ABS coming on at last moment of stops--no code errors 2011 4wd 3500 express van

[pfacdb]

2011 Chevy 3500 express (Roadtrek 190) van with Stabilitrak and recent 4wd conversion has ABS tripping at last moment before stops--obviously at low mph. Front end Dana 60 and both front and rear differential with Detroit trueurac. Mechanic says no error codes, but when disconnecting brake sensors (folks I'm not the mechanic, so just trying to translate what I understand), the ABS being triggered momentarily at stops does NOT happen.

Speculation is that it could have something to do with the truetracs (geared not with clutches). I don't really understand why that would give disparate info to the sensors, thinking that a "skid" is happening. I'm not a car mechanic, however, so that may mean very little.

I realize that Ford's are van du jour for the most part here, but perhaps this might ring someone's bell.

Thanks so much.

 

[Jakes01234]

I'm not up to snuff on these vehicles but it sounds like a bad abs/tone ring.

 

[expomazda]

Most likely a bad wheel bearing. Will cause the tone ring to not read correctly.

 

[Jakes01234]

That makes sense, I'm trying to remember a few years back why my 3500 did the same thing and I do think it was a wheel bearing.
So I would go with that.

 

[BajaSportsmobile]

The ABS system uses wheel speed sensors out at each front wheel and one at each rear wheel (in some cases, one at the rear differential for both rear wheels) to compare wheels speeds at each wheel. The system expects am exact readings from each sensor. What sensors and reluctor wheels were used in the conversion?

 

[pfacdb]

reply to Ramsey

Hi Ramsey,

I'm not sure what sensors Boulder Offroad uses in their conversions, and until a few minutes ago, didn't know a reluctor wheel was. I can get this info from Erin or Don on Monday, if you think it would be helpful.

It seems that it would be imperative to scan the sensors while driving and stopping, to see which sensor, or sensors are dropping out speed wise. While I have spoken to Don about the possibility of rust or crud in the sensor holes (they kept the rear axle on 2011), I think the sensor monitoring while driving would be the logical first test. It could be rust or crud in the sensor holes, a bad sensor in the new front, or I suppose even something during conversion that got in the sensor holes in the front axle.

I do know that the steering wheel pulls definitively left upon stopping, thus I understand this indicates a problem on the right. Could be front or rear?

Any other questions you think would be pertinent to send along to Erin and Don, please let me know.

Thanks
Dana

 

[BajaSportsmobile]

The first thing to do is get a scanner that can read the ABS System. It will tell you the wheel speeds at each sensor and show any problems, and indicate when the ABS pump is being activated. The pulling could indicate a little air in one caliper and some systems measure differences in brake fluid pressure. This shouldn't be to difficult to diagnose.

 

[Bikersmurf]

Wheel sensor would be my guess. Had similar problems with our Durango... Ended up replacing a couple sensors to fix it.

I've also read the ABS computers in Fords are prone to problems due to cold solder joints.

 

[pfacdb]

a bit more info

To Ramsey, the front axle from super duty ford 250/350 08+ and it has unit bearings.

After moving scan--all four sensor show same mph while driving, but right rear drops 1 mph less than the others at the last moment before stop. They switched the rear sensors and still does the same thing. I mentioned the idea you suggested of air in break lines.

There still is the hypoth that it could be due to the Eaton Trueurac's.

They will continue to dig into the issue; tone rings and holes for sensors are apparently fine. Any other ideas--esp. about relation to geared Eaton Truetrac's would be appreciated.

Thanks
dana

 

[BajaSportsmobile]

The sensors are at the individual wheels - the differential should have nothing to do with it. When you are traveling in a straight path the differential would not even come into play.

I find it interesting that the Ford front axle has the same output as the Chevy units it replaced. Would not have bet on that being the case.

 

[1stDeuce]

Your van definitely thinks one wheel is locking as you come to a stop.
Get the scan tool back in there and drive VERY slow, so all sensors read zero. Then go slightly faster until you see three sensors up and one still at zero.
That zero sensor is the one with an issue. Too much air gap between the sensor and tone ring will cause a high dropout speed. So will metal particles stuck to it. If you swap sensors side to side, see if the problem side follows or not. If it does, replace the sensor. Of not, you may have to shave a few thousandths off the sensor mounting surface.

Also, check to see if the front and rear speeds match when you first drive off after starting. If they don't, its because the front tone rings have the wrong number of teeth. The system will correct it as you drive, but it may lead to some sensitivity for the first mile or so after you start the truck.

That it pulls left indicates ABS is sensing a locked wheel on the right.
Finally, it has nothing to do with your truetracs.

 

[pfacdb]

thanks for your response about the ABS!