So I found a donor tachometer that I'd like to put in my euro 300TD turbo.

The car was originally equipped with cruise control, so it has a sensor on the balance wheel. It also has what I think is the tach amp (the little garbage can thing on the left fender that the grey tach sensor line runs to), but there is no plug for the tach behind the instrument cluster.

So the question is: What parts do I need to install a tach?

Crank balancer, tach pick up, tach amp, tach its self.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Hi Doug,
Is your donor tach from a Mercedes? If so find the MB components that BgKast listed. If its just an aftermarket tach what you need a way of providing it with a square wave signal that it will interpret as a standard spark ignition system. Basicly you are "fooling" a regular tach into thinking its on a gas engine. I have a homemade prototype system on my 240D that uses a Hall effect sensor to accomplish this. I was thinking of offering a kit, plans, or possibly a fully assembled product, but I would need to know there was a market before tooling up to make the pc boards and such. These pics show the sensor, the control box, and the tach

The sensor on the harmonic balancer actually runs the egr system, and it has nothing to do with the cruise control.

Cheers, Chris

Yeah, I'm taking one out of a 300D.

I need to make sure I'm not missing anything here.

The crank balancer and tach pick up are the magnet (?) on the balancer wheel and the grey wire that goes to the left fender, right?

The tach amp is the little round garbage can looking thing on the left fender, right?

I do know what the gauge looks like, however.

I'm most concerned about the wiring from the gauge to wherever it goes. I'm sure I can just follow it to its end once I start pulling it out (I'm taking the entire dash and windshield out of the donor car, so the wiring should be relatively easy to trace).

The car was a grey market import so it never had a factory installed EGR. It did have a very poorly installed EGR, that did absolutely nothing. I have since removed it.

I read somewhere on this forum that the cruise control got some of its signal from the tach pickup.

You got it.


The cruise gets it's signal from the speedometer.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

The back of the tach should have a plug. Being from an '80, there ought to be a two prong plug. '84 and '85 used a different set up, getting signal from trans. If there are three prongs on back of tach, that is the newer style. You will need an amp, trash can looking thing. The wire coming from the balancer will plug into the bottom part, (three female affair) to amp, to wiring harness to tach. The amp connects the wire from sensor to harness.

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83 SD

84 CD

I've heard conflicting reports on all this. I also have an 84 Euro 300TD Turbo. All it has is one big clock instead of the clock/tach combo. Is there a way to retrofit a tach/clock into my dash? What do I need to look for to see if it will work? My car does not have cruise control if that matters. I've heard that all 84s and 85s had no tach and it's very difficult to retrofit one...just wondering if anyone has done this successfully.

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'84 Euro 300TD Turbo, 192K

The dash cluster, once out of the car, you'll see, the different gauges can be swapped out from the back.

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83 SD

84 CD

http://www.peachparts.com/shopforum/showthread.php?t=198203

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

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!

If I am not mistaken, I believe your '84 euro TD should have a pickup sensor on the front balancer and a diagnostic port on the driver's fender.

You will need a 2 prong tachometer from an '82 or '83 300D or CD, or '81 -'83 TD. If you look on the diagnostic port on your wheel well, you notice holes with numbers associated with them. #2 is ground and #6 is switched power and leads to the firewall. You will need to add in a wire for #3. Get the wire and plug for the back of the tachometer (instrument cluster) from the donor vehicle. The green-black wire from the tachometer plug goes to the #3 prong on the tach amp and the red-black one goes to switched power that you will need to find.






You should post pictures of your engine bay and balancer area so we can determine if you can do this.
.

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1983 123.133 California
- GreaseCar Veg System

Ah-ha! This is precisely what I was looking for. I do indeed have the pickup on the crank balancer with a gray wire running to the diagnostic port on the driver side fender that's in your picture. My balancer has the little cylinder sticking out of it for the pickup as well (saw both of these the other day when I was checking timing chain stretch).

My diagnostic port does not have an amp, just a cap, so I will need to procure that as well I guess...

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'84 Euro 300TD Turbo, 192K

The tachs on cars that had 'em in 84-85 used a different pick up, from the transmission. Which is why you need the proper tach, a two pronged unit. You also need a wiring harness. Make sure the balancer has a pin on it, should, but check anyway.
That diagnostic port harness is what you're looking for, plugs into the bottom of the socket, the amp connects the two, then you have a power lead, and two wires to the tach itself.

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83 SD

84 CD

I think I have a couple of things to add to this discussion.......

Be sure that you get the 2 wire tach, and not the 3 wire. The 2 wire will work with the simple system (pickup at the harmonic balancer, tach amp, and simple wiring). The 3 wire tach requires the pickup at the bellhousing and the EGR computer to actually create the signal to drive the gauge.

I've been trying to "reverse engineer" the 3 wire tach systems to try to come up with a way to drive the 3 wire gauge with a modified version of digital tach amp that I worked up. The purpose is twofold - to allow 3 wire gauges to be used in older cars with the simple system AND to allow the simple system to be retrofitted into cars that have suffered EGR computer failures, since replacement EGR computers are becoming unobtanium. So far it's been slow going, as I don't have a working 3 wire system to work with.

On another front, some of the guys that have done OM617A engine swaps into their W460 Gelandewagens have asked if there is any way to adapt the tach amp to directly drive their gas engine tachometers. I have had success with a modified version of the digital tach amp. This link will take you to a page with some video of setup working:

http://www.mbelectronicupgrades.com/Project-Aftermarket_Tach.html

The first video is the modified amp set up to produce 3 pulses per crank rev to match what an M110 would present, directly driving the VDO tach from a 280GE.

The second video is a typical aftermarket tach being driven the same way, but in the 4 cylinder setting (2 pulses per crank rev). Thanks to Mr. 4x4_Welder for letting me borrow his Sunpro to get this working

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Bob
'82 300D Petrol B-G Metallic

You do good work Bob. Keep it up.
The two prong w123 tachs are slowly disappearing. So having a three prong option would be great.




.

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1983 123.133 California
- GreaseCar Veg System

Thank you, DeliveryValve (and everyone else). Finally got the tach and amp installed - works like a charm!

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'84 Euro 300TD Turbo, 192K