ABS light on after gear swap

Heartbeat Hauler · August 11, 2004

Hi guys,

Brand new here and it's my first post. My brother has a 1999 1500 4x4 and switched from 3.73s to 4.11s(front&rear), had the PCM tweaked to record the gear change, but the ABS light stays on. We are stumped. Yukon gear, the place where he bought the ring & pinion, has never seen this problem, The guy that installed them does this everyday and has never seen the problem, finally the guy that programmed the PCM has not found any answers yet either. Can anyone give us any idea what is up? We have driven the truck with a scanner plugged in and the speedo/odo seem to be right on with the rear, the problem is that the front 2 abs sensors are reading different from the rear and different from each other. Any clues at all?

JP

cbrxx · August 11, 2004

Is the tone ring on the GM's inside the axle housing on the rear? Basically, there is one sensor in the differential for the ABS on both rear wheels, with a separate sensor for each of the front wheels, outboard at the wheel hubs. I haven't really looked at our 04 Yukon, but my 99 Dodge is like that.

So, perhaps, this causes the front and rear to read at different speeds now due to ratio change. Easy way to check is call parts counter to see if there is a different tone ring based on axle ratio. I have the parts diagram for my Dodge, and it looks like a different tone ring part number based on axle ratio.

Steve

Heartbeat Hauler · August 13, 2004

cbrxx,

Well the problem is that the Chevy 1500 isn't offered with a 4.11 gear ratio. So I'm not sure if the GM parts counter can help. What I do know is that the reluctor is attached to the carrier and the carrier was not changed. The 4.11s were added to the front diff as well, so you would think all things being = , everything should be hunky dorey. The install guy is a friend and he has done many of these upgrades, same application and parts, and has never had any problems. However, I know that when you swap rear diff gears in an ImpalaSS or Caprice, they use the same gear assemblies, you must change the reluctor. Having said that, when changing those gears you are going from a 3.08 to a 3.73 or 4.10 so a new reluctor may be needed because it is a larger change in ratio. I was thinking this was the problem, but I talked with Yukon gear and they say a new reluctor is not needed and that it is a programming problem. I can't believe my brother is the only one doing this so there must be an answer somewhere. Hopefully something will come up in the next week or so. It apears that the programming parameter for the front ABS stuff is relatively new and not a lot of info is available. The PCM guy is looking into it, but he doesn't seem positive about what he will be able to do. Thanks for the reponse though.

JP

Heartbeat Hauler · August 13, 2004

Ummm...correction to the post above, the ABS reluctor for the rear is located on the output shaft of the transfer case. According to GM parts department here in Evansville.

JP

cbrxx · August 13, 2004

Sounds like that could definitely be the problem. The transfer case will be spinning a certain percentage faster than it was before the gear swap, at any given road speed. The front sensors will have the same readings as prior to the swap, if those sensors are out at the wheels. The PCM sees this constant difference in readings, which is probably greater thatn an acceptable range, and triggers a fault.

If unable to adjust the PCM, you might be able to fool it. Look into one of these external modules for motorcyle speedometer recalibration. They recalibrate the the signals when you change sprockets, as many current bikes get their speed readings from a similar "speed" sensor on their transmission output. You set up the "black box" with a fixed percentage error correction based on the gearing change. Perhaps you could adapt one to the GM ABS sensor.

Here's one such item:

Yellow Box

Good luck!

Steve

mazingo · August 14, 2004

Then is a sensor adapter that will solve the problem. It is some where in one of these topic I just could not find it but I saw it some where. It just plugs in and adjusts the differences I think. Or get a tuner and adjuust the axle ratio in the pcm.

Heartbeat Hauler · August 15, 2004

Thanks guys, I will check that out. One thing though, even though the rear tires are spinning faster, due to gear swap, won't the front tires realize that same speed increase, due to the front ring & pinion being changed also (4.11s in front too)?

JP

Shotgun · August 15, 2004

This is normal after changing gears .It will go away after you program your ECM for the newly installed gears. I would have them reprogram it or check it to see that the right ratio is entered in the PCM. If this fails I would think that maybe the wrong ratio gears are installed.

Frank S · August 15, 2004

Well the problem is that the Chevy 1500 isn't offered with a 4.11 gear ratio.

If it's a NBS truck it was offered with 4.10's under RPO GT5,if memory serves me,which most likely are the gears that he changed to.

cbrxx · August 16, 2004

Thanks guys, I will check that out. One thing though, even though the rear tires are spinning faster, due to gear swap, won't the front tires realize that same speed increase, due to the front ring & pinion being changed also (4.11s in front too)?
JP

No, I think the speed sensors on the front are out at the wheel hubs, one on each side, so they measure wheel rpm's after the gears and would not be affected.

Steve

redvett · August 16, 2004

Assuming that the front & rear tires are the exact same size then the only thing that could be wrong is either a bad front tone ring (check is both are reading the same all the time) or the calibrations are wrong for the application.