heyo all,
i recently swapped an 86' 300SD motor into a 85' 300d. now im getting to the little issues and next up is the tachometer. i took the reader off my 300d and stuck it on the SD motor. the tach is off by a percent (i dont know it for sure but pretty much when im cruising at 70mph it shows 1800-2000 rpm. it shows an idle of just above 500. i have heard about a tach amp??? idk what is the correct way to do it but any help is greatly appreciated..

thanks, Jack

Which motor did you install?

The 86 model would be a OM603 6 cylinder. And it would be from an SDL, not an SD...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

i wasnt there when the motor was purchased but my understanding is its a 1986 300sd motor. i could have misunderstood and it is an 85... like i said i just installed the motor i didnt buy it myself.

now that i think of it it makes more sense that its an 85'

thanks. JAck

The easiest way to tell is just count the injectors...if there are 5 it's a 617 and if there are 6 it's a 603.

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

its a five cylinder. it would be a squeeze to get the 603 into this car. anyway im just wondering about the tach

The '85 did not use a tach amp. Pre-85 (or possibly pre-84) vehicles used a VR sensor mounted to read a pin on the harmonic damper and the signal is routed through the tach amp and then to the tach module in the cluster. The 85 on the other hand takes its signal off the flywheel teeth and routes it through the EGR controller and then to the tach module in the cluster. Some have reported that the signal can be routed directly to the tach module, bypassing the EGR controller completely. In my swap, I kept the EGR controller mostly because I thought I would need it to pass the BAR inspection. I don't know if there is a difference in the tach module itself between the 85 and the earlier but I suspect there might be - and your experience might point to this reality.

Here's the diagram



Hope that helps.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

MB didn't change the 126 engine bay going from 617 through '85 to 603 from '86.

Are you using the ring gear and tach sensor from the old engine or of the replacement engine? Did you touch the rear axle or change tire size?

Sixto
83 300SD

i took the sensor off my 300d motor (original motor) and bolted it to the SD motor. there is a hole in the adapter plate that the sensor sticks into. off the sensor comes two wires with a clip connector on the end which i clipped to the wires coming from the tachometer gauge in the instrument panel.

i just recently put a 3.07 differential in the car (from the original 2.88) and that did not make any difference in the tach.

my guess is that the flywheel in the SD motor is different than the 300d one therefore it is reading off.

It wouldn't....tach measures engine speed while the differential would affect car speed (speedometer)

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

... is why I asked the diff question.

With the resources available in the forum, check the (AC?) signal from the ring gear sensor and compare with another member's similarly setup 300D.

Does the IP have an RIV port? I recall a member, maybe mach4, fashioned a tach or at least an rpm signal driven by the RIV lug.

Sixto
83 300SD