Odd ABS problem

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Sep 17, 2007
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140
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Michigan
My 1997 Monte Carlo has developed an odd problem. My ABS gently activates in the final 10 or 20 feet of a stop. It is definitely the ABS. Interestingly, the ABS light does not illuminate. I checked all of the components, and discovered the passenger frint reluctor or tone ring is broken: I was able to turn it by hand on the axle. There was no other discernable damage. Pulling the ABS fuse cures the problem...and disables the ABS completely. I considered having the ring replaced, and it is very expensive...so, I glued the ring together, assuring that it could no longer spin freely on the hub (it really didn't spin easily, but the epoxy assures it cannot spin). This made no change in the problem. I'm a former master mech, but have been away from the practice for a very long time. My knowledge on this topic is minimal. I will take a guess: the problem appears to be one of signal strength, rather than the damaged reluctor (as it appears to have the same gap and position as the others--but, I don't discount the possibility that the reluctor is the culprit). Because the signal (voltage) from the reluctor/tone wheel decreases as the wheel speed decreases, I believe that maybe there is increased electrical resistance somewhere in the system, which is dropping overall voltage. Therefore, the voltage from one of the sensors is being read by the computer as lower than normal, but not low enough to pulse the ABS continuously. Yet, as speed approaches zero, the voltage drops faster than it should, thereby pulsing the ABS. The increased resistance could be: Increased gap between the tone ring and sensor Bad sensor (but, it seems that the sensor would either work correctly or not...I'm just guessing!) Any connections along the circuit to the computer. Heck, I'm not even sure which wheel is causing the problem. So, I guess the easiest thing to do is to check the air gaps, then check and clean all connectors. Next would be individual sensor tests: I would guess there are published values for voltage output, but where to find the info? I could check all of the sensors and see if there is one reads redically different from the others. I could be full of [censored]...but if anyone can help, I sure can use it!
 
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Joined
Dec 19, 2004
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beaver land EH?
Hi, While I'm by not means an expert, but with strong electronics diagnostics background, here's how I would approach your problem: Now, if you are to assume that your reluctance rings or your sensors at fault, you may get different readings from the wheel speed sensors. OK, trying to lay out a better, more easily understandable diagnostic steps for you: Background: ABS systems, in general, requires signal feedbacks from all 4 independent wheels, a reference from engine RPM, and depending on how sophisticated the ABS system is, some may have yaw sensors, transmissions sensor(to detect which gear it's in, sometimes signal coming from the ECU that deals mainly with engine controls), EBD, etc. Now, Assuming that your system is the less-sophisicated ones and we'll try to simplify things further: (making the assumption that your ABS control module and computer controls are in good working order) -disregarding the engine speed/transmission sensor, that leaves 4 wheel speed sensors on all 4 corners. These sensors have a small coils wounded to them into a former (could be a small bobbin style or it could come as a big ring surrounding the outer bearing/hub area), and then there's a toothed gear that generates the pulse. The coil itself is a sensor towards the reluctance ring which serves as a pulse counter in the most precise fashion. Clearance between the reluctance ring (toothed ring) and the coil is not that important anymore, so long as they don't rub against each other. While crude resistance checks can give you a pretty decent idea as to how good electrically the coils are, unfortunately, any internal breakage of the coil, intermittent conductivity due to moisture/corrosion of the coil windings, the lead wires betwen the coil former back to the module (past the wheel arch, etc.) and even as far up as intermittent/corroded sensor connection between the ABS sensor module and the sensors can be the cause. Occasionally, you may come across a situation where due to rust and debris, the clearance between the reluctance ring and the coil may be closed due to ferrous debris and this will call for a complete disassembly and clean up before you can put them back. Now, back to the subject of sensors. Most common fault that may lead to partial/intermittent/complete failure that may give false signals to the ABS module would be corroded connections, broken wires/leads between the coil sensors and the ABS module, intermittent coil windings due to broken/damaged insulations, etc. While there is not hard and fast way to diagnose this other than to rely heavily on the ABS generating fault code, the other rather tedious trick is to pull each and every sensor off the car and then test it on the bench, with one technician holding on to an ohmmeter while he other one starts bending the leads until you get a discontinuous reading (even just a flicker means bad sensor), or perform a thorough cleanup and then perform visual inspections for damaged insulation on the coil former or the package itself. Since these units are generally sealed, there's usually no proper means of restoring them back to normal, reliable working order. While you may resort to using used parts from wreckers (which comes w/ very little guarantee other than the typical 60days replacement guarantee) that it's going to solve your problem nicely, and new parts are fairly expensive. either way, if the sensors are the cause of the trouble, you have to replace them in order to get it to working properly again. Another way to test it is to use a high sensitivity scope (oscilloscope) and read the pulse form but that falls into a different subject. Hope that's not too overwhelming for you. ABS tends to be a PITA to service/diagnose... Q.
 
Joined
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Test for a signal at the sensor connector from the ring you fixed. Compare it to the good one on the other side. [You have to spin the wheel off the ground with your multimeter hooked up.] This should identify the problem in 1/2 hour or less.
 

Kestas

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The Motor City
We use oscilliscopes to assess the signal from a wheel bearing hub unit. It's not that sophisticated; it either generates a decent wave form or it doesn't. We spin the bearing by hand. The gap between the tone wheel and sensor is important. I wonder if the fact that the tone wheel is broken has an effect on the signal.
 
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Sep 11, 2004
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snowblind in TX
 Originally Posted By: Kestas
I wonder if the fact that the tone wheel is broken has an effect on the signal.
We have a winner. When the tone wheel breaks it changes the "flux" on that section of the wheel. All it takes is a tiny hard to see crack. I ran into this all the time on FWD Fords. Constant ABS activation on the last 10-20 ft of a stop was the classic symptom. The hard part about diagnosing problems like that is they don't leave codes, since the ABS is technically doing what it is supposed to, and scan tools won't catch the "blip" that causes the problem. Good call on the scope.
 

Loogie

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Sep 17, 2007
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Michigan
I wondered if this wasn't the case. I will check to assure that it isn't simply increased resistance somewhere, and pull the fuse if everything looks ok. Many thanks!!!
 
Joined
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beaver land EH?
increased resistance is a sign of coil insulation breakdown, or something worse. You shall seriously consider doing as complete set of testings as possible (aside from replacing the reluctance ring) to ensure that the ABS would have engage properly afterwards. Yes, scope works in diagnosing/differentiating pulse pattern and signal strength (comparison between all 4 sensors) but it cannot tell you if your coil has elevated resistance, or intermittent resistance reading due to beakdowns or physical damage. A bit of systematic diagnostic approach and an ohmmeter does. My 2c's worth. Q.
 
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