1991 300SL ABS relays

[Garista]

Figured this out from the web:

ASR II system checks:

The most common cause of power loss in these systems is a blown fuse. If you have checked all the fuses and they are all good, then you need to look for a loose one or a bad connection or break in the wiring. Another possible cause of power loss is a faulty relay.

The throttle control components of the ASR II system = the throttle potentiometer (R25), the throttle drive motor (M16) that communicates with the Electronic Accelerator/Cruise Control/Idle Speed Control (EA/CC/ISC) and the ABS/ASR control module, it also has the wheel speed sensors.

R25 proper 7.2V reference voltage. Specs: 6.5V at closed throttle, decreasing to 0.70V when at WOT position.

The safety switch in the actuator changes at 5.9V.
Safety switch in the pedal position sensor correct control voltage at about 6.2V.

Test the power feed at the ABS control module from the over-voltage protection relay (OVPR). You need to see less than 0.3v drop from battery voltage on pin 1 of the ABS/ASR control module.

This is what I am up to at the moment! When it stops thundering and hailing.

 

[Garista]

So, thinking about the potentiometer that is (?) connected to the throttle cable which the ASR uses for signal. Apparently, if it gets too much resistance at idle, the ASR light comes on. A lot of the electrical stuff around this area, the ASR pump, the wiper motor, were pretty furred up, so maybe it needs cleaning? Car has no limp mode though and no problems accelerating at all, very smooth.
Mine looks different to those on Youtube I can find. It has quite a few connectors on it.


I am kind of a little worried about disconnecting it all. I guess just take that pin out? Will I face any terrible problems putting it all back?
Another question is, are there therefore two potentiometers on this engine, this one, plus the one on the throttle body? Or is this just a mechanical thing?

 

[Garista]

Yeah another question I have is does the tempomat need the ABS system to work? If so my ABS is working. I notice it slows me on downhill if I have set the speed slower, this is by the ABS braking, no?

 

[Garista]

The thing in the last pic is talked about on German R129 websites, a bit. The car was not sold in US or France.
So I am trawling and translating them. It might be the problem.

 

[Garista]

OK I took off the ASR potentiometer and checked it. It was clean and good. I lost a spring in the process, where could it have gone, I dunno. Just disappeared. Car is fine still though.
I got some advice from a garage about another garage who is good with electrics, so maybe I will go there. I feel it's something simple that I'm missing as I am an idiot like that sometimes.

 

[Garista]

Anyone know if the ASR brake fluid accumulator pressure regulator has electrical connections to test, it doesn't look like it.

 

[Richard Moakes]

Anyone know if the ASR brake fluid accumulator pressure regulator has electrical connections to test, it doesn't look like it.

No, it’s purely mechanical, the pressure sensor is in the ABS block

 

[Garista]

No, it’s purely mechanical, the pressure sensor is in the ABS block

Thanks Richard.

Well, it seems ok.

I've been working on the ABS/ASR wiring. Testing the loom from the ABS hydraulic unit plug to the ABS ECU plug and the Jetronic ASR plug.
Most have continuity, wire 10 from ABS plug goes to both ECUs, but wires 6 and 13 do not have continuity to either plug. I am wondering if they go elsewhere? I'm trying to trace them. Might be the problem? I now need to know what colours they are as I can't tell from the plugs or looking at the wires unless I take apart the plug.

One other thing, I found the Jetronic ECU plug has one missing tooth. The pin is present on the Jet ECU. It does not look broken off, is this correct, a disused pin hole? I've been looking for images but no luck so far.

Thanks again

 

[Garista]

As far as I can make out, wires 13 and 6 don't go to the ECUs but to the fuse box, then warning lamps, so I would not get continuity on them anyway I suppose. The fuse is ok. But not sure which one it is, possibly 6, I can't read the diagram. Fuse box label does not refer to brakes or ABS.

 

[Garista]

As far as I can make out, wires 13 and 6 don't go to the ECUs but to the fuse box, then warning lamps, so I would not get continuity on them anyway I suppose. The fuse is ok. But not sure which one it is, possibly 6, I can't read the diagram. Fuse box label does not refer to brakes or ABS.

Yeah fuse six has continuity from plug wires 6 and 13. So it checks out ok.

 

[Garista]

So, the only weirdish thing is the missing pin in the plug to the KE Jet ECU.
Does anyone know if this is correct or not?
It is the second from end pin in the shorter row of twelve pins.

