YaHoo!

I have a 93 C2 and although my ABS warning light works and is not illuminated I decided to test the ABS by slamming on the brakes to see if the wheels would skid. It turned out that the front wheels locked up!

I did my research and knocked out these items with no result - the fronts still lock up:

• Changed brake fluid a month ago (not sure if the ABS was working correctly then or not),
• Cleaned all ABS electrical contracts to the battery and ground,
• Cleaned the wheel sensors (which were already pristine), the associated plugs and inspected all the sensor wires for damage,
• Inspected fuse #16 and relay #34 on the main panel as well as the 2 relays on the ABS itself for damage,
• Pulled the #34 relay while the car was running and the alarm sounded in the car and was immediately silenced once returned,
• The ABS warning light illuminates with the other warning lights upon start-up but not otherwise.

One observation, when I test drove the car after completing the items above I slammed on the brakes, the fronts locked up then I'd release and reapply the brake and it seemed the ABS was working as I could feel pulsing through the pedal very briefly before stopping.

I have a few questions:

• Did the removal of the #34 relay, alarm sounding and then silence after putting it back in place confirm that that relay is good?
• Any way to really test the relays on the ABS unit itself?
• Because my back brakes aren't locking up I suspect there is a pressure differential in the ABS that may be the root of the cause. I believe pressure is controlled by the opening and closing of the solenoids but I'm not certain and, if it is, I have no idea what to do to correct it- any thoughts or suggestions would be appreciated.

For good measure here's a couple of pics of cleaning the wheel sensors.

tdiquattro

When mine failed there were no warning lights, checked everything just like you, it turned out to be the ABS pump. The pump is an electric motor that spins and works 2 pistons driven by a cam on the end of the motor. Both pistons were seized in their bores. Tiny things about half inch diameter.

Anyway, the pump was the problem, I repaired mine. The only way to know is to remove the motor from the pump and have a look at the end.

hope this helps,

tdiquattro

YaHoo!

Thanks tdiquattro! Hope I don't have to drain the system and break down the ABS motor to fix the problem but will if needed.

I just got back from a drive to test the system again - the ABS never ever kicked in for the front wheels and the backs never skid. I then pulled: the fuse with the car running- the alarm sounded; relay 34, and again the alarm sounded; 1 of the 2 relays on the ABS and the alarm sounded; then I pulled the the 2nd relay on the ABS and no alarm. Maybe that's significant? More to research...

Keep the ideas coming!

ThomasC2

It seems like ABS-pump failures are getting more common, maybe a problem due to age. I recognize the symptoms you describe and it is always the pump being the culprit.

I don't know if it's a good thing to actually "exercise" the ABS pump once and a while when out driving. Most drivers probably never use the ABS and the cars sits in a garage for long periods.

A new pump is very expensive. You can repair them if lucky or find a used one. But the used stock is probably getting smaller over time.

Thomas

robt964

There has got to be a way that you can jump the motor allowing you to test run it? Maybe strip it down in situ (without moving the lines) until you can identify the wires feeding the pump? Thats the route I think I'd take.

As ThomasC2 said, I bet a great many out there won't actually be working now having sat there for 25+ years and maybe operated only a handful of times.

Edited: Looking at the wiring diagram Sheet 4a (page 97) and the 'ABS Hydraulic Unit', it appears this is possible. There are two relays that live on the pump. One has 7 pins, the other has 4? On the diagram, its the relay on the right (4 pin) that switches the motor. You just need to bridge terminals 9 and 12. Check the wiring diagram.

robt964

I've no idea of the implications of running up the motor in isolation so for precautionary measures I'd disconnect the ABS ECU first and only fire up the pump long enough to confirm it spins. If you don't like jumping relays, or are unsure of the pins to bridge, the other approach is to pull out the relay, pop-off its plastic cover, refit it then pinch the contacts closed.

boxsey911

My ABS pump also failed. Same symptoms i.e. locking up without warning lights was the only sign. With standard brakes the fronts will always lock up first. You'll only get the rears to lock at the same time if you do a very hard emergency stop. I managed it on a dry road...there was a lot of tyre smoke!

ras62

I had no ABS and decided to check the system. Once the pump was "exercised" as Rob964 described the system worked fine and I've had no ABS problems since. I make a habit of infrequently triggering the ABS on a wet road a. for peace of mind b. like any elec/mech system it should be tested / used.

Dr Mitch

I was at a DE event this past summer and during the emergency braking exercises my fronts also locked up - no warning lights (I also have big blacks up front and 993's in the rear - wasn't sure if that somehow played a role) - sounds like I need to have that motor looked at as well...

Goughary

Most of us don't bleed and flush the abs system enough. Brakes get bled often (for some), but we tend to neglect the rest. I suppose this is why we are seeing more failures. Brake fluid takes on water and we get lots of corrosion in the system. In my c4 the insides of my diff slaves were terrible...had any previous owner just done a full bleed once per year, minimum, I wouldn't have had issues there, or in any other part of my system that I replaced...

If you have to replace the abs pump and any other parts in the system, full bleed minimum once per year.

Cars with fresh fluids last forever. Well...almost...lol.

tdiquattro

Just to add, the motor doesn't fail, it is the pistons on the end of the motor that stick/rust. So the motor will run, but isnt moving the pistons. There isn't any gasket between the motor and assembly so water/damp is free to get in to the cam/pistons.

I replaced the 2 pistons and motor (with cam) with items from a merc 190E ABS pump (£15 delivered from ebay for a whole 190E pump!!) easy fix.

I can post a few more pics and a how-to if you like.

Cheers

robt964

I speculate that the price of merc 190E ABS pumps is about to rocket - Nice find. Was it pot luck or did you know the same parts are used on this vehicle?

boxsey911

I think you're being modest there mate. I remember when you contacted me about the ABS problem and we discussed that you were getting the same symptoms as I'd seen. My hat goes off to you about how resourceful you were in finding the Merc pump and how you fixed it so quickly.

I'm sure everyone here would be very grateful for a 'how to'.

YaHoo!

It's starting to look like the pump might be the issue.

ras62 how did you exercise the system? While road testing yesterday I locked the wheels up several times hoping the repetition might loosen/exercise the system (the pistons that tdiquattro refers to above; no such luck I only produced smoke and skidmarks!

tdiquattro and robt964 great info and suggestions, thank you-

tdiquattro a "how to" would be very much appreciated!!!