Hello everyone. First post here. I'm prepping my daughter's 1993 Explorer for a long trip next week and I need to diagnose and fix a problem with the ABS system. Here is what I know.

I have driven the car at least 10 times in the last week and here is what happens. Not all the time. Mostly when coming to a stop light, as I approach 10-15 mph, while my foot is still on the brake, all of a sudden, a sound like a "pump" vibrating appears and the brake pedal vibrates, and sometimes it pulls to the right. But it ALWAYS happens, if I am turning to the right at slow speed and step on the brake.
At faster speeds, if I step on the brake..SOMETIMES..it scarily pulls to the right very hard. And sometimes, the brake pedal acts like when you are holding the brake hard when bleeding the brake system, and then quickly drops to the floor like when you open the bleeder valve. But quickly recovers when I take my foot off the pedal, and then reapply pressure. And then again, sometimes, I step on the brake, and even though my foot feels the pedal depress to the point where the brakes should work, they don't. Again, if I let off, and reapply foot pressure quickly, now the brakes work. Kinda scary.

However, here is what I've found so far. Not only was she having a problem with the ABS, the right front wheel bearing was starting to make noise. Now, she did take the car in to a repair shop to have the ABS problem diagnosed, but they told her the problem with ABS might be due to the wheel bearing problem, as the wheel doesn't run true when moving and turning, so that had to be fixed before they could diagnose the ABS problem.
Ok, being a long term, non pro, home mechanic, I pulled the right front wheel, caliper, and rotor to replace the bearing. At that point I could see the the rotor (I think) had scraped against ABS sensor, although I didn't see any damage to the rotor, I could clearly see scratches on the face of the sensor. Well, I went ahead and replaced the wheel bearing and tightened everything to spec, and replaced the wheel and took it for a test drive. Well, the same thing happens as I described above, but only occasionally, so it appears replacing the bearing did nothing to solve the ABS problem.
I just watched a video on checking the continuity on the sensor itself, but haven't done it yet. However, I would think that since the problem doesn't happen EVERY TIME I come to a stop, that there might be some other problem. That is why I came here. The thing is, we don't have the money to have a shop repair this, and I am fairly adept at standard diagnosis and auto repairs. But this problem is kind of out of my sphere of experience, and would hope someone may suggest something to test, or check. I do know the master brake cylinder is full of fluid, and all the brake pads are good. I will check the continutiy on the damaged sensor but thought maybe I should replace it, which, due to financial constraints, I really don't want to if it doesn't need replacing due to a possible other problem. So..there you have it. Any ideas would be appreciated. Thanks

rick

 

Make sure all the ABS cogs are on the toner ring, then perform the continuity tests. I don't perceive there is any issue with the sensor, but you may have a damaged wire.... Philip

 

Hi Phillip. Thanks for replying. Well, I'm not that familiar with ABS systems. Can you tell me where this "toner ring" is located? I've never seen anything about this part. Thanks.

 

It is the back part of the rotor, that would be part that rubbed on the sensor.... Philip

 

Ok, thanks a lot Phillip. I'll pull the rotor...again.

 

Make sure you check out the LEFT front wheel as well. If the ABS module loses the signal from the left sensor, it will think the wheel has locked and dump the pressure in the caliper. This will cause the truck to pull to the right, since that is the wheel that is doing the braking.

 

Wow! Ok.
Actually, I was going to repack the left wheel bearing today also. So yea, I'll definitely check everything? And thanks for the "heads up". Makes perfect sense. That's what I like about forums like this.

hmmm, not knowing how the ABS system actually works, I'm curious. When you say.."since that is the wheel that is doing the braking.", I don't understand. Does the ABS system module somehow "disengage" or control the hydraulic pressure at the brake cylinders ..if a sensor isn't working? Which would make sense.. I just hadn't thought about it...hmmm

( note to self...Before embarking on repairs of complex systems...remember your Dad's axiom ...LOOK-THINK-RESEARCH-LEARN !!!)

