Thanks to all of you last time, I was able to solve the problem with my heater last winter. However, I've had an ongoing issue with my ABS system that I really can't pin down. Hopefully I'll be able to figure it out with some advice from here.

For some background, I've got a 1991 Volvo 940 GLE sedan. It uses a 3-way Bosch ABS unit and for a long while, it worked perfectly fine. The first time it malfunctioned was when I bought new front tires. As I slammed on the brakes to test them, only to find only the driver's side ABS worked. I could hear the very obvious tire squeal/dragging from the passenger side.

I ignored it, and kept driving. Then, I changed my brake pads, and lo and behold, my driver's side front wheel ABS stopped working. My passenger side wheel still didn't work.

I tested the system in the winter on the snow, with me outside watching as my Fiancee drove the car and hit the brakes. The ABS doesn't come on on the front, but every single time, the ABS would activate on the rear.

This is where it gets confusing. Its gotten to a point where it will never work on the first hard application of the brakes. But, if I stab the brakes, locking the tires, lift slightly, then reapply brakes immediately, the ABS will come on, but not for a long while. It will cut out again before the car comes to a stop. It doesn't matter if its low speed or high speed.

At the moment, there are occasions of false modulation when rolling to a stop, but its not extremely disruptive. Some days though its worse than others.

Finally, the list of things I've done on the car, in order from first to last directly related to the brakes are:

1. Front Passenger side Wheel Speed Sensor (no change)
2. Front Driver's side Wheel Speed Sensor (no change)
3. Front and rear pads and rotors (ABS on driver's front wheel stopped working)
4. Brake fluid flush (no change)
5. Checked relay and fuse under steering wheel (no issues)
6. Checked relays and cleaned contacts at ABS unit (no issues)
7. Spliced in new wiring at rear axle sensor (no change)
8. Replaced both front wheel bearings and ABS tone rings (no change)

So after all of that, I've hit a brick wall. I'm guessing its possible there may be a grounding issue on my car, but which ground would I need to take care of? Other than the ABS, my car runs great!

 

I have 0 experience with abs.
Do you think the module could be bad/going bad? Try getting another one from a junkyard, and swapping them out? Ground wires are definitely stubborn little punks. could be an issue.
I think I remember some guys on turbobricks having issues with the ABS modules and they just deleted the system all together.

 

is the ABS fault light on the dashboard illuminating ?

you can read the ABS fault codes via the diagnostic block under the hood, at least on the newer models, see Brakes: ABS

theres a bunch more ABS troubleshooting info on that page, above and below that inpage link.

 

Thanks for the input. I'm suspecting the module might be going bad, but how does that explain the intermittent nature? I've also read that the module very rarely goes bad and I don't want to spend the money when its not needed.

Pierce, I've read that website about ABS brakes top to bottom multiple times, and I've gone through everything! The fault light illuminates at the start during its self-diagnostic, but apart from that, the light usually stays off. I say usually, because on occasion it comes on, but goes off on restart and stays off for a good while.

My on-board diagnostic box has no pin-outs for the ABS, so I can't even get any codes.

The last step is a diagnostic of the pin-outs at the ABS computer. I've got the Chilton's manual that does a pretty good job at explaining the diagnostic steps. Hopefully the problem will make itself known.

 

'91 don't have code diagnostics for the ABS like later models. The ABS light is it.
If your ABS light is OFF then your system is OK; you are unclear about when it comes on but I gather is it is normally OFF...

 

the codes apparently started with the 1992 updated ABS actuator that has the rectangular slab of alloy on top.

your encoder wheels are clean, and you've checked the gap from the front wheel sensors to the code wheel?

does your speedometer work reliably? I believe the speedo pickup is also used for the rear axle ABS channel.

 

Pierce,
the tone rings (or encoder wheels) are very clean. The new wheel bearings have only been installed for a few days. The air gap is perfect. The Speedo works well...now. Previously, it used to be slightly erratic, but it has sorted itself out somehow. I didn't change a thing. It used to jump around all the time, but not anymore. I have pulled the rear axle speed sensor and verified the resistance as well as its cleanliness. I also spliced in new wires and a new connector at the rear axle.

