'85 300TD tach issues (Don't worry I've searched the forum)
 

Hi all,
trying to get my tach working again. I know it's not vital but I'd really like to have it working - as I get my trans and engine dialed in I'd like to have that reference for myself and I have never really tried to tune any engine.

I've searched the forum A LOT, here is what I've done:
- pulled speed sensor and wiped clean
- checked voltage from speed sensor, checks out fine
- changed OVP relay with another from a PnP car. Mine was really rattling. Not sure the 'new' one is working
- OVP relay fuse (it was blown)
- Pulled connector off the EGR computer, cleaned the ***** out of the pins and pin connectors, and replaced the female connector on the bottom which looked the worst

Known issues:
- OVP relay. From a PnP car so can't really tell if it's working as it should
- There is one part of the instrument cluster circuit board that has burned through. Very minor, no damage to the board, however that burned through spot is behind temp, oil and fuel gauges which are all working fine.
- Lots of corrosion on the EGR computer pins and connectors. Nothing seemed to have penetrated the module itself but there was definitely moisture in the area

After all of that, when I am accelerating, the tach needle is all the way to the LEFT, when I let off the gas the needle swings all the way to the RIGHT. So clearly the gauge is getting some information, the speed sensor is putting off what it should, the strange gauge behavior is not what I would suspect from a bad OVP relay. All this seems to me to point to a bad EGR computer.

For what it's worth the previous owner disconnected the vacuum lines to the EGR, which I have not plugged back in. Not sure what it's worth.

Am I missing something? Anyone have a similar experience?

The system is pretty simple really. Here are the components

[1] Variable Reluctor sensor (82) - On the transmission (reads flywheel teeth)

[2] EGR Computer (80) - processes signal

[3] Tach (a) - signal to gauge

[4] OVP (85) = provides power to controller




http://www.peachparts.com/shopforum/...1&d=1336267768

You can't really test the VR sensor without a scope as it puts out A/C current. But these are generally bullet-proof - the least likely suspect. Just check for good wiring and make sure the sensor is solidly in place. (Actually there is a way to test the VR sensor - see below for a simple VR tester setup)

You can look for 12v on pin 1 to verify the OVP (or take power from an unconditioned source for testing).

You could pull a tach unit from the JY to test the actual gauge (or pull the unit and test it in a known good vehicle, if you have a friend with one) If those all test good, you're most likely point of failure is the controller.

Because of the corrosion you report on the contacts, I'd probably bet on that being the problem.

VR Tester

When I was working on my mechanical to electronic speedometer conversion I rigged up a VR tester, like this

http://www.peachparts.com/shopforum/...on-trans09.jpg

It was just a convenient hole saw with a number of nuts taped to the circumference. (I used 4 for the speedometer because there are 4 teeth in the trigger wheel at the rear of the transmission) Nuts aren't the best because they have a hole in it that could create the appearance to the system of two teeth. In your case I'd suggest using something solid with as many "teeth" as possible for the trigger wheel and about 1/2" high (the one on the flywheel - flywheel starter ring teeth actually) has around 50. Then to test, take the VR sensor out of the intermediate plate and hold it close to the make-shift trigger wheel "teeth" and watch the tach. You should see a smooth rise as you increase the speed on the drill (assuming everything else is functional).

... I'm picturing you using this tester...
"ok, hold the rpm steady, steady, a little closer... AHHHAHHHAHAHAHHAHHAHH!!!"

I chose not to use an angle grinder to "safety" the edge, but you bring up a good point and that is I should have issued a disclaimer - so consider the post "disclaimed". :)

Is the way to test the OVP relay to pull it, figure out which pin is pin 1, then push the relay back in leaving a gap for the multimeter probe?

Seems that assuming the relay passes that test I can narrow it down to the EGR controller or the gauge itself.

Since I got a good multimeter reading from the speed sensor, would using the hole saw of destruction test give me any new information? Guess it is worth trying before replacing the EGR controller.


Aside: Anyone know what #80 in the diagram is doing? Just says switchover valve - what is being switched over?