Ok, thanks a million FireMe. That really helps with my diagnosis.
rick

 

Hello everyone. Well, got a couple of more questions. I did some research and found a video on testing the sensor circuit with a simple continuity test, once the wiring connector has been disconnected. However, this was on a REAR sensor of a late model Explorer. I assumed this same test could be done at the front sensors. However...see *** below.

Video: How to Check Continuity of Anti-Lock Brake Wires | eHow.com

But after viewing the video... something occurred to me. This test was simply using a continuity test light, testing each of the two wires one at a time, reversing the polarity on the second wire, right? It shows if the light came on when testing each wire, then the circuit was good. However, in the video.the technician claimed that if both wires checked out good(light comes on)...then the sensor was BAD?? Well, exactly what tells him this? Seems to me...you would have to measure the sensor itself..no?

That's why I came back. I saw some resistance checks for other cars, but can't find the resistance spec for the front sensors. I want to know whether or not the sensor is bad BEFORE I pull the rotor/etc. That is..if the wiring continuity test checks out....however....

***Ok, after finding and disconnecting the sensor wiring connector at the left front wheel, I tried this test . HOWEVER, the tech in the video didn't say whether or not the ignition switch should be on or off. I tried it both ways..and I got NOTHING. So now, I don't know what the heck is going on other than maybe you can't do this test. But if so...I haven't got a clue now.

But as to the sensor itself, should I find the circuit is good(????) here's another thing I'm wondering about,... the sensor housing on the back side of the axle knuckle(I think that's what it's called) is PLASTIC. I found one thread where someone said these break easily when trying to remove. These things are $42 bucks, and I don't want to break one UNLESS I know the thing is bad in the first place...right?

SO, does any one know where I can find the resistance spec for the front sensors? I looked all over the net...arrrgggggrrrrr. But, in the meantime, I guess I'll go ahead and pull the rotor anyway since I need to repack the left wheel bearing. One other thing...how the heck do you remove these sensors???? Drive em out?? Oh, I did remove the 6mm bolt from the sensor..but man..they are TIGHT!! Anyway, I have to get this fixed and I'm kinda clueless at the moment. So if anyone can offer any more info...it sure would be appreciated.

Thanks guys. and btw, sorry for the

 

Gak..something else just occured to me. Ok, on one of the offroad forums, one guy suggested, if need be, simply disconnect ONE sensor, which supposedly, will tell the computer something is wrong...and it will shut down the ABS system. Now, if this is true...then why wouldn't the computer shut down if a the circuit inside the sensor is OPEN??? Instead of reading it like a locked up wheel, like FireMe suggested. Wouldn't disconnecting the sensor be the same thing?

Reason I ask is...my wife and daughter HAVE to have this car to make a long trip on friday. I just thought..in a WORST CASE SCENARIO, if this disconnect thing does work(shuts down the system), since the only time the ABS thing is useful is while driving in snow/ice, and there's no snow/ice in Tahoe this time of year...then why not?? At least they can make the trip.

But I thought I'd toss that one by you guys first. Just in case

Thanks

 

That was not a good exaample of an ABS test

Check this out, Testing ABS sensor with multimeter - YouTube ...

and this, http://www.youtube.com/watch?v=Tf9yg...eature=related ............. Philip

 

Cool. Thanks Philip. Now if I only had a digital multimeter. My old analog meter sucks.

 

It will work just as well with the old analog, look for needle swing... Philip

 

Say what?

ABS functions under ALL driving conditions, even dry pavement. Most of the time ABS operation is trasparent to the driver, and most people do not know how often the ABS system intervenes to help keep the car braking in a straight line.

If you consider your wife or daughter to be expert drivers then I would support your option to disable the ABS system by removing the fuse or relay in the fuse box as indicated in your owner's manual. However, if they don't have experience driving short-wheelbase, high center of gravity vehicles without ABS and being able to control them in a skid, such as when freeway traffic suddenly stops, then I would urge you to use caution. Not trying to be a doom-sayer, but as a firefighter I've put many people in body bags for car accidents that could have been avoided, and I'm sure most of them thought they would be just fine on their trip.

