A different kind of ABS problem

#1

I’m working on my brother-in-law’s Spec E30, 1987 model. The ABS hasn’t worked since he put the car together, and he can’t remember if it ever worked since he bought it. THIS car only triggers the light after it’s been driven several yards, which would typically indicate a bad wheel sensor. So I stocked up on wheel sensors at the junkyard since the insulation on his was crumbling off and swapped them out, one by one. The light still comes on. I also performed the “disconnect all but one” test, only to find that neither rear sensor sets off the light. So now I’m suspecting the entire rear signal circuit is compromised somewhere. However, I don’t know how to do a check on this. I thought with the key on there would be 12v going to the wheel sensor leads, but I could not find any power at the front ones (which work). So I’m kind of stumped as how to proceed. This car has had a massive amount of work done to it over the last several years. The rear suspension was completely apart, a full roll cage was installed (the wiring harness runs along the left rocker), lots of welding has been done, etc etc. I have checked the teeth on the impulse ring and they look find (and rather clean, compared to the fronts). I’ve checked and rechecked the sensor connections (which are inside the car and quite clean). I think (but am not now sure) that I tried swapping in the ABS brain from my car last year. Looking for ideas on how to track down where the break in the system is.

#2

Did you do a continuity check on the wiring from the rear sensors to the ABS computer? That would require grabbing some spare wire and aligator clips and rigging up some 8’ jumper leads.

I don’t know that the sensors are powered. The passing teeth would create a low voltage square wave signal on the wire.

Be advised that there’s a number of guys here that know more about ABS than I, so I’m kinda winging it here.

#3

Scott is right, you won’t find any voltage at the wheel sensors, but I think you do have wheel sensor issues. If the pump wasn’t functioning or the relay was bad, the light would never go out. A quick and easy way to check each sensor and associated wiring is to remove the connector at the ABS control module and measure resistance across the two terminals that connect each sensor. Pinout follows:

Left Front: Pins 4 and 6
Right Front: Pins 21 and 11
Left Rear: Pins 8 and 9
Right Rear: Pins 24 and 26

Resistance should be ~1000 ohms; IIRC, the rears have slightly higher resistance. If a sensor is bad, resistance will be very low (short circuit) or infinite. If you find wheel sensors that are not right at the main connector, run the same test at the wheel sensor individual connector (the yellow ones). If resistance checks out at the sensor, then you have a problem with the wiring between the sensor and the main connector. You can check this wiring pretty easily with a voltmeter by shorting the yellow connector and checking for continuity between the pins at the main ABS control module connector. It would be possible to replace this wiring pretty easily at the connector if you had a donor harness.

If all sensors and wiring check out correctly, you could have one of the front sensors reversed. The left and right sensors are the same on the rear, but the fronts are specific to the left and right. They mirror each other in shape. If you have one of each, you might try swapping the left with the right. Before you try any different sensors, make sure you check for 1000 ohms before installing.

The attached picture shows the ABS control module pinout at the main connector.

[file name=ABS_pinout.jpg size=55760]http://spece30.com/media/kunena/attachments/legacy/files/ABS_pinout.jpg[/file]

#4

Fish, pls go start a thread in resources on ABS. Put this stuff in. Then we’ll get Jim to chime in, and we’ll also need the specifics of the Patton sensor test in there.

#5

Start with the Taylor/Patton stupid series of test, then move to the real diagnostics, then to complete system add-on like Bratton did. Throw-in the tech stuff from Gress, the electrical how-to from Yoda Levie and tips from others…gotta love this place for the how-to.

Thanks, Scott, for trying to get us organized.

RP

#6

FishMan wrote "If the pump wasn’t functioning or the relay was bad, the light would never go out. "

I don’t want to hijack this theread but my light is always on… I checked the relay and it appears to be OK

What tests can I do to isolate the problem?

Thanks!

#7

The first thing to do is to check the wiring from the ABS module to the rear sensors. Fishman’s advice is good. But the way I’d go about it is to disconnect the sensors and check the wires to the rear connectors for a short to ground. Then short the connector and see if that shows a short circuit. Check each of the rear sensors for resistance standalone and then connected.

If the light comes on with ignition and doesn’t go off when the engine is started, the relay, ABS power, or the module is to blame.

#8

fishman, numerous thanks. That’s exactly the kind of testing procedure I was seeking. I probably won’t get to mess with it until the weekend, but I’ll report what I find.

The front sensors may possibly have been swapped a few times by now and it’s possible that they could be mixed, but aren’t they offset where the bolt goes through? I’d think they would only fit one way (or at least look like they belong in there). How do you tell which goes to which side? And, for general knowledge because I like to know these things, how would the fronts being reversed effect the rear?

#9

The front sensors will only install one way even though the same part number is used for both sides. The front sensors don’t affect the rear, at least in the sense of disabling the rear sensors. Whis is the case you have.

#10

The fronts will install in either side though. I’ve learned this the hard way… When properly installed, the mounting tab will be oriented straight down and the wires will be angled slightly forward.

