I have a 1984 300TD and it's from California. I'm not sure if it was made as a california model, but read somewhere that in 1985 W123's and 1984 300TD's they changed the way the tachometer gets it's information. I've had 126's in the past where I had to replace the tachometer pickup at the engine and also one where I had a bad tachometer amp on the fender.

My car doesn't have a tachometer amp. It has the housing for one, but just an empty cap. Not knowing about the change, I ordered a tach amp and plugged it in and no go. Before I go replacing the pickup wire, is there another place I can check before spending more money on unneeded parts?

__________________
Current fleet-
1993 Mercedes 300SD
2004 Chevy Avalanche 4x4
2009 BMW 535i xDrive
1995 Saturn SL2
1993 Mercedes 300SE (parts car)

Previous Benz's-
2001 Mercedes SL500
1983 Mercedes 300SD
1980 Mercedes 300TD
1996 Mercedes E300D
1999 E300DT
1984 300TD-T
1988 Mercedes 560SEL (parts car)
1979 300SD
2003 E500
2004 S430 4Matic
2001 E55
2003 E430

Look under the passenger side kick panel. On my '84TD there is a fuse there and it was burned out. Changed the fuse and have not had a problem since.

Good luck....

Most likely you've got the tach pickup on the intermediate plate of the engine/transmission. If so the signal is routed through the EGR computer to the cluster. If you search a bit you can find how the EGR can be eliminated and the signal passed directly to the tach module in the cluster.

It should be an easy fix. If it's the EGR, it can be eliminated. If it's the sensor that's one bolt (and they very rarely go bad). If it's the wiring, that can easily be replaced. And if it's the tach module, that's like two screws to swap out. The EGR module gets its power from the OVP (over voltage protector) and it has a fuse on it...that would be the first thing to check. And sometimes the cable to the box gets corroded and needs to be cleaned up.

__________________
Current Stable

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

Great! Thanks I'll check that out tomorrow. Car is a GREAT car. picked it up for $1,500 and it is completely rust/rot free from Cali now living in NJ which is rare here on the east coast. My 1980 was totally rotted out

__________________
Current fleet-
1993 Mercedes 300SD
2004 Chevy Avalanche 4x4
2009 BMW 535i xDrive
1995 Saturn SL2
1993 Mercedes 300SE (parts car)

Previous Benz's-
2001 Mercedes SL500
1983 Mercedes 300SD
1980 Mercedes 300TD
1996 Mercedes E300D
1999 E300DT
1984 300TD-T
1988 Mercedes 560SEL (parts car)
1979 300SD
2003 E500
2004 S430 4Matic
2001 E55
2003 E430

On mine, when I was investigating a suddenly dead tach I pulled the connector off the EGR control unit and a bunch of water poured out, so keep a towel handy.

I bypassed the control unit and it's worked great and stopped blowing fuses.

-Rog

84's might not have an EGR computer, in which case the wire goes from the pickup straight to the cluster. That's how mine is. No tach amp, no computer, no OVP.

http://www.peachparts.com/shopforum/diesel-discussion/376694-84-w123-no-tach-amp.html

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

I know neither CA nor federal 1984 had the "tach amp". The tach pickup moved to the transmission bell housing. The front crank pickup is re-termed "diagnostic" and the plastic connector a "diagnostic port" which dealers might have connected to. I would strip out all that front stuff except hard-wired to the engine harness.

The 1984 CA injection pump has the "rack position sensor", which runs to the "EGR computer". If federal 1984's have that, they probably also have the EGR computer. One guy who traced the tach wiring found that it goes thru the EGR computer but isn't affected by it, thus simply a wiring convenience. I don't recall if CA or federal. Others say it runs straight to the dash cluster. Either way, if you ohm'ed the wires it should appear to be a direct connection. Another way to resolve is see if the PN for the dash cluster is the same between CA and federal.

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

Turbozeke, if you are looking to unload that tach amp, I'd buy it. I'm also in NJ!

Federal '84's don't have that EGR computer. At least, mine doesn't and never had one either.
I think 85 federals do have it, which would make the federal '84's an oddball configuration as far as the tach goes.

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

Did you ever trace out the wiring or get your tach to work?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Nope, I gave up on my tach. Other things popped up, like the axles, and so the tach was back on the low-priority list. It's still on the list. I'll get to it. Someday.

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

I took off the passenger side kick panel and I do in fact have a blown fuse in the OVP. I replaced it with a 20amp fuse just to check things out and even with the car off, as soon as I plugged the relay back in, the fuse blew instantly.

I unplugged the harness from the EGR computer and put another fuse in and it still blows instantly. All the contacts are pristine in the EGR and the harness too.

I wired the pins in the harness that I've seen in another post and the tach WORKS perfectly! Now I can keep it like this (which I will for now), but I would ultimately like to find out why the OVP relay is popping. It can't be the EGR computer because it blows even when that isn't connected.

My wait to start light doesn't work either (checked the bulb already) but the glow plugs do cycle and she starts with no issues after the normal glow period. Maybe it's just a coincidence?

__________________
Current fleet-
1993 Mercedes 300SD
2004 Chevy Avalanche 4x4
2009 BMW 535i xDrive
1995 Saturn SL2
1993 Mercedes 300SE (parts car)

Previous Benz's-
2001 Mercedes SL500
1983 Mercedes 300SD
1980 Mercedes 300TD
1996 Mercedes E300D
1999 E300DT
1984 300TD-T
1988 Mercedes 560SEL (parts car)
1979 300SD
2003 E500
2004 S430 4Matic
2001 E55
2003 E430

Per this well-written article https://www.ifixit.com/Guide/Mercedes+W123+Over-voltage+Protection+Relay+and+Fuse+Replacement/22843 your OVP is toast and needs to be replaced.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thank you. I've seen this before but never read all the way through. Guess I'll pick one of those up.

Here's more info on the OVP http://www.peachparts.com/shopforum/diesel-discussion/255537-ovp-relay-when-does-trip.html

I'd do more test to be sure it's a bad OVP, and not something else that is causing the OVP fuse to blow before buying one. I don't think they're cheap.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked