The next job I was set to do really was a leap into the unknown - rebuilding the ABS pump. The car has always had an ABS light on, which usually means a sensor wire is down, but on a 964 often means the pump has seized. But, the good news is that they can be repaired fairly easily and very cheaply using a second hand pump from a Merc 190E or similar. These are in plentiful supply, don't suffer the same issue as the Porsche unit, and cost about £30 on ebay. My car came with one, so even better.
Removing the pump is easy, it is mounted in the front boot behind the battery. The only bit that is a pain is that it has five pipes that go from it, through the bulkhead, to the rest of the brake system. Three of these go into a bulkhead connector which is female on both sides, so has 3 pipes in, 3 pipes out, and the connector is secured by an M6 bolt allowing you to undo all the unions easily. Well, you could if they hadn't turned to chocolate, so the standard operating procedure of cutting them off tight to the union and then removing on the bench applied. Those on the boot side were fine, as protected from the elements. These 3 feed both front corners, and the rear circuit, going to the T-piece up the side of the gearbox shown earlier. Simple enough.
The other 2 go to the master cylinder, and each one goes through a cylindrical bulkhead connector which is not secured at all. So unless you are Mr Tickle, or have a helper, you cannot hold a spanner under the car and inside the boot at the same time.
Fun ensues.
Kinked pipe results, another to replace letter.
Anyhow, the unit out on the bench ready to pull apart.
So, armed with a How to guide, taken from an american Porsche site, I set to it. Now, I have not done this before, and expecting the Porsche one to be in a right state and difficult to separate, I started with the Merc one. This also gave me the option of backing out if I ran out of talent
It genuinely is straightforward, remove the relays, disconnect the earth strap, remove the main power lead and then undo the bolts securing the relay board to the main body. I then cable tied this out of the way. Note, the donor pump had been cut out of the car.
To then get to the bits we are interested in you simply remove the two very long torx bolts that secure the motor to the main pump, and carefully remove the motor. This is then revealed...
So on the end of the motor is a rotating cam and this operates two pistons in the pump. These are made up of a two piece piston inc 2 springs and a ball bearing. The order of them is critical, hence the careful disassembly. My plan was to remove one very carefully, suss out the order for the next build. Unhelpfully, my build thread had left this crucial info out, so it was imperative that I got this right.
There was plenty of crud in there, but no corrosion at all
Access to the pistons is via 2 very large (10 or 12mm) allen headed bolts on the side of the pump. Once removed, you should be able to push out the pistons into the chamber in which they operate. Mine needed a good tap to free, but once removed were found to be in perfect condition. Thank you Mercedes!
For anyone doing this, the order is large piston - contains the two springs set inside each other, the ball bearing then goes in (using the larger spring as a seat) and then the tiny piston presses into the larger one. I coated it in a little brake fluid, and you can then operate the tiny piston into the larger one. The rubber o ring stops it falling out, and the springs allow it to operate.
So, with a bit of confidence in what I was doing, I set about the Porsche pump.
And then removing the motor reveals....
Thick with orange rusty crud. In fact, the pistons were seized solid and the tiny pitons were fully inserted into their larger counterparts and seized there (so much so that I can separate them).
There are a few theories as to why this happens. The most credible is that the engine bay heat of the front engined Merc means that any moisture evaporates, whereas on the 964 it just causes rust.
So, I cleaned up the chamber, lubricated with a bit of fluid and inserted the rebuilt pistons. With the small outer piston compressed, you should be able to very carefully refit the motor. Of course, once the pistons have any pressure on them, they expand out to touch the cam on the motor.
Now at this point I looked at the cam on the Porsche unit and was not very happy with the rust on it. It had been seized so long that it was completely pitted (would love to know how many years it got through an MOT with the ABS light permanently on...)
I looked then at the Merc motor, which was in much better condition. Would you believe it, whilst the pumps are different between the two cars, the motors share the same Bosch part number!
So I took a punt and put the Merc motor on my pump.
I replaced the damaged pipe and job done. I reckon it took 2 hours or so end to end.
Now I must confess, a day later I was clearing up the garage and found a ball bearing. My heart sank, was it from the ABS pump? Having come this far, and having not fitted it back in the car I decided it was better to take apart now and double check, which i did, and all was in order. I think the strip down and reassembly was done in 30 mins only this time round.
I later found out when I used my 3/8ths socket set it was the ball bearing from the hex socket. Grrrrr
