Tach install

[BATComm1966]

Looking at one of these, I have a 300TD swap, the sensor lead from the pickup on the crank shaft ( front of engine, see Pic) is still on the engine.. Just wondering if I could use that sensor/pickup for this tach??

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[cornemuse]

Should work, they're just simple wire coil sensors (sense disruption in magnetic field at the groove in that pulley/harmonic balancer, what ever it is)

[BATComm1966]

That's what I was hoping....

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[jay_bob]

Or find a tach/clock instrument from a diesel 123 and switch it out for the clock part of your cluster. Gas tach won’t work on a diesel because it counts the sparks.

[BATComm1966]

Originally Posted by jay_bob

Or find a tach/clock instrument from a diesel 123 and switch it out for the clock part of your cluster. Gas tach won’t work on a diesel because it counts the sparks.

Looked into that, have to have the tach Amp and the wiring harness, my 240d didn't have one from the factory so no wiring harness...

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[BATComm1966]

Originally Posted by jay_bob

Or find a tach/clock instrument from a diesel 123 and switch it out for the clock part of your cluster. Gas tach won’t work on a diesel because it counts the sparks.

I've found an 83 300Td, has the gauge cluster and the Amp on the inner fender and all wiring, question is what wiring do I need, I have the pickup and wire on my engine now that is attached to the front crank pulley, but that's it (see pics) what would I need off the 83??

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[BATComm1966]

After doing more research, the 85 300Td I have in this 81 240D would get its tach signal from the speed sensor reading from the flywheel, I believe the sensor is still there.. But.. The signal goes through an EGR computer first (which I don't have on the 240) then to the gauge, don't know if not having this and trying to do a "direct wire" from the sensor to the gauge would work.. Found a video that the guy did just jumper the leads of the EGR computer plug in to get his to work, so I'm assuming that would be the same as a direct wire.... Anybody got any input on this??? I'd like to keep it stock looking but hate to fork out the $$for a gauge cluster just to find out it won't work....

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[ykobayashi]

Heads up that the 85 is an entirely different animal than the other 80’s turbos.

The tach amp is behind the 85 glove box with a 10a blade fuse on top.

You can probably use the 83 turbos amp and the 240 pickup but you have to do your own wiring for 12v, god, signal low, & amplified signal. Sounds like you already know this.

Images of amp in post 31 of my 85 thread.
Got a 1985 300D turbo

Edit - ugh like usual I spoke too soon. The little silver box under the fuse may not be the tach amp. The fuse supplies power to the tach amp electronics which is likely in the EGR controller that you do not have. I’ve been searching around and I cannot find the thread about redesigning the tach amp. They had pinouts and schematics for the 83 version which can help you with your rewire job.

Here is the schematic. A little fuzzy but it has all the pinout numbers for the wiring.
How the Tach Amp works

[BATComm1966]

Originally Posted by ykobayashi

Heads up that the 85 is an entirely different animal than the other 80’s turbos.

The tach amp is behind the 85 glove box with a 10a blade fuse on top.

You can probably use the 83 turbos amp and the 240 pickup but you have to do your own wiring for 12v, god, signal low, & amplified signal. Sounds like you already know this.

Images of amp in post 31 of my 85 thread.
Got a 1985 300D turbo

Edit - ugh like usual I spoke too soon. The little silver box under the fuse may not be the tach amp. The fuse supplies power to the tach amp electronics which is likely in the EGR controller that you do not have. I’ve been searching around and I cannot find the thread about redesigning the tach amp. They had pinouts and schematics for the 83 version which can help you with your rewire job.

Here is the schematic. A little fuzzy but it has all the pinout numbers for the wiring.
How the Tach Amp works

I'll go through this also, I just have the 85 engine, everything else is the 81 240D...so I have nada on the wiring side for this... That's why I was hoping if I took everything out of the 83 300d parts car I would have what I needed... Thank you for the diagrams.. I'm out messing with it now, I don't have a sensor lead coming off the flywheel area that I can find anyway...

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[ykobayashi]

Originally Posted by BATComm1966

I don't have a sensor lead coming off the flywheel area that I can find anyway...

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You know what’s funny, I went through the same thing on my 1982 240d and I recall there was some kind of pickup there. It was funny because the car didn’t have a tach...just a big nice clock. But I recall there was something down there. Maybe it was a stud on the balancer? Memory failing me. I feel like there was a cable with a diagnostic plug at the firewall...but I may be mixing up my cars.

Good luck. Probably best to pull everything you can from the 83 300d for the transplant.

[BATComm1966]

Originally Posted by ykobayashi

You know what’s funny, I went through the same thing on my 1982 240d and I recall there was some kind of pickup there. It was funny because the car didn’t have a tach...just a big nice clock. But I recall there was something down there. Maybe it was a stud on the balancer? Memory failing me. I feel like there was a cable with a diagnostic plug at the firewall...but I may be mixing up my cars.



Good luck. Probably best to pull everything you can from the 83 300d for the transplant.

Yup... Mine has the pickup on the crank pulley also... But I'm thinking the same, grab everything off the 83 and it "should" bolt on and work...

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