EGR ECU question - PeachParts Mercedes-Benz Forum

dcraigk · #1 06-07-2017, 07:13 PM

New to me 300CD. It came to me w/o a working tach. The pink label ECU was in the car when I got it (not the one in the picture). I've tried 2 other used pink label ECUs and neither make the tach work. Today I tried the off white one and still no tach.

The tach will work if I jump the wires on the ECU plug.

Both the pink and off white ECUs are reported to be for a 1985 300D/CD/TD. I can't find any info on the internet that tells me the difference between the two.

Mxfrank · #2 06-07-2017, 07:31 PM

You likely have a bad overvoltage protection relay. It's a tall can with a fuse on top, probably near that computer. Ignore the fuse, ignore advice about resoldering the board. Just replace it.

dcraigk · #3 06-07-2017, 07:37 PM

Thanks but I already tried a new KAE OVP.

Rogviler · #4 06-07-2017, 08:10 PM

Why don't you want to just jumper the tach wires permanently? It's what I did when I found the connector full of water. But my EGR stuff is non-functional. Not sure how it relates if you wanted to keep it all.

-Rog

dcraigk · #5 06-07-2017, 08:32 PM

My first choice is to keep it all as it should be. I might have to go back to the jumper wires.
What happens to all the other things that go through that plug?

Mxfrank · #6 06-07-2017, 09:42 PM

The first thing I wiuld do is check whether ir not the computer's other functions are working. The easy way to do this is to T a vacuum gauge into the EGR line. The EGR pod should see no vacuum at idle or at WOT. In between, vacuum should be proportional to engine load.

dcraigk · #7 06-08-2017, 10:21 AM

I'll check the EGR today. I did it yesterday but I didn't realize the car should be at operating temp.

I went and checked the ohms of the new engine speed sensor that attaches to the trans and it's not testing within spec. I'm not great with a multimeter but whatever ohm setting I used the reading wasn't close to what it should be.

86-300sdl · #8 06-08-2017, 10:52 AM

Quote:

Originally Posted by dcraigk

Thanks but I already tried a new KAE OVP.

I've had 2 new KAE brand OVP units be dead out of the box. Third has been working like a charm for two years now.

dcraigk · #9 06-08-2017, 11:14 AM

Is there a test for the OVP? All I see in the service manual is when the ignition is powered, pin 15 sends power to the ECU.

For the speed sensor, the manual says to separate clutch (connection?) and test the resistance with a multimeter. The reading should be 1.9 = or - .2 ohms. If not renew.

I'm getting .57 from the new one. Someone in another thread suggested they thought the sensor can be adjusted, but I don't see how and the manual doesn't say anything about adjusting.