A/C, ABS LIGHT & Tacho probs?

[cifp48]

Greetings,

This is my first posting on this forum, Chris told me about this site, I am very excited to come accross it and very thankful to Chris! Chris, remember me?? from Houston,TX?? about 300SDL??


I have just recently aquired a 1987 300 SDL with 96k original miles, 1 owner car, with all services from MB dealer and records in the glove! I think I was lucky until I get to this situation. This car was bought when it had a small damage on its Drivers side fender which damaged the spindle, both the fender and spindle were replaced. Its starts pretty good, and I wanna say it runs ok too, but I have my ABS light on constantly, the Tacho does not work and the A/C does not blow cold air ( does blow hot air, Freon is good), after my reasearch on the forums and under my hood, I noticed there were two relays missing, I figured out they were OVP and Refrigment compresser & cut off relay (both next to each other between the 2 firewalls). Installed a brand new OVP and a used compressor relay, now the tacho works intermitantly and the ABS goes off with it, I have noticed that both these work sometimes when I go over a pot hole of there is a sudden jerk due to uneven road or so. So sometimes when I am driving it, or stopped at a light, the Tacho comes on and the engine kinda revvs a little. What could be causing it?? BUT even when the tacho ABS work, the A/C does not blow cold air?? Have a look at this pic below.

Any suggestion would be very appreciated. Sorry about all the details, its just I believe, dignosing a problem is a bigger problem!!

Thanks
Mehdi

1987 300SDL 96k

[sixto]

The Klima relay and OVP certainly play a role in the problems you're describing. I don't know why those relays would be missing from a car that has seen proper maintenance. First thing I'd do is make sure there is a good fuse in the OVP relay, then clean the sockets and pins of both relays. It should be easy to take a wire brush to the pins. I don't know how to clean the sockets. Maybe spray Caig Deoxit into the sockets. Hopefully that does it.

The reason for the AC compressor not coming on could be the used Klima relay. Maybe it's one that doesn't work with Diesels or there's an tired solder joint or two.

Tracing a loose wire in general is not easy. A really wild guess is that the voltage regulator brushes are worn and driving over bumps compromises the contacts. But if that were the case, you're have more problems than you describe when the power goes down. You'd probably see more warning lights than you see now.

Sixto
95 S420
87 300SDL

... and I'll say it one more time, an 87 SDL will not leave you stranded because of an engine electronics failure.

[cifp48]

Thanks so much for your reply. Finally somebody replied!!

Well, the reason why the relays were missing were that this car came from an auction and you know how it is, when a car is at the auction, people steal stuff from emm either for their use or so that the vehicle does not sell at a good price!!!

The vehicle was hit on the left fender, it damaged the spindle. both the fender and spindle were changed.

The OVP is brand new from Merc. The refrigrant compressor cutoff relaye (next to OVP) was bought as a used one. But it also came from a 87 300D. The problem is, the A/C goes on automatically when the car starts but it blow hot air, not really hot but not cold either, and when I change it to hot, it does not blow any hotter air! We check for freon and added some 134 to the R12, so freon is good. But you see the pully on the compressor does not run. How would you suggest I trouble shoot and track the problem??

About the intermittant working of Tacho and ABS light, this only comes on when either the car is cold on the morning just after I start, it might hicup a couple of times when the Tacho works for a few seconds and the ABS goes off!! but then through out the day, it does not work unless if I go over a pot hole, not surprissingly, it only happens when the left wheel goes in a pothole not the right??!!! the same one that was hit.

BTW, is it safe for me to drive the car when the RPM does not work and the ABS light?? I dont mean ABS wise, I mean engine wise. coz the engine idles a lill better when the ABS goes off and the tacho works!!

My main problem right now is to trouble shoot the air conditioning system, boy its hot in Houston!! I took it to this Merc guy and he said the compressor needs to be checked?? that kinda does not make sense, keeping in mind the car has only 96k miles ORIGINAL.

Oh boy, this is a long posting!!!

Would appreciate your comments.

Thanks
Mehdi

Since you are seeing the problem when hitting a bump, it just has to be a bad connection or shorting wire somewhere. Unlike other make cars, I've never seen a chafed wire on an MB. They do a good job running wires through bulkheads and such. That does not mean that there could not be a wiring problem in the middle of a bundle, but it is highly unlikely from my limited experience.

This leaves connections as sixto pointed out. Check where the wire is crimped on lugs. If you can have somebody watch the instrument while you wiggle connections or tap on the relays, you might find the problem.

This might be a really tough one to track down.

Best of luck,
Larry

[stevebfl]

Maybe not so hard. Engine rpms are critical to the Klima relay. Engine rpms are criticla to tach operation. Engine rpms are critical to ABS and posibly un noticed, engine rpms are critical to idle speed stabilization.

The unusual thing is that diesels have no ignition so how does one get engine rpms? Turns out the OVP is critical here because it powers the EGR transducer which takes the crank sensor inducted AC pulse and converts it to a square wave DC rpm (engine speed) signal. Fix the rpm problem and you have probably fixed everything.

I would scope the induced crank signal versus eng speed signal where ever it don't work and work back to the EGR transducer or the crank sensor.

[sixto]

You added R134 to an R12 system?

Sixto
95 S420
87 300SDL

[cifp48]

Thank you all for your messages.

Yes Sixto, I did add R134 to R12?? would U recommend drain the R12 and fill with fresh R134??

Thanks
Mehdi