ABS light/codes

[4evrbad]

hello, I have a 1994 lt1 caprice. I just had the rear end re-built new axle new bearings posi and 3.73 gears. Since the work has been done my abs light pops on after driving 2-3 miles. I took the car back and they said the car has code 28 wheel frequency error. code 21 right front speed sensor and also code 25 left front speed sensor. All in the front end. isnt the speed sensor in the tranny in these cars and just one sensor? anyone have a clue?

[96capricemgr]

Did you get the right ABS reluctor for the 3.73s?? You can not reuse the stock one, the reluctors are gear ratio specific.

[4evrbad]

I asked the manager at the shop that same question about the reluctor and told him that I thought they were ratio specific and the reason for my abs problem, he replied and said he's done hundreds of 10 bolt 8.5's and never changed the reluctor and the abs light never came on. I then said so its just a coincidence that my abs light came on with the gear change? and he hooked it up to the computer and gave me those codes saying it was the front end and not the rear.

[j cAT]

Originally Posted by 4evrbad

I asked the manager at the shop that same question about the reluctor and told him that I thought they were ratio specific and the reason for my abs problem, he replied and said he's done hundreds of 10 bolt 8.5's and never changed the reluctor and the abs light never came on. I then said so its just a coincidence that my abs light came on with the gear change? and he hooked it up to the computer and gave me those codes saying it was the front end and not the rear.

the transmission and front sensors must agree..if not it will report a slip/slide condition...with the transmission reporting a different rotation speed with the different diff rear diff ratio this will cause this..


so the rear wheels are one sensor then the two front sensors must all report to the pcm the correct same rotation..if not dtc..

is this a GM shop or a hack shop ??

[4evrbad]

apparently a hack shop....... what is dtc? So the sensor that is in the rear differential is disagreeing with the front 2 abs wheel sensors on each wheel. So the problem is not the speed sensor in the trans or the abs sensors it is the rear sensor in the diff. why would that code not pop up and instead show the front end is the culprit? anyway to show or prove this to the shop published info etc??

[j cAT]

Originally Posted by 4evrbad

apparently a hack shop....... what is dtc? So the sensor that is in the rear differential is disagreeing with the front 2 abs wheel sensors on each wheel. So the problem is not the speed sensor in the trans or the abs sensors it is the rear sensor in the diff. why would that code not pop up and instead show the front end is the culprit? anyway to show or prove this to the shop published info etc??

there is no sensor on/in the rear diff..
transmission....is where it is mounted..

[96capricemgr]

I think j-cat stopped learning anything sometime around 1980.
Ther most definetely is a ABS sensor in the diff housing, midway betweeen the pinion yoke and driver's side axle tube. It reads a reluctor pressed onto the pinion.


The b-body is a three channel ABS other 8.5" 10bolt, not all ABS 10bolts use the same system. If the reluctor is on the ring gear or at the wheel then they do not need to be changed with the gears since they are reporting wheel speed. Since the b-body reluctor is pinion mounted it does have to be ratio specific because the gear ratio chang, changes to count per tire revolution.

Let's first understand how it all works with stock say 3.08 gears and say a 20 tooth reluctor(Ballpark guess on tooth count) the abs system will count about 61 teeth for one tire rotation 3.08x20=61.6. Ok so now you change the gears to 3.73s and now with the same 20 tooth reluctor the abs sees 74.6 teeth per revolution of the tire.

It gave you a front code because it thinks that both front sensors are malfuntioning not counting all teeth.

So when you swap from 3.08 top 3.73 gears you need a reluctor with fewer teeth to try and maintain the same tooth count per tire revolution. Given the above example 3.73s would need roughly a 16-17 count reluctor to give you the right tooth count to match the front.

I hope that someone taking the time to break it down into "how it works" has helped you. It should be done more often on car forums, too few actually understand the hows and whys. Most shops are staffed by parts swappers who have little to no understanding of how it all actually works.

[j cAT]

my error that is correct it is mounted on the diff in the pinion area.....

1980 what had abs....not caprices..

this is called the " rear axle speed sensor ring" in the OEM repair manual.



I believe the part you need is "3.73 RELUCTOR B-BODY 26018946 "...

[4evrbad]

Thanks for the detailed response. I recieved the new ring today it has 18 teeth should work. It was hard to find (orginally sold at dealers for $10-15), GM made these for only 2 years for the cadillac brougham with a tow package. I guess no after market companies decided to make these.

[96capricemgr]

There is actually someone making those for many ratios BUT they are individually manually made so they are I want up around $100, since they are a press fit the internal dimension in particular is critical and not something easily accomplished.