#11

Couldn’t have done it without you guys. There were actually 2 problems: one dud sensor (and you just knew it was the ONE I didn’t swap out because I was out of spares) and the fronts were reversed. The fronts as stated must have the wire exiting the sensor angled toward the front of the car. This looks wrong, but it isn’t. If you have them in backwards they will pass the “unplug all but one” test but it will fail when they are all connected. That one I’d have never figured out on my own.

Thanks again guys! :slight_smile:

#12

If the light comes on with ignition and doesn’t go off when the engine is started, the relay, ABS power, or the module is to blame.

My light stays on. I have swapped 3 good looking relays and same issue. Does ABS power mean no power to the system like a blown fuse or something worse? As for the module can i try to pull one from a junkyard car? New ones are really expensive.

#13

[quote=“RacerDan” post=56495]If the light comes on with ignition and doesn’t go off when the engine is started, the relay, ABS power, or the module is to blame.

My light stays on. I have swapped 3 good looking relays and same issue. Does ABS power mean no power to the system like a blown fuse or something worse? As for the module can i try to pull one from a junkyard car? New ones are really expensive.[/quote]

Dan - I have some working ABS parts left over from when trying to diagnose my system. You can try some of my parts (ABS brain/ABS relay/ABS pump) and if it solves your problem, we could work something out. Let me know.

#14

[quote=“RacerDan” post=56495]If the light comes on with ignition and doesn’t go off when the engine is started, the relay, ABS power, or the module is to blame.

My light stays on. I have swapped 3 good looking relays and same issue. Does ABS power mean no power to the system like a blown fuse or something worse? As for the module can i try to pull one from a junkyard car? New ones are really expensive.[/quote]
It could be the alternator or wiring. If the ABS module doesn’t detect charging, the light wont go out. Modules seem to be pretty robust so I doubt that’s your problem. It’s more likely to have three bad relays…

Pull the connector of the harness and check wiring as below:
-Pin 10, 20, and 34: Ground.
-Pin 15: 14v with car running. This is the charge indicator from the alternator.
-Pin 25: 14v when you activate brake switch; no voltage with brake switch open.
-Pin 29: .65v; this is the green red wire on the negative side of the warning light.

If the voltages read as above, it’s likely the relay or control module. My money is on the relay. I’m not sure how to check a relay without opening up for inspection. There is a light gauge fuse that often blows out. You can solder it back together or buy a new $40ish relay.

#15

I replaced the ABS pump and the light is still on. The electrical pin tests all passed except pin 15 was getting 13V. Is that a problem? Or is all good and i am looking at the computer module as the culprit now? There are 2-3 clicks at the ABS pump when key goes on and the main relay is good.

#16

Does the light come on with ignition and never go off? That would be the ABS fuse, control module, or pump/relays.

#17

The light goes off for about 2 seconds before moving then comes back on. Before i replaced the pump and it’s relays it never went off. Is there an ABS fuse? I checked the main fuse block and nothing is blown. The main relay looks good and tests out okay. I am figuring the module now.

#18

If the problem is the sensor then the light will trigger when the car starts moving. I can’t tell for sure if that’s what you meant to say. But if that’s the case, search here for an old thread on how to figure out which sensor it is by trial and error…assuming the sensors seem to have the right resistance when checked.

#19

Troubleshooting the ABS system isn’t very difficult. It is a stand alone
system that only gets input data from the brake light switch, wheel sensors and
system power. The parts of the system are the ABS module located next to the
steering column, the ABS relay mounted above the ABS module, the ABS unit in
the left front of the engine bay, and a wheel speed sensor at each wheel.

If the ABS light comes on with ignition, but doesn’t go out when the engine
starts, a wiring problem, bad module, or bad ABS relay are the likely
candidates. A failed or weak alternator will also cause this because the
system voltage won’t rise enough. There is a fusible link inside the ABS relay
that can be blown. The relay can be repaired, or better yet replaced.

If the ABS light comes on when you turn on the ignition, goes out when the
engine starts, and then comes back on before the car is moving the cause could
be a bad module, a bad ABS unit, or a wiring fault.

If the goes out after engine start and only comes one when the car is moving,
there’s a problem with one (or more) of the sensors. The simple test to find
out which sensor(s) is the cause is to disconnect all but one of the sensors
and drive the car. If the light comes on, that sensor is sending a speed
signal to the ABS and is good. Repeat with each other sensor to find the
one(s) that don’t cause the light to come on. Since the ABS module has no
speed input besides the wheel sensors, if only a bad sensor is connected the
module can’t tell the car is moving and thus doesn’t fault on missing speed
signals from the other wheels. Once a bad speed sensor is found, if a new
sensor still doesn’t work, there could be a wiring fault in that circuit or a
bad input channel in the ABS unit.

#20

We have an 87 E30. ABS light is always on.

Relay replaced - and works.

Module swapped w/ 2 others - no change.

Tested the three ground circuits - all 1.3 ohm resistance to ground

Tested the 3 power pins suggested. All gave close to voltages listed.

Shorting the ABS pump shows the pump working.

Have swapped a couple front sensors so far.

Any ideas on what to check next.