Tach Work '85 300D - PeachParts Mercedes-Benz Forum

rmmagow · #1 10-06-2012, 05:48 PM

I'm aware of the wierdness of this system for 85. I disconnected the sensor wire connector at the firewall. I get OHMs but no apparent voltage with the engine running. I have removed and cleaned the sensor from the bellhousing to no avail. The rest, OVP, EGR Klima I think might be OK. Should I just replace this sensor wire first before getting concerned? Expensive little sucker, abou $60.00!

cullennewsom · #2 10-06-2012, 06:21 PM

Tach sensor gives a pulse, not a DC voltage.

Doktor Bert · #3 10-06-2012, 06:22 PM

Tach amp most likely culprit...

mach4 · #4 10-06-2012, 07:24 PM

Quote:

Originally Posted by Doktor Bert

Tach amp most likely culprit...

No tach amp on that engine. It gets it's signal off the transmission and the EGR controller feeds the gauge.

rmmagow · #5 10-07-2012, 10:20 AM

I know it's a pulse, but from my readings around the forum I thought that this pulse would cause a DV Volt Meter to read about 4V while the engine is running. Even with a pulse, it should make the meter at least twitch no? I have a digital VM and it just stayed flat at 0.

mach4 · #6 10-07-2012, 11:00 AM

Quote:

Originally Posted by rmmagow

I know it's a pulse, but from my readings around the forum I thought that this pulse would cause a DV Volt Meter to read about 4V while the engine is running. Even with a pulse, it should make the meter at least twitch no? I have a digital VM and it just stayed flat at 0.

In my opinion the sensor is the least likely to be the problem. It's extremely simple and tough. The components of the system, in the order I would troubleshoot, based on the probability of being the problem, are

1- OVP provides power to the EGR computer. Check the fuse, then check power on pin 1 of the EGR computer
2- EGR computer - the plug connector can get corroded causing poor or no connections. If the connection is good see if you can borrow a known good one for testing.
3- The gauge itself. Put a small voltage across the gauge and it should deflect. Alternately swap in a known good module for testing. You have to open the cluster but ultimately it's three screws and three wires you'll need to solder (+,-,GND)
4- Wiring. Trace all the wires for damage and do continuity checks where possible
5- The VR sensor on the transmission. Check it with a digital oscilloscope .. or just grab one off a junk yard vehicle.

Good luck

rmmagow · #7 10-07-2012, 08:06 PM

Quote:

Originally Posted by mach4

In my opinion the sensor is the least likely to be the problem. It's extremely simple and tough. The components of the system, in the order I would troubleshoot, based on the probability of being the problem, are

1- OVP provides power to the EGR computer. Check the fuse, then check power on pin 1 of the EGR computer
2- EGR computer - the plug connector can get corroded causing poor or no connections. If the connection is good see if you can borrow a known good one for testing.
3- The gauge itself. Put a small voltage across the gauge and it should deflect. Alternately swap in a known good module for testing. You have to open the cluster but ultimately it's three screws and three wires you'll need to solder (+,-,GND)
4- Wiring. Trace all the wires for damage and do continuity checks where possible
5- The VR sensor on the transmission. Check it with a digital oscilloscope .. or just grab one off a junk yard vehicle.

Good luck

1. I know where the OVP is in back of the glovebox. I'll put in a new fuse. The EGR. I found it in back of the access door. Do I need to remove that entire kick panel to access the connector? Power on pin 1. How is this checked? Everything connected then back probe to pin 1, VM on the back probe and ground the other side of the VM and read voltage?
2. The gauge will twitch once in a while. In fact, after I completed a brake job the other day (and I know this is unrelated) I started the car and the tach worked for about two to three minutes than shut off again. So I think gauge is OK.
4. Will attempt.
5. Where is and what is the VR sensor on the tranny? MB diesels are rare in my drive to junk yards but I'll see what I can do.

Thanks Much

my123ca · #8 10-07-2012, 10:26 PM

Make sure the OVP is plugged the correct way. It can be easily reversed if the guide isnt there anymore.

DeliveryValve · #9 10-07-2012, 10:28 PM

I don't know if you seen these diagrams I posted a while back..

Some links for good reading..
1985 300D CA Emissions Tachometer repair

1985 300 D Tach Problem

.

mach4 · #10 10-07-2012, 11:19 PM

Quote:

Originally Posted by rmmagow

1. I know where the OVP is in back of the glovebox. I'll put in a new fuse. The EGR. I found it in back of the access door. Do I need to remove that entire kick panel to access the connector? Power on pin 1. How is this checked? Everything connected then back probe to pin 1, VM on the back probe and ground the other side of the VM and read voltage?
2. The gauge will twitch once in a while. In fact, after I completed a brake job the other day (and I know this is unrelated) I started the car and the tach worked for about two to three minutes than shut off again. So I think gauge is OK.
4. Will attempt.
5. Where is and what is the VR sensor on the tranny? MB diesels are rare in my drive to junk yards but I'll see what I can do.

Thanks Much

[1] Yes you will need to pull the access panel to access the controller. Unplug the connector and put a Voltmeter on pin 1 as shown and the other connector to ground.

[2] I agree, and this points to a loose connection somewhere - I'd guess the connector to the EGR controller

[5] See image below

Good luck