Lev,
I would like to believe my ABS system is fine, but even with the light off, my wheels will very audibly and physically lock up. As I've mentioned, its intermittent, but I can get the ABS to reliably activate if I release and reapply my brakes. On first hard application of brakes, ABS never comes on. After I release with a second quick follow-up application, ABS works. Sorry if I'm unclear about when it comes on, but the light will only very occasionally turn on. Otherwise, it is normally off. Even with noticeable ABS issues, the light stays off.

Essentially, the ABS works when I hit the brakes and reapply it. Otherwise, if I don't let up on the pedal and just do a hard panic stop, the wheels will lock up and ABS will never kick in.

 

hmm. is the brake switch functioning reliabily? if its not closing, its quite possible the ABS would assume you're not stepping on the brakes, and not do anything.

there is also a ABS surge protection relay, if that is erratic, that could cause strange behavior. this relay is under the dash somewhere near the ABS Control Unit, it has a blade type 10A fuse in it too.


I do have the 740/early940 brakes & ABS troubleshooting manual, but I can't scan it. its got a LOT of test procedures involving various connectors and a volt meter...

 

Pierce,
I don't see how the brake switch is NOT functioning properly. While inside a dark garage, my brake lights turn on very reliably on every application of the pedal. The bulb failure relay is fine, and my cruise control works fine as well.

I checked the fuse to that relay you mentioned, but I guess its not a bad idea to check the relay itself.

I was doing a diagnostic on the pin-out of the connector at the ABS module, and noticed something odd. The Chilton's manual says that on Pin 7, there should be 12V. If there isn't, they say the ABS failure bulb must be out. The fix is to replace it. On my car, there doesn't seem to be 12V on pin 7, but the bulb works for sure and lights up everytime on start-up during its self-test phase.

Any opinion on what else might be going on with Pin 7?

 

there's at least 3 different ABS units, early style (up to 87), later early style (88-91), and late style (92+). the first two can be exchanged, the latter can't, at least not without replumbing the brake lines. because of this, and Chilton's track record, I'd take everything in that book with a grain of salt.

now, I've never heard of this being an issue on the Volvo side of things, but Mercedes from the same era use a very similar "OVP' (Overvoltage Protection) relay, which are notorious for causing a wide range of erratic symptoms, and the driveway mechanics suggest replacing them as a matter of course every 10 years or so. on the mercedes, this OVP is on ALL the power to the ECU/ICU/ABS/etc

 

the Volvo ABS/Brakes service manual warns you NOT to stick a volt meter probe into the connector plug at the ABS module as it can damage the pins, instead they say to pull the back off the connector, so you can probe it from the wire side.

with the ABS module connector unplugged, pins 10, 20, 32, 34 should be 0 OHMS to ground (tested with ignition off). any of these grounds are bad, it says to check wiring integrity, and ground connections at left A post. if pin 32 is not grounded, it says to check again with a new 'valve relay' (this is plugged into the ABS modulator unit under the hood).

with ignition on, pins 27, 28 should be battery voltage. pin 29 should be 0.5 to 1.0 volt. with ignition on and brake pedal pressed pin 25 should be on. if 25 is bad, check brake light switch. if 27 and/or 29 is bad, faulty valve relay. if 28 bad, faulty motor relay (also at ABS modulator).

switch ignition off. plug ABS control module connector back in. unplug hydraulic modulator connector. switch ignition back on.

on ABS modulator plug, pin 6, 7, 10, and 12 should have battery voltage.
if 6 is not +12, check wiring.
if 7 is bad, reconnect plug, verify ABS light lights (seems oddly worded, I'm paraphrasing).
no juice on pin 10 is surge protector faulty
no juice on pin 12, check wiring.

ignition off, plug modulator plug back in.

(goes into sensor testing).

 

This is great information. I will check it out. Thanks!

 

under the cover on the modulator, there's 2 relays.

the smaller one, closest to the hydraulics... thats the Valve relay.

the tall/longer one is the Motor Relay.

 

So according to your diagnoses, if Pin 7 is not showing 12V yet the ABS light is functional, what could be the issue? Looking at the wiring diagram, the ABS light should not turn on if there is no voltage through Pin 7.