YaHoo!

Hi All-
I have a 93 C2 and an ABS that doesn't work. The warning light works but doesn't illuminate to indicate a problem with the ABS. I know it doesn't work because I can lock up the front wheels (not sure why the back ones don't lock up).

Here's what I've done to try to eliminate the problem:

• Changed brake fluid.
  
  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,
  
  Removed the ABS pump (see pics below) and cleaned the pistons,
  
  Inspected and cleaned the pins on the large plug on the ECU (also applied a little dielectric grease on them to prevent any corrosion in the future),
  
  Jumped the abs motor (connected the #9 pin on the pump directly to the battery) and the motor runs,
  
  Jumped the fuse box relay R34 between terminals 30 and 87 and can hear one of the relays on the ABS click but the motor does not turn on, should it?

I assume that because I the warning light is not illuminating that ball the sensors and related wiring is OK (I also inspected them as mentioned above).

Based on the info above I'm suspicious it is an electrical issue. It seems like the motor is not being told when to turn on. Any ideas on items to check are welcome.

Breaking the pump open was pretty fun and informative but required 2L of fluid afterward to bleed all the air out. Here are a few pictures of the process- note the waxy build up on one of the pistons- I thought that was the source of my issue but the system still didn't work upon reassembly. I melted the wax away by putting the piston in a bath of denatured alcohol.

Look Mom No ABS

ABS motor, left, and pump, right, with pistons exposed.

Strange wax found on one piston.



Clean piston parts

Mr.Alex

I wonder if that is the whitish wax that was a culprit for blocking lines in 993's.

https://rennlist.com/forums/993-foru...-abs-pump.html

alexjc4

I had similar symptoms to this and the issue was in the ABS unit, the dedicated bosch ABS test tool was used to discover the ABS unit was building pressure, holding pressure but not then releasing. I think the part of the ABS pump you rebuilt (kudos) are different to the components that caused my issue.

justin-in-athens

See if you can find another lister fairly nearby with an abs controller you can borrow to see if it is the culprit. If it is, you might source one from a parts yard, or send it to a DME rebuilder to see if it has failed internal cold solder joints.

YaHoo!

Alex- was that on a C2 or a C4? The pump on a C4 is a little different in that there is an additional pressure boost pump and a pressure valve.

I'd love that and it should be simple/fast. Alternative could be to my ECU in someone else's car. Any takers out there? I'm in Davis, CA.

Bet it is albeit the symptoms of that is a soft pedal which I never had. If I continue to be stumped then I may end up flushing the system with either Dot 3 or denatured alcohol after some research to confirm those would be ok to use.

I'm still convinced it is something related to powering the the motor on the pump- it is not knowing when to turn it on. Another key piece of info may be that it does not do the system test at ~6mph. Just rechecked the ECU plug again last night and it looks brand new, no issues.

Any other suggestions? Thanks everyone.

alexjc4

Yup c4 but the brake circuit is separate from the booster and pdas hydraulics. The actual bmc/abs/calipers circuit is the same.

YaHoo!

Hm, can't imagine what they replaced. I looked at my motor/pump last night and the only other thing I can see on it are the three solenoids which don't look like they can be removed or replaced.

alexjc4

They replaced the whole unit, but it was those solenoids that weren't doing their job as I understood it at the time. They had the car for a month trying to figure it out!

YaHoo!

If brake fluid passes through the solenoids, which I think it does, then before dropping the big bucks on a new pump I could run denatured alcohol through the system as described above in an attempt to clean any wax in the solenoids and perhaps have them function properly.

Alex did your unit run the self test?

alexjc4

I agree, i think my issue was likely caused by that waxy stuff somehow making the solenoids stick, my issue happened just after i got the car and was fixed under warranty so I didn't have much of a say in the process, I'd definitely try the alcohol route.

Not sure what you mean by self test, the warning light behaved normally ie it came on with ignition and then went out. They used the bosch tester which allowed them to trigger each stage of operation: build, hold, release, pulse - at which point they located the issue.

YaHoo!

Thanks, this info is helpful. Interesting and maybe related- when I had all the brake lines off the pump I blew through the orifices (blowing into the pump) to ensure there was clear passage- they all were clear but some passages allowed easier or more air flow than others. I wonder if the flow resistance in all of them should have been the same.

Our cars run an ABS system test at around 6MPH to make sure the system is operating.

So my next question to the Rennlist experts is if I'm getting a high pressure reading due to the "clogged arteries or solenoids" would that somehow prevent the absent self test at 6 mph? In that test for a C2 I assume the pump runs and one can hear it- is that a correct assumption?

Thanks all.

YaHoo!

Anyone have other suggestions or the answers to my questions in bold above? Thanks.

ras62

The self test is just the electronic side, power ok, ecu ok, wheel sensors ok. The pump and internal solenoids are not operated during the test, there is no feedback from these items to the ecu anyway that I'm aware of which is why the Bosch tester is so useful.

boxsey911

+1. When I had a wheel sensor problem the warning lamp (and buzzer) stayed on and I eventually tracked down the sensor (with ras62's help). But when a solenoid valve failed in the ABS pump there were no warning lamps. The only warning was the wheels locking up. The Bosch ABS tester eventually diagnosed the problem and a used ABS pump was fitted and solved the problem.

The same happened to rennlister 'TDIquattro'. He fixed his problem by transferring parts from a mercedes ABS pump which is almost identical to the 964 one.

justin-in-athens

I don't have a whole lot of input, just thought i'd share this pdf of the PDAS system.