 

[Garista]

So, the only weirdish thing is the missing pin in the plug to the KE Jet ECU.
Does anyone know if this is correct or not?
It is the second from end pin in the shorter row of twelve pins.

I think it might be pin 15 "Circuit 30a from Overvoltage relay (Calif.) Check Engine Light (Federal)" but that is US and a different model diagram. But it is 15.
This is German car so...
I think if pins are not used they usually still have connectors in them. ???

 

[Garista]

Maybe it is normal, some plugs have no contacts in them I suppose.

Annoying. Star diagnosis starts its ASR troubleshoot section with 129 061, mine is 129 060, I've been wondering if this is the way the car was built, it seems to be before ASR was introduced? Or I dunno.
.

 

[Garista]

Next need to test wiring from OVP to ABS ECU, and the connector to the ECU for poor connection. According to Star diagnosis.

 

[Garista]

Unfortunately the Star Diagnosis starts with M104 engines and does not refer to M103.

 

[Garista]

The voltage to the OVP plug is 12,42, so not bad. Going to test from OVP to ABS ECU.
The Star diagnosis says if both lights come on and stay on, then it is either the OVP, which is new, or the switchover solenoid valve in the hydraulic block. But this is another block. So must be wires somewhere.
On ABS light alone it refers to Circuit 61 which is the generator (alternator), so I cleaned those connections. I might change the brushes. Maybe it is not charging enough.

 

[Garista]

Red wires from the ABS/ASR Hydraulic Unit solenoid valve relay (the 6 pin one) go direct to the fuse box, fuse 6, then to both warning lamps, then from these lamps wires return to the HU relay, via plug pin 8 and 10, but are joined to 2 wires coming direct from the ECU pins 17 and 29. The ECU is (I think) meant to switch off the lights once car is running by signal on this circuit.

The wires all seem good, have continuity and get good voltage. The OVP is good. The wires from HU relay direct to ECU I have already checked for continuity and are good.

So, the lights are being activated from the HU relay via fuse 6 and 2 red wires.

But I don't know how to check the other wires after the lights that return to the HU relay, they are GY/RD and BR/YL and which branch to the ECU. And I don't know how to check the ECU independently. But with it disconnected, then start car, there is no change (DO NOT disconnect while ignition is on!!).

And, I now have two sets of HU solenoid relays in good condition. I have tried both with no change. It is this relay that receives the signals back from the ECU for the ASR and ABS lights to switch off after ignition, I think.

So:

ECU could be bad,
both relays could be bad.
Or there is a wiring problem between the relay and the lights. But for it to be both lights on, two wires would need to be affected, which seems unlikely unless they branch off somewhere together. They have to both go through to the dash cluster.
I already checked these connections earlier I think!! (X 30 or something, a connector behind the ECUs) They were good.

Am working on this.
It seems to be pointing to the ECU.
Problem is, Mercedes dealer said it was all functioning ok. It can't without the ECU.
So maybe a voltage drop in this circuitry that just puts these lights on but leaves it working????? Seems unlikely.

 

[Garista]

Annoyingly the Mercedes diagnostic tests use a machine that goes between the plugs and the ECUs, so the test numbers they give are not much help (as far as I get it).
Anyway checked more wiring and it all seems ok, but there is a voltage drop from the battery to the ABS ECU of about 1 volt, which is not great.
Before buying another ECU or anything else, I will visit the specialist. I can't find anything wrong.
Anyway, I've cleaned most of the electrical system while I tested it all.

One thought is that it has stored in memory the fault but I don't know how to clear it safely. I have tried leaving the battery disconnected. The usual way is to drain the system fully by connecting the pos and neg leads to each other, with the battery disconnected of course. Should I try it?

 

[Garista]

OMG, I am an idiot.
I should have checked the voltage regullator on the alternator ages ago, but the battery tester said it was charging fine.
But I took it out just now. It seemed like it was the original one, very old, and the brushes are almost gone. I think this could be the cause, don't know for sure yet but the MB Star diagnostic does say something about circuit 51.
Anyway it has to be changed, not very expensive.
Ordering one. Will let you know.
I have booked it into a garage specialist end of June, maybe I can fix it before.

 

[Garista]

OK I got the voltage regulator and installed it and the lights are still on, but the battery drain has completely gone and the electrics are strong now, so some progress. The old regulator had one brush nearly gone and the other really low.

I am thinking that maybe the rear lights are wired wrong somehow, maybe the fog light. Looking for a wiring diagram for early LHD r129 rear lights.