85 300D Tach Issues - PeachParts Mercedes-Benz Forum

rmmagow · #1 07-01-2013, 11:10 AM

I am going to quote a message one of our members sent to another related post:

[QUOTE=rcounts;1833709]For those wondering about the 84 and 85 models and the tach amp, my 300 CDT (production date 01/84 - must've been one of the first one off the line that year) there is NO tach amp under the cover on the assembly mounted inside the DS fender. If you remove the cover and look at the sockets, the #3 socket (they are numbered) doesn't have the metal female pin in it.

On this style the tach gets its signal from a different sensor mounted at the BACK of the engine in the bellhousing of the transmission - just behind and below the bottom of the oil filter assembly. The sensor is held in place by an allen-head screw and it takes a 5mm allen key to remove it.

My sensor is bad and needs to be replaced. You can test it by unplugging the 2-wire cable connector at the center of the hood opening just in front of the cowl. If you follow the wire from the sensor up to the connector it is easy to find.

I ordered and installed a new sensor wire to no great change. I used an OHM meter set to 2K to read the resistance in the new sensor and got a digital reading of 573, the OHM reading on the old sensor is 1900 +- some. With the engine running I can't get any kind of voltage reading with the old or new sensor installed. The OHM reading (1900) on the old sensor was made with the sensor installed, the reading on the new sensor wire was with the wire not installed, just a test out of the box.
I know it is a pulsed signal from the flywheel, something akin to an ABS sensor type pick up right? I am pretty sure my tach issues are related to not getting a signal from the sensor. Tach used to work, then intermittently worked, then failed. Are there different versions of this sensor wire? Am I using the OHM meter correctly? Set to lowest DC volt setting, I never see anything with the engine running. Did I maybe receive a bad-out-of-the-box sensor wire (BERU)?
Thanks Much

funola · #2 07-01-2013, 11:36 AM

if by sensor you mean the one near the harmonic balancer, then the resistance should be around 80 ohms.

jay_bob · #3 07-01-2013, 11:50 AM

The sensors used for the tach (and ABS) are what is known as a variable reluctance sensor. There is a coil of wire and a metal core inside. When the coil is excited with a low power ac signal, a varying magnetic field is created.

In free air, the magnetic field is constant and does not vary. Bringing a ferrous object near the magnetic field causes it to change. This change can be detected by the excitation circuit. It's kind of like throwing a rock into a pond.

BTW this is how the sensor at a traffic light or parking gate works that senses the presence of your car. On the car, the ferrous object is the pin or tooth on the circumference of the wheel (harmonic balancer, flywheel, hub assembly, differential gear, etc). The traffic sensor just looks for an entire car.

Therefore, reading the sensor on ohms will only give you the dc resistance of the coil, which may or may not give you a good indication of coil health. Also attempting to read dc volts while the sensor is being actuated won't tell you anything because the coil is not being excited.

About the only meaningful test is to monitor the function of the coil with an oscilloscope while it is runnning.

rmmagow · #4 07-01-2013, 12:09 PM

I found a boat-load more info about the Tach testing procedures. I think if I set my meter to AC Volts it should see about 4V with the engine running, I was using the DC setting. My OVP fuse is OK, don't know about the OVP itself. I found the small door for getting to the EGR computer but I think I have to remove the entrie panel to access the connector to check for corrosion. AFIK my A/C is not functional at this time nor has it been for a few years. I might be getting close to a solution. This may well explain my odd-ball Cruise behavior, accelerate to 60, hit cruise, car continues to 65 mph then stays there correctly.

mach4 · #5 07-01-2013, 12:12 PM

The sensor is probably the last item that would go bad.

Here are other items to check. The VR sensor feeds an A/C signal into the EGR controller on your car. The EGR controller they sends a signal to the tach module in the cluster. The EGR controller gets its power from the OVP (Over Voltage Protection) Relay.

The very first thing to check would be the fuse on the OVP. Then you can pull the plug off the EGR controller and check for voltage on pin 1 (see image) If you've got good voltage there, you can turn your attention to the other components, which are not that easy to check. Check the plug on the EGR controller - some have reported corrosion there that prevented the component from working. You can check for continuity of the VR sensor and wiring by checking pins 16 and 20. To check the tach module itself, you'd almost need to source one at the junk yard or borrow a cluster from a known working car. Same for the EGR controller.

To test the sensor you'd almost need a scope. Theoretically you could rig up a trigger wheel and run it against the system, but you've got a technical problem and that is that the sensor takes its signal off the flywheel teeth - that's a lot of cycles per second to try to emulate with a make shift trigger wheel. (I rigged up this kind of a system when I was trying to set up my speedometer, but the speedometer only uses a 4 tooth wheel to it was a lot simpler)

Not saying it couldn't be the sensor, just that they almost never fail - no moving parts; no electronics; no power; nothing to wear, just a coil and a wire and a an in-line connector.

funola · #6 07-01-2013, 01:09 PM

the resistance test is a good test. it tells you whether the sensor is open circuit or shorted or the wrong one. not sure about 1985 but for 1983 the sensor is 80 ohns.

I did a lot of work with the 617 tachs, including un potting a few bad tach amps and repairing them.

if you get 80 ohms on the sensor I'd say it is 99 percent good. most here don't have an oscilloscope. you can take a DVM, put it on the most sensitive AC scale (milli or micro volts), connect it to the sensor, rev the engine and you should see the voltage going up.

edit: you're not going to see 4 volts out of the sensor. its been awhile but I recall it somewhere around 100 to 300 milli volts depending on the setting of the sensor to trigger gap.

yuke · #7 07-01-2013, 03:49 PM

Eliminate a potential problem and replace your OVP relay it is the one that has a fuse in the top of it in fuse box on my 85 300SD it fooled me in that the fuse was good but the relay was bad after replacing the relay not that expensive viola tach worked and continues working to this day.