm sure with all the tests it describes I'll find the problem.

I still don't understand why changing the VSSB doesn't change my speedometer though. I drove around yesterday with the Tech2 attached and all 3 speed sensors are showing exactly the same speed. The speed signal to the ECM was the same as the 3 to the 4WAL system. They also matched up with my GPS speed but my speedometer was still showing 3mph faster. Maybe I can just move the needle to adjust it.

 

[Will L.]

Yeah, GM gauges are not the most accurate in the world. I had a 99 3500 gasser that was off by 4%. Factory tire size, 4 dealerships couldn't do a thing about it, and since it was reading faster than I was going they felt no liability in it so gave me the brush off.

 

[NVW]

He's refferring to ABS codes, not ECM codes. GM used the same 2 digit codes for both, but depending on which module the codes were read from determines which code it is. And code 35 is the one you need to focus on as it says an erratic speed sensor signal can trigger the code 65, and 35 is for erratic speed sensor signal. If the ECM is not giving any trouble codes for a speed sensor problem, then I would say your VSSB is working. The VSSB sends out 2 different speed sensor signals, a 4K PPM signal for the ECM and radio, and then a 128K PPM signal for the ABS. I would be checking the wiring between the VSSB and ABS module if it was me.

Thanks Ferm, I should have looked back to the thread beginning to see what it was about.