Tach compatabillity
 

Hello,

I have aquired a tach from a '72 300sel 6.3. I would like to make it work in my '71 280se, which I am converting from the old inline 6 gas to turbodiesel I removed from a wrecked 1985 300d. What can I do to make it work with the diesel engine? Any advice is greatly appreciated.

That depends on what type of tach setup it has. The diesels (up to 1984) used an AC coil sensor at the crank, fed to a tach amplifier on the fender, and then to the gauge. What do the cars you have use?

The 300d was a '85 and I believe it read off of the flywheel, I also have parts from a '83 300d so whichever setup is the easier I will more than likely go that route.

The '83 setup will be easier. The 84 and 85 models get the tach feed through a computer box.

Will the tach gauge from the 1972 300SEL 6.3 V8 gas be compatible with the tach amp and related parts for the 1983 300D?

If they have the same type of connector, then I'd assume it'd work. The '83's have a two-pin (or three, one is an extra ground) connector.

The 6.3 tach has two wires; a red and green. I think the metal casing is a ground.

And where is the rest of the 300sel?

I wish I knew; I purchased the gauge cluster from ebay.

I doubt it would be compatible. The tachometer for the gasser reads ignition pulses - 3 or 4 per rev depending on how many cylinders you have. The diesel tachometer gets one pulse per rev. You'll want to find a tach from a diesel and transplant it.

I am quite impressed with your knowledge of electronics after reading the thread you started regarding how the tach amp works; anyways, do you think it is possible to make a tach amp that takes one pulse from the crank and sends four to the tach? If possible, what is the best way to go about doing so? The 300sel 6.3 V8 tach is the only type that will fit in my car. So if it can be done, regardless of difficulty, I would like to make an attempt to make it work

Also I was wondering how the late model diesel tach works; I know those read off of the flywheel instead of the crank but I am not sure how it reads 100+ teeth and converts it to a pulse for the tach.

techguy512 has produced a tach amp replacement that uses a small microcontroller (computer) to do what the old analog part did. It may be that he can adapt that to convert the signals. The issue is that the tachometer expects a steady stream of pulses, and not bursts - so his algorithm would have to be somewhat complex. It would have to behave like a Phase Locked Loop.

It will be much, much, much easier (not to mention cheaper) to retrofit a diesel tachometer. It looks to be a straight conversion with no extra holes. Some rewiring is required, of course.

Maybe not so much. First thoughts would be to go to the next controller in the series. 2k of code space, and two independent timer/counters. One to measure input pulse rate and one to create output pulses.

We would need to know with a great deal of certainty lots of details about the gasser tach (input pulse rate, input signal levels, etc, etc).

Did the w115/w114 diesels have tachs? I was under the impression that a tach was never offered as an option for that body style which uses a similar gauge cluster as the w108/w109s

I removed the tach from the cluster; it has the following on it:

12V / 24000 Imp.

230 / 29 / 3

8Zyl. / 4Tkt

There is a blue stamp on the unit that reads:

225
10.68

This particular tach unit was used on the m100 engine.

Hope that helps; I greatly appreciate everyones help so far, thanks.

Steve Brotherton has/had a VDO service shop. He should know how to make it work.

Or ask the folks at www.paspeedo.com

The Rube Goldberg in me says to have a machine shop fit as many crank triggers on the crank damper/pulley as needed to have a proper reading.

Sixto
87 300D