Hi everyone, with the car running and driving much better now, my focus has turned to the electrical problems, starting with the tachometer. My SD has an 81-84 engine swapped into an 85 chassis. As far as I'm aware these use different pickups for the tachometer signal (on the crank pulley for 81-84 and on the bellhousing for 85). The tachometer amplifier/diagnostic port is still present. Has anyone been able to adapt the earlier tachometer to work on an 85 car? I assume some signal processing would have to be done as there's only one reluctor pin on the crank vs many teeth on the flywheel.

Get a tach amp and run wires to the tach. Its just 2 or 3 wires (depending on the tach) Signal+ground or Signal+power+ground.

Yes that’s for the most part correct but I think the 85 gauge expects a different pulse frequency since it triggers multiple pulses per revolution unlike the pin on the harmonic balancer. I think if you do this you’ll also need a tach from an 82-83 turbodiesel with two pins rather than three.

I take it the pickup on the bell housing is gone? It may be easier to try to fix the 85 system rather than install an 82 system. Just a thought.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

I don't know about the 300SD, but I have a 1984 and 1985 300D. Both use the tach pickup on the transmission bell-housing. See if yours has that. It is on the L side near the top. It routes thru the EGR Box (behind R kick-panel) on both, I recall. If the OVP relay above fails (common), the EGR Box doesn't pass the tach signal thru. I removed both and jumper'ed across the EGR connector, using a post you can search for.

Both engines still have the pickup on the crankshaft damper, which runs to a connector on the L inner fender. There are no electronics there and it is termed "diagnostic port". Apparently dealers could connect an instrument to read rpm. Earlier cars had an amplifier which plugged into that connector. I would take out that clutter but the cable is hard-wired to a harness, so would have to snip wires to remove it.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

The EGR computer controls the tach and replaces the old tach amp. It sends out the same signal.


Number of pins makes no difference to anything. The third pin is just a +12v circuit.


Its as simple as plugging in a tach amp to the socket and running wires to the tach.

Thanks for clarifying that. I’ve always wondered about the pulse frequency between the two and three pin gauges.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

You sure about that? The EGR-computer cars have 144 pulses per revolution of the engine from counting flywheel teeth vs the single pulse of the crank damper pin of the non EGR-computer cars.

Let's not confuse mechanical similarity of the gauge connector with electrical compatibility. A 6 cylinder gas tach from a 126 looks identical to the 6 cylinder diesel tach and even uses the same plug, but they cannot interchange due to the frequency of the engine RPM pulse they receive.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Because computers cannot manipulate an input signal into a desired output signal for parts compatibility...


Have you actually done it like I have?

Because they receive a signal from the ignition system and do not require an amplifier as the diesel does. You'll notice most still have the same "diagnostic port" as the diesels, minus the output harness, like the 85 diesel models.

Well that pretty much says it there. If it works it works. This is good to know because a lot of 80s gassers have the diagnostic ports and pickups just like the diesels.

I’ve been meaning to use the diagnostic port on my 85 to run the tach on the dash. I held off because I thought I’d need a two pin tach. Now I just need a 2.5mm socket contact for pin 3 of the diagnostic port. I already have an amp.

EDIT -

Hey @Felching I'm really confused now. I took your word for it that you have "done" this, but can you tell us exactly what you did and what you got working? You took the diagnostic signal off the firewall plug and ran it to an 85 tachometer gauge and you got a signal without replacing the gauge with the older two pin 82 type gauge?

Take a look at the image below. That is fresh from the three pin plug going straight to my gauge in my 1985 300D at idle. My signal comes from the bell housing sensor and presumably it reads the teeth on the ring gear like Diseasal says.

Here is my problem. On my 82 300SD at idle I get a 14Hz square wave as an input to the gauge. If you look below at my scope capture, I'm getting a 1.84KHz square wave. How the heck is an 85 gauge supposed to read the right idle rpm when it gets 14Hz from the diagnostic port than instead of an 1.84KHz wave coming from the bell housing.

Something doesn't click here. What is crazier is this (14Hz : 1.84KHz) is almost exactly a factor of 144 like Diseasal mentions is the number of gear teeth read out on the 1985.

I felt something was wrong when I know guys are bypassing the EGR computer with jumpers and getting their tach to work on 85s. That means the EGR computer is nothing but a pass through for the rpm signal. I guess the sensor is putting out a clean square wave from the get go like a hall sensor instead of the variable reluctance sensor on the balancer? There is literally no electronics between the sensor and the back of the tach in the 1985 car. No tach amp.

Please explain. I don't understand how your "done it" works. My take is the tach gauge in the 85 is a totally different circuit inside...unless it is so smart it can tell that there is a Khz signal coming in instead of a Hz signal and can adapt by itself. In 1985? No way.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles