1991 300SL ABS relays

[Garista]

Thought it better to start a new thread in electrical. I have the ABS light on problem and have been diagnosing it.
I am now testing the two black cube relays atop the hydraulic unit after cleaning the innards of it, it was dirty, and bleeding the system, twice. I have found that these relays have no resistance between PIN 85 and 86 on either of them, other checks are ok, they click and switch power. I think this is wrong but not certain, can anyone confirm?
One is expensive the other not so much. Thanks!

 

[Garista]

Found it was a loose or bad connection in the instrument cluster, I think I have fixed it in the process of trying to take it out and putting it back in properly, it seems to have remade the connection and the abs warning light has gone off. I have bled the brakes a lot and cleaned the hydraulic unit and checked the carbon brushes in the abs motor, and all is good, still not sure about the relays but I took them to a shop and they seemed to think they were ok, and Mercedes dealer said the brakes system was fine. So I just have to convince the CT person it is fixed.

 

[Richard Moakes]

Found it was a loose or bad connection in the instrument cluster, I think I have fixed it in the process of trying to take it out and putting it back in properly, it seems to have remade the connection and the abs warning light has gone off. I have bled the brakes a lot and cleaned the hydraulic unit and checked the carbon brushes in the abs motor, and all is good, still not sure about the relays but I took them to a shop and they seemed to think they were ok, and Mercedes dealer said the brakes system was fine. So I just have to convince the CT person it is fixed.

If the ABS light comes on and then goes out, I would hope the CT would pass

 

[Garista]

Well I now have the abs and asr lights on because I renewed the bulbs in the cluster.
I got out the hydraulic unit again and it is on the bench.
I cleaned it all, the relays are good. It was a little dirty.
I want to test the switchover solenoid A7/3y5. I have separated it from the unit. I think it might be bad. It has 2 electrical wires, yellow and black. Any idea how to test? Or are they just sensing the pressure?

 

[mattkh]

I admire your expertise.
Why are you ruling out the speed sensors and the brake light switch,,??

 

[Garista]

I admire your expertise.
Why are you ruling out the speed sensors and the brake light switch,,??

I replaced the two front wheel sensors with new, and the brake light switch, I followed the good advice on this site.
I have not touched the rear ones though.
The Star diagnosis thingy says it is probably either the OVP relay or this solenoid if the lights come on and stay on until ignition off. I have checked the OVP, I have two, one newish and it seems to work fine. The car drives with no problems.
I wish I was an expert! But I am certainly getting to know this car.

 

[Garista]

I just cannot find a switchover solenoid for sale separately. It seems a shame and expensive to replace the entire hydraulic unit. The solenoids are obviously meant to be able to be replaced, the top of the hydraulic bloc can be taken off to get to it without too much trouble, you need to drain the brake fluid down to about a centimeter from the bottom.

 

[mattkh]

Forgive my ignorance, the relay on top of the ABS unit must be for the ABS motor.
BUT why are there 2 relays on it.??

 

[Garista]

Forgive my ignorance, the relay on top of the ABS unit must be for the ABS motor.
BUT why are there 2 relays on it.??

I think one is for the return of the fluid to the reservoir, the other I forget at the moment... something necessary...

 

[Garista]

So now I am trying to check the rear sensors, got the car on ramps, found them up near the rear diff. Not easy to get to but possible. I cannot see any way to undo them, the connectors are round and I cannot see any holes for hex socket etc. Any ideas? Maybe they need a clean.

 

[Garista]

Looking at the ones available online they must have a torx nut I guess but I can't feel it.

 

[Garista]

Ok I could get to the bolts just about, but it is too difficult to get enough leverage on them. Access is very very tight. So I just hope it is not them!
I am again suspecting the OVP relays and have ordered a new one. Other than that it will need a new abs hydraulic block I guess, or a computer.

 

[Garista]

The guy who sold it to me kept jump starting the car. Do Not Do That! Recharge the battery, preferably with it disconnected, or you risk at best blowing the fuse of the OVP, or destroying the OVP, the solenoid, or even worse the computers. All of which are not cheap.

 

[Garista]

OK an update. Been working on the SL, fixed other things, but been thinking about this. I put new dash warning lights in and the ASR lamp came on as well as the ABS. So maybe it came on first, and this is the problem. Also, when I open the SP port on the ABS hydraulic unit, nothing comes out (car running), and I do not hear the ASR pump working. Well maybe my problem is this pump, as I think Richard suggested already.

On my LHD car it is not easy to get at, situated under the brake fluid reservoir and the master cylinder, which both have to come out to get to it. So I guess more brake bleeding is in store for me as well if I do this job. I notice this area of the car has had the most share of small problems, so maybe was stood in the sun there for a while or something like that. Anyone done this before on a LHD 300 SL r129?

 

[Garista]

So took out the precharge ASR pump; it was not so hard but had to take off the master cylinder and reservoir and it is very tight especially trying not to bend the brake lines. Took the pump apart and cleaned it, maybe it seemed a bit stiff, it was in good condition otherwise and I bench tested it and it ran fine. So I put it all back, with everything cleaned.

I have to bleed the brakes again although I didn't lose much fluid, Warning lights still on.

One thing I found was the positive supply wire to the unit has continuity to ground when the car is off, when ignition is on it has about 8 volts, does anyone know if that is correct? Seems wrong to me but I dunno. Does it mean it is shorted to ground? The negative side has nothing going on.

 

[Richard Moakes]

I know the precharge pump runs when there is low pressure in the accumulator cylinder near the ABS block, if memory serves, I had the ignition on, opened the SP port, some fluid flowed out under pressure and then the precharge pump kicked in and there was a steady fluid flow until I closed the SP port, the pump stopped shortly afterwards.

 

[Garista]

I know the precharge pump runs when there is low pressure in the accumulator cylinder near the ABS block, if memory serves, I had the ignition on, opened the SP port, some fluid flowed out under pressure and then the precharge pump kicked in and there was a steady fluid flow until I closed the SP port, the pump stopped shortly afterwards.

Yeah, that doesn't happen for me. So I reckon I've got a short circuit in the wire powering it. Maybe it is draining the battery as well, it seems to go down too much. Tomorrow I will investigate further! Perhaps the wire goes to the ABS computer.

 

[Richard Moakes]

Hopefully some help….

 

[Garista]

Thanks Richard.

Looking at the wiring to the ASR pump, I have exactly the same type of connector to it and to the brake reservoir, the reservoir connector has the ground labelled on the connector as pin 1, and the other is pin 2, Number 1 has continuity to ground. I am assuming the reservoir is wired correctly and is how it should be with pin 1 always ground.
The connector for the ASR precharge pump is the other way round though;, wire 2 is grounded while wire 1 reads as nothing. But when I run the car, wire 2 has voltage, but a bit low, but the other still has nothing.
I'm quite confused, ha ha. Has it been wired wrong, or is there a short? Or is this correct? I know the pump works fine out of the car.
If the wires in the connector were reversed by mistake, wouldn't there be power on the other wire, number 1 when the car is running??
At the moment the minus side of the pump is not being grounded by the wire to it, on pin 1. Maybe the computer turns off the power to the pump when it senses something wrong so I get no power on the other wire pin 2?
You can't plug the connectors in wrong because a plastic stopper prevents this, but you can open them up.

 

[mattkh]

You can't plug the connectors in wrong because a plastic stopper prevents this, but you can open them up.

Does the very original thread have any pictures.
If not, then pictures showing the colors of the wires to the connectors will be a help. Thanks.