85 123 chassie 300d Califonia version, Tachometer does not work - PeachParts Mercedes-Benz Forum

bergmjh · #1 10-28-2009, 05:54 PM

I am trying to bring a tachometer back to life and not having much luck. As I understand the working of this tachometer from other articles in this forum the rear crankshaft speed sensor sends a signal to the EGR control module. The EGR control module sends a signal to the gauge in the dash.
I did find a bad fuse in the over voltage protection relay that provides power the the EGR control module. No luck when the fuse was replaced. I also tested the relay and I do get 12V out on one terminal with the ignition turned on. I believe I have a bad sensor or the EGR computer is bad? I did get a 1800 ohm reading on the rear crankshaft sensor.
1. Does anyone know the proper resistance value for the sensor?
2. How do you check for a bad EGR computer?
Thanks John

ImBroke · #2 10-28-2009, 06:18 PM

I would disconnect the EGR computer, clean the contacts, lube with dielectric grease and reinstall. The contacts can corrode after time and not get good contact. I have two 85's, one Fed and one Cali. It's dark now, so I can't get you the reading you want. Perhaps tomorrow.

Jeremy5848 · #3 10-28-2009, 06:26 PM

It sounds like you have done your homework, thank you. The bad fuse suggests that something (a voltage spike) happened that increased the voltage across the Zener diode in the OVP, thus blowing the fuse. If the new fuse did not blow, then the problem may have been transitory.

The OVP is supposed to have ground on pin 5. Pin 6 applies 12 volts to the relay, pulling it in and sending voltage to the EGR module via pin 2 and to other loads via pin 1. If OVP is good, with the ignition on (and maybe not even with the engine running) you should have 12 volts on pins 1, 2, and 6 and ground on pin 5. If you do not have 12 volts on pin 2 then the EGR module will not work and the tach will get no signal. If so, you could have a bad relay coil or corroded/burned contacts in the OVP, which can be cleaned. An ohmmeter should register a few hundred ohms across pins 5 and 6, which are the relay coil connections. If you put 12 volts across those pins (+12 to pin 6 and ground to pin 5) you should hear the relay "click."

The only way to test the EGR module is to replace it, unless you happen to have the antique test gear that Mercedes designed for it. I don't know if dealers have that anymore.

The rpm sensor has a factory-new resistance of 1900 +/- 200 ohms so yours is apparently good. The output of the sensor with the engine idling is supposed to be 4 volts AC or more (increases with engine rpm). That is measured at the two-pin connector on the firewall near the transmission dipstick. There are two connectors held onto the firewall with plastic clips. The one you want is the smaller of the two with two pins.

Jeremy

DeliveryValve · #4 10-28-2009, 06:37 PM

This might help along with Jeremy's info.

Wiring diagram.

RPM sensor test.

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bergmjh · #5 10-28-2009, 07:06 PM

The over voltage protection relay was the culprit Thanks for the help. John

ImBroke · #6 10-29-2009, 10:16 AM

Subscribing for those diagrams.