Here's a diagram of the connector I created to help in my swap

http://www.peachparts.com/shopforum/...ect-380-73.jpg

I would just pull the plug and apply a multimeter to pin 1 and see if you've got good voltage.


Here is diagram of the vacuum showing the #81 Switchover Valve, which applies vacuum to the Air Recirculating Valve.

http://www.peachparts.com/shopforum/...um-diagram.jpg

Those are two of the most helpful pictures I've seen. Thanks so much for taking the time to do that!

It should be around 12V, right? Will it be DC or AC? As I understand the speed sensor generates AC, which is why we need the OVP relay. It looks to me like power is getting to the EGR controller through the OVP relay.

One more wrinkle that just occurred to me - in the process of sorting all this out I have driven the car with the computer both in and out and can find no difference in how it runs. The trimming plug has something to do with idle speed, is that correct? Could this point to a bad EGR computer?

Wow those are great modifications to those diagrams I originally scanned and posted. Thanks for doing that!



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The OVP (Over Voltage Protector) relay is to prevent damage to the voltage-sensitive controller should a spike occur. So yes, it will be close to 12v DC. It is the source of power to the EGR controller. You could take a 12V source and hook it directly to pin 1 for testing purposes (a motorcycle battery would be a safe option)

The VR sensor will generate a wave form similar to the following, so it is an AC signal.

http://www.peachparts.com/shopforum/...1&d=1336321679

Yes, the trimming plug is a fine-idle adjustment. I don't recall a post on the forum where someone has even done this... I could be wrong though.

I think the controller is the most likely source of the problem, but also the most costly, so I would test each component to rule it out (however, you have to have a good controller to test the VR sensor or access to a scope)

Just checked some things out with a multimeter. I am still a little confused about AC and DC in this application so I checked values for both types. The female connector for the EGR computer is getting a steady 13V DC when the car is running. The AC value was .02-.03V AC. The connector that goes to the tach gives 6V DC, and 5-6V AC when the car is running. It's backwards though, at idle the voltage is about 6V AC, but when revving drops down to 5V AC. That seems to be the problem, though granted my limited understanding of how the system is utilizing voltage is apparent in this post.

Am I right to think it is actually the EGR controller at fault here? Anyone in Maine with an 85 and a working tach mind if I swap by to confirm?

Is there a way to just bypass the EGR controller entirely?

Don't get distracted by the AC thing. The only reason for mentioning it is that you really can't measure it - it's just a signal that goes positive to negative every time a tooth passes the VR sensor.

If you've got 13v from the OVP you're good there. If the connectors are good, you can assume that the EGR controller is getting power.

The voltage to the tach is kind of meaningless without a chart of values correlating voltage to RPM.

I'm leaning toward the EGR controller at this point, but we still don't know for sure if the VR is putting out or whether the tach itself is ok. Best course of action would be to find a known good EGR controller to swap in and test, or conversely, swap yours into a known good car. Either way you'd know. You mentioned corrosion on the connector, so you almost need to test both of those scenarios, because your controller could test good in another vehicle and still not work if the connectors are corroded. Same thing the known good controller could show bad because of corrosion (bad connectors).

There is a work around, and that is to get a tach amplifier from the junk yard and hook it up using the engine speed sensor on the front of the engine. You'd need tach electronics from an earlier vehicle as well as the output is different. I picked one up when I thought I might have to go that route. However, if you do some searching around, you will likely come to the conclusion that there enough problems with that system, that you'll be better off solving your existing problem.

http://www.peachparts.com/shopforum/...3-tack-amp.jpg

Here's some more info that may help down the road - http://www.peachparts.com/shopforum/2901193-post244.html

Sounds like at this point I'm down to finding someone else with a working tach and trying my EGR controller in that car.

I've just moved to this area so until I find a buddy with a benz I think I'm shelving this project.

Thanks for all the help everyone, especially mach4!

Did you ever fix your tach?

Mach4, in the diagram, do you know where the OVP wire that's marked "c" goes? In the pin outs, it says to circuit 30. What is circuit 30?