Ford does not publish any resistance specs in the manual for your vehicle, they require a function test similar to the first video above posted by Aquanaut. If the gap between the tone ring and sensor tip is less than 0.070 inches and the sensor does not produce a consistent voltage, then it is to be replaced. Of course, you have an intermittent problem, so you may have to rig wires from the sensor into the cab and have someone drive while you monitor the functioning of the sensors until they act up.

Personally, I would tell you to either take it to someone that has the knowledge, experience and equipment to repair it or just go out and buy a sensor and put it in. Third choice would be to just disable the system, but that would have to be your call.

 

I spoke too generally. Yes, I understand this. Sorry.


Hello Fireme. Thank you for the reply. Under any other circumstances, I wouldn't do this. My daughter has driven this car..in TAHOE..in the snow, ice etc..for 5 years. She lived there for 20 years, driving cars with NO ABS. She knows how to drive. However, here is the deal. She HAS to make this trip. And we have NO extra money or time. Here is my rational. (Besides, I still tried to fix this, but due to circumstances, I haven't even found the cause yet)

1. What if there were NO ABS. People have driven like this since cars were invented.

2. It's summer. Good weather and good roads.

3. With the ABS disabled, the brakes act normally.

4. I simply disconnected the left front connector to the sensor. The ABS light came on and this tells me the system is now disabled.

5. To check, I did a long test drive, up and down hills, with tight curves, and in down town traffic to make sure the original problem was not occurring in any way shape or form, as this problem was actually MORE dangerous than driving with out ABS.. IN FACT, on one of the sites that describes how the ABS system works, there was a link to a study by the Department of Transportation, whereby they discovered, that while more accidents were prevented in vehicles equipped with ABS, there were actually MORE fatalities. They didn't have a clue why. This tells me something as well.

6. She's a good driver.

7. I would make this trip with the ABS disconnected in a heart beat.

8. Some people INTENTIONALLY put a disable switch in the circuit.

So, bottom line...if I leave the ABS connected, the problem is potentially MORE dangerous( as I described in my first post), than with it disabled.

This is not to say I like it. Yes, if conditions were different..I would opt for not even driving it until fixed. But all things considered, I had to make a decision.


However, I'm still perplexed about something. IF, a sensor goes bad...via it or the wiring having an "open" condition, would not the ABS light come on, as this is the same thing as a disconnect, which I proved it does come on? And if it DOESN"T come on if ONE front sensor goes bad regardless of the condition ...does not that create a potentially dangerous condition as I described which you explained why it may be happening? (ie..computer incorrectly diagnoses wheel rpm-dumps pressure-yet does not shut down system nor turn on light-which it would, IF an open condition occurs in wiring...no?)

This is what I'm trying to figure out. If the left front sensor is bad, WHY didn't the light come on, as I proved it wasn't an open condition in the WIRING, as disconnecting it MADE the light come on. ??????????

The fact is, in MY opinion, as designed, the system FAILED to warn my daughter of a potentially dangerous situation, as it was only discovered by virtue of the problems I described. This tells me, for thousands of other people driving the same vehicle, the system designer has potentially placed them in danger in the first place. Please, if you disagree, explain why so I may further understand the potential pitfalls of this ABS system. Perhaps my lack of understanding how this system fully works is faulty itself. However...personal experience driving this vehicle is the support for my opinion.

Advice acknowledged and transmitted to daughter.

Thanks.
rick

 

Not as simple as you think..

Yes the light illuminates on an open circuit, but that is only a small part of the supervised circuits....

When you first turn on the key and power up the ABS supervisory module, It runs a POST (power on self test), it also expect to see a small current return on the wheel circuits (small resister in wheel sensor). If the return value is not within spec the ABS module illuminates the dash light and disables the ABS.. What may disallow the ABS to activate, an open circuit, a shorted circuit, a short to ground or a high or low reading (without any of the above mentioned faults..)

If the POST passes and the engine is started the ABS control module then expects to see a modulated signal from each wheel, if all is within spec it is happy... should one corner slip or not match rotational speed (bad sensor or low tire pressure) the ABS will adjust the hydraulic pressure (equalize) to permit a safe stop...and if required illuminate the ABS warning light...

This is by no means a complete synopsis, but enough that you are aware that yanking a connector is not the fix.... Philip