How do I test the EGR relay?
 

On the 85 300D with a non working tach, I am trying to test the EGR relay. The fuse is not blown, but I am unsure of how to actually test the relay it is part number 126 5404745.

Anyone?

If you attach a vacuum gauge to the vacuum line going to the EGR valve and Rev-up the engine to approximately 1,400 RPM you should get a vacuum reading of around 20. At idle it should be zero. If you get the above results, you can safely assume that your EGR relay is working correctly.

What are you trying to fix?


Phil

I am trying to figure the procedure to test the EGR relay inside the car. The 1985 model has an electrical relay that is used in conjunction with the EGR computer.

The actual EGR is not the problem.

The system not working makes the tachometer and A/C not function.

The procedure that I outlined DOES NOT test the EGR valve itself, but rather determines whether or not the EGR relay is working correctly.

I do have the procedure for performing a functional and diagnostic test of the whole EGR system, including the associated relays, and components but its only applicable to my '95 E300 Diesel. I subscribe to "ALL DATA", but I can only get the procedure for my specific model. It may be worth while for you to subscribe as well. Its around $20.00 per year. I find it extremely worth while. The Mercedes CD is another alternative.

I am not sure if you have a KILMA relay, but if you do, than this will be a common point for the symptom that you describe. Another possibility is the engine speed sensor. (may need to be cleaned) or defective wiring. The problem is unlikely to be the EGR relay itself.

Phil

I had someone test the acutal relay and it is dead. I took it apart and the coil is not closing the switch. So I manually closed the switch with a piece of plastic to see the effects. It does operate the vacuum on the egr when this is done, but does not still make the tachometer work. :(

As I understand it, the tach must give a signal to the klima relay and then the relay will give power to the ac compressor lead.

Replaced the relay and the tach works now.