W124 - a four problem puzzler!

[Keith]

Hi
I've been grappling with 4 problems which I thought were the result of a u/s OVP relay. I have now tested the relay and its AOK, so I am now stumped.

Can any of you guru's tell me whether these are related issues and if so what should I suspect if the OVP is not the problem.

Here we go:
Car is 1995 E300 Diesel which I have owned for 5 years:

Problems
1. No cold air - has been like this for over 12 months, have recharged twice with no benefit

2. Tacho is erratic (always has been) and lately has stopped working altogether unless i accelerate hard when it springs to life briefly

3. Engine runs badly after starting from cold this winter but is fine when warmed up and restarts perfectly. (have recently had all the glow plugs checked and all are AOK)

4. Cruise control is erratic (always has been) , sometimes works sometimes doesnt but not related to the tacho working or not if you see what I mean.

Work done:
glow plugs - tested , one replaced all now AOK
OVP relay - fully tested and AOK

I am sure there is a connection here, can anyone point me in a new direction , I had thought the OVP would be the issue but seems not.

would really, really appreciate any pointers
cheers
Keith

 

[Myros]

OVP only

I thought one of the buggy bits about the OVP was that it looked and felt fine if you tested it, but still weaseled away and dropped you in it when it felt like it. Not being any more electronically advanced than when I left school, I went for the good advice of changing mine for a new, not fitted with a fault at the factory version. £47 well spent, and many thanks to Mjtray.

 

[Keith]

OVP , tach amp or Klima at fault?

thanks for the advice. Maybe I should do the same but I'd rather be sure. I took a look at the KLIma relay as this also uses the tach signal and compares compressor signal as well as powering up the compressor. I bridged the compressor output to +12 at the Klima with it removed and bingo the compressor comes alive so obviously something is stopping the Klima doing this. I guess it could still be one of the following:

OVP (bench tested ok)
Tach signal
Klima

Problem is it could be any of these if I replace them all its going to be £££'s.

Can anyone tell me how to diagnose which it is without replacing all of them.
I only have a multimeter by the way in terms of diagnostic tools.

thanks
Keith

 

[turnipsock]

There seems to be three versions of the Over Voltage protection relay kicking about. 5, 7 and 9 pins.

There is a fuse built in as well.

The pricipal seems to be based around a 22V zenor diode that cuts out a relay if the voltage gets to high.

I'm not 100% on what this unit is supposed to do but it looks a two minute job to replace this. You could always try one out of another car just to see...

How was the OVP tested?

 

[Keith]

tested using a procedure from a MBClub magasine

I found a test procedure in another forum which had been published in a MB club magasine , not sure if in US or where . But it just used 12v on certain pins and seeing if the relays flicked over at the right time which they did.

Also I dont have any other electrical misbehaviour other than A/c and tacho itself. from reading other posts it seems that you get other things like ABS warning lights etc if you have an OVP problem.

I am going to make another post asking for help with the tacho sender and follow the trail from there because I think 2 other relays Klima and EGR could also be culprits so starting with the tacho sender hopefully I can find the right on.

Unfortunately dont have easy access to another MB to swop parts having sold my R129.

thanks for responding
Keith

 

[turnipsock]

My thoughts about the testing were concerns about the quality of the relay contacts. If you check these with a meter, there is is no appreciatable current running through them and would show up as working correctly.

The Over Voltage Relay seems to cut the power to all the modules when the key is turned off.

 

[Keith]

good point

good thought turnipstock will bear that in mind if I start drawing blanks on the other components - OVP will remain on the suspect list!

k

 

[graham fawkner]

Borrow my old OVPs

After a similar nightmare with my 1991 200TE I fitted a s/h ovp. This changed the characteristics of the fault quite markedly but confirmed I was on the right track. Purchased and fitted a new one which has 99% solved the problem. I now have two surplus dodgy OVPs which any one is welcome to borrow if it will help. They are both Part No 201-540-32-45 and have 7 pins one dated 1990 and the other 1992. I believe the 1992 model to have less faults. My new one has a different Part.No.and 9 pins.They are easily posted. Graham.

 

[Keith]

positive progress!

Guys - thanks for your input and Graham thanks for the offer of OVP loan.

In the process of searching for the RPM sensor and disturbing the wiring it now seems to work AOK and starting difficulty has gone away. I am pretty sure it was the sensor or the wiring ( runs in a pretty hostile invironment behind the block). If I get a recurrance of the starting problem I will get the sensor out and clean/rewire it/replace it.

Thanks for the loan of the dodgy OVPs, if I have further issues I will probably replace it anyway since it seems most of them go and mine is 10 years old.

You may be intrested that my aircon problem I traced all the way back to lack of ground signal from the PBU. this is strange since all the other functionality of the PBU is OK so it must have ground itself. I took it apart but wasnt up for unsoldering and resoldering it so I am now on a hunt for a guaranteed used one. In the meantime I have installed a switch that puts the ground signal onto the correct wire from the PBU so I have guaranteed a/c if the PBU wont play ball. It works - cold air .

thanks for your input, any ideas on whats up with the PBU or any DIY repair would be welcome.

Keith