Tach Help 1985 300SD CA

[idrivea2002golf]

I've been reading through several forums and pages looking at issues with tachs in the W126 cars. I'm at a loss on mine as I don't see to have any of the components in areas others describe. I have a 1985 300 SD with CA emissions. I do have ABS and I also have the Klima relay. Not sure if those make a difference but they've popped up on other pages so I thought it worth mentioning.

• My tach does not register at all.
• I do have a functioning blower motor for my HVAC however, no AC. I don't know if the AC is charged though so I can't say if it is the tach causing it not to work. Before I go looking into the AC system I want to fix the tach given the ACs dependence on the RPM signal.
• The EGR computer behind the passenger kick panel does not have any corrosion.
• I do have the Klima relay in the car.
• I do not have the tach amp in the circle thing, if I pull the top it is just empty pins.

I've seen pages mentioning the OVP relay. I am trying to test this but I can't seem to find it in the "usual" places. I've read it's behind the passenger air vent and accessible through either the glove box or removing the passenger knee board. I don't have anything there other than some klimateregel computer.



Others have stated it's behind the battery between the firewall and the "fake" firewall. Nothing there but the coolant tank.



Others have pointed to it hiding attached to the fake firewall to the inboard side of the brake booster. Nothing there either.



I do have an OVP relay in my fuse panel. I don't know if these are the same OVPs though, if someone could confirm. If it is the correct relay then the fuse is OK.


Some additional info on the AC, the auto speed blower does seem to function as it will increase/decrease blower speed according to the temperature differential.

[ykobayashi]

It’s the red thing in the lower left hand corner of your image with the 10A fuse in the top. Check the fuse to see if it’s burned.

Oops you said you did that my bad

Okay other things that could be wrong. Your connection from your rpm pickup to your tach could be corroded. There is a video on YouTube of a guy finding this break on an 84/84 SD. I think the plug that was bad is right behind the valve cover. Trace the wiring from the sensor on the bell housing (right near the oil filter) on up eventually to the dash. I’ll se if I can dig up a video.

ETA - ok, it wasn’t a vid. It was a guy I helped on FB with a bad tach on an 84. He had a broken wire here. It was the sensor wire coming up from the bell housing pickup.

Check this. This wire should eventually find its way up to the tach. It’ll probably get routed through the EGR computer where there are also some potentially bad connections.

[idrivea2002golf]

I'll take a look at the connection. I've seen the connector in a few videos but for some reason my firewall is different. Where that connector should be is a computer which looks a lot like a modern day ECM.



I'll take a look at the sensor in the bell housing as well.


If I understand what you're saying correctly, there is only one OVP per vehicle?


Is there a way to test the OVP without the tach working? Other than the AC compressor. Although I suppose I could charge it and see if that system works.

[ykobayashi]

I’m not sure how to test the ovp. I’m not sure that red fused relay is the ovp…but it sure looks like the one behind my glove box on my 85 300d.

The ECM you’re looking at is likely your EGR computer. Pull the big plug and look for corrosion on the connector. I think the rpm signal comes up from the bell housing, goes up through the EGR computer and eventually finds its way to the tach.

If you can follow the wires you may find a break or a corroded connector.

[idrivea2002golf]

The picture you posted looks more like mine wiring arrangement. Although that's a federal emissions car. The connector is above the passenger wheel. I thought that was the correct one and you've confirmed it. I'll take a look when I get back to this in the morning.

It's got me wondering if the videos I've been watching were the 300D and not SD.

[ykobayashi]

Did you see this thread? Really confusing. I think there were some changes with the 83/84 models.

Do you have the little two fuse relay up near the brake master cylinder?

Overload relay location???? 83 300sd

[idrivea2002golf]

That was the thread that got me onto the confusion about the Klima circuit, which I have. Apparently it was rare in 1985 but became standard in 1986. As was ABS which I also have and is apparently pretty rare in 1985. I do know it was a July 85 build date.

I'm not sure what the build date trigger was but apparently in 1984 they started a CA emissions model of the diesels. This move caused them to get rid of the Tach Amp in the little round block and move it to this electrical confusion.

Unfortunately, I can find lots of information on the 300D and 300TD but it seams like the SD isn't as well covered. Obviously most, if not all, components are the same but finding their location has been interesting. Made worse by the fact I have (well, had. I removed it) the trap oxidizer.

There's even a video of a guy putting jumper wires across the connector for the EGR system to bepass it entirely. Might give that a try tomorrow too. As part of the trap oxidizer delete I also plugged the EGR valve port.