Overactive ABS

[pete c]

Since I bought my 93 mtx, I have noticed at times the ABS activating when it has no business doing so. During gentle braking at less than 20 mph it will do it frequently. This is a little scary when it kicks in as you are pulling up behind someone at a light. It never does it at higher speed (more than 20 mph).The ABS performs well when it is supposed to.

It does it most frequently when I am pulling into a parking spot. I believe this is because the car is sensing the different wheel speeds encountered during sharp turns. I have read before that ABS is supposed to be disabled at very low speeds. I know it to be a fact on '90 300SEL (I slid one into a parked car on packed snow). Should the SHO ABS disable below a certain speed.

BTW, I don't get an ABS light. It goes out right after starting.

 

[jelloslug]

Do you have the right size tires on the car? Some times that will throw off the ABS computer.

 

[scott c]

I also have this prob and have aked here in the forum before. No help! I seem to only have it on my left front and originally thought it was the sensor. A buddy of mine says the voltage readings were wrong !? I mechanic neihbor of mine says its the "pedal travel switch" which I disconected and it seemed better although Ive noticed its worse in the rain. I would love to find the source once and for all! yes its a scary feeling !!

 

[pete c]

yes, I have the stock 225/55/16 tires on stock slicers. Just how would the car know if I something different, so long as all 4 tires were the same?

 

[Mr Anonymous]

pete c:
yes, I have the stock 225/55/16 tires on stock slicers. Just how would the car know if I something different, so long as all 4 tires were the same?

Well, technically the stock tire size is P215/60/16, but the 225/55/16's are very close and shouldn't cause the problem you're experiencing.

I suspect you have a caliper hanging up which is causing the ABS sensors to detect a false lockup condition.

 

[pete c]

My original question stands. How can the car know what size tires it's wearing? It would definitely sense a difference between front and rear tire sizes, but, assuming they are the same all around the computer wouldn't have a clue if the rolling diameter was changed unless it's got some kinda gps or ground tracking radar to measure tire rpm against. I am pretty sure I don't have that feature.

Still would like an answer on whether ABS is supposed to disable at low speeds.

 

[jelloslug]

A wild change in OD of the tires will throw off the speed sensor and make car think going faster or slower that it really is. This normally won't cause major problems with an ABS system, but sometimes it can especially if you had different sizes on each side. The speed sensor works off of wheel RPM, so a smaller tire will spin more times for a given distance that a larger tire.

 

[projectSHO89]

The speed sensor works off of wheel RPM, so a smaller tire will spin more times for a given distance that a larger tire.

Ever see the difference in sizes when using a "donut" spare?

So far, I haven't seen an ABS concern while driving with one.

Steve
89

 

[pete c]

jelloslug:
A wild change in OD of the tires will throw off the speed sensor and make car think going faster or slower that it really is. This normally won't cause major problems with an ABS system, but sometimes it can especially if you had different sizes on each side. The speed sensor works off of wheel RPM, so a smaller tire will spin more times for a given distance that a larger tire.

yes, a major change in od will cause the actual speed to vary from the indicated speed, but, the computer won't know the difference.

 

[Sho-N-Go]

Here is the problem look at the ABS tone rings on the front axles they are on the farthest most point. you will see the abs sensors on the spindle. The ring will be cracked on either side or both. The part is only about five bucks at ford. and just take the axle out and heat the new ring and put it on

 

[Sho-N-Go]

Well did you find anything out?

 

[pete c]

haven't had a chance to look at them yet. I'm gonna do a new set of front pads and turn the rotors on monday. I'll take a look then.

 

[pete c]

tool boy,

yup, cracked both sides. Looks like it's time to pull the half shafts. Goody goody. Any recommendations on replacing other stuff while I'm in there besides ball joint/tre boots? I am also doing new pads, front brake lines ($tainle$$??). Are stainless worth the $$$$$? This is no track car. I don't plan on much in the way of rubber/handling upgrades, maybe a set of tokicos for christmas. Also, the front rotors need turning. Since these are prone to warping even when new and these have lots of rust in the vents, should I just go with new rotors? Turning cost 20 bucks, new rotors 20 a side.

 

[gregr]

Here'r a link to the SHOFOrum.com How to article on how to fix these:

http://www.shoforum.com/ABSrings.htm

I had the same problem. Fixed it right up.

Careful, 3.2 & 3.0 have different size trigger rings!

 

[pete c]

the 6 inch PVC sounds like an excellent idea. I will do it next time. What I did was slid the ring on, tapping it gradually at different places until it was partially on. I then reinstalled the half shaft into the knuckle. I then tapped the ring all the way on tapping it a bit, turning the shaft about an 1/8th of a turn between taps. It is far enough on when it is aligned with the sensor. I wonder why they don't use a stamped steel ring as they do in the rear. It is cheaper and less prone to cracking.

 

[pete c]

I replaced the passenger side ring with a new one from ford. I replaced the entire driverside halfshaft with a rebuilt (the boot was ripped). The rebuilt halfshaft's ring had no cracks, but, the ring was worn. It looks like it was rubbing against something.

I no longer have the intermittent abs chatter at 5-15 mph like I used to. I now have chatter just as I am stopping at less than 5 mph. And it happens every single time. I did bang up the passenger side abs sensor in my unsuccessful attempt to remove it from it's mount. Can I check the sensor using a ohmmeter? Would a damaged sensor cause an ABS light? Help.

 

[Ian]

From what I hear, the vehicle speed to the computer (that it gets from the VSS) and the axle speed that the computer gets from the ABS transducers have to match within a predetermined parameter. If they don't, you will get an ABS warning. From what I've heard, the difference between stock and aftermarket tires cannot exceed 1/4". Also you may have one tire slightly smaller dia. than the rest which will, along with a non stock tire which will just compound the problem.

Ian R.

 

[Ian]

Don't have ABS on my car, an 89 but work on vehicles with. Check clearance between ring, or sun gear, and the sensor. The sensor produces voltage as each tooth of the gear passes it (AC) which the computer gets. If the sensor is or wiring is damaged yes that could be the problem. If you can, you can put a meter on the leads, turn the wheel,axle, and check for voltage on the meter using AC at the lowest setting, or use an analogue meter on DC (on the lowest scale) and watch the needle move back and forth.

 

[pete c]

Thanks, Ian.

I'll throw my fluke on it and watch the voltage fluctuation. Do you know what spec is? Is it actually an analog signal? Or two distinct voltages? I would think the later would be true as I believe the abs sensor is nothing more than a magnetic switch. It will see either a short or an open.

Any other info would be appreciated. If I do have to take it to someone, recommendations in the ct/w.mass area would be helpful.

thanks,

pete

 

[RStalveyARFF]

Check to see how tight the axle nuts are. If they loosen a bit, you will see a sub 5MPH chatter. Don't use impact, I personally use a 18" 1/2" breaker bar, and a 24" cheater bar on top of that.