Can somebody tell me what modulates the advance of the needle on the analog tachometers in our instrument clusters?

I want to replace my tach which indicates 0-6000, with one whose face indicates 0-7000. If I replace the face of the replacement tach with the old face which indicates 0-6000 r's, will it be accurate? Or, if I leave the 0-7000 face on the replacement, will it be accurate? The arc of the tach is the same on both faces, the difference being that the unit marks are closer together.

Please, knowledge, not speculation.

My big question is what diesel engine are you running that pushes anywhere near 6000K?

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1987 300TD 147,000 miles- Palomino leather interior, 1995 facelift and body cladding, E350 wheels, Rebuilt suspension and sport springs, rebuilt turbo, New Monarch injection pump and injectors....and the list goes on and on...

6000 is OEM for the 921. 2.2L

what is a 921 2.2L?

most of the tach's in the MB diesels are limited in their RPM's by the fuel governor in the IP... so unless you are modifying this, a larger Tach is not needed.

could you give us more info on what you have, and why you want a higher peak?

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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!

I don't WANT a higher peak, just a new tach. It's a 601 engine. The question isn't what I want but whether a tach which indicates 7000, with an arc the same as the 6000, would be accurate as a replacement, and with which face? I'm guessing the new tach with the old face with the same arc as the 7000 face, would be accurate.

I don't know what controls the swing of the needle and whether there is some restriction or modulation in the head units. The tach is out of a '91 gasser but I don't believe the source has any bearing on the question.

keep in mind that gassers have an electronic pulse generated buy the spark... so their tachs may be driven off that. although in today's computerized world, I wouldn't be surprised to find crank trigger being the signal source...

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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!

Yeah, I know, but the signal will be the same at the head unit where I don't know if there is any modulation. I don't think so but I've pulled my cluster out 18 times now in the last 8 weeks and I don't want to make it 20 this week if I put in the new tach and find it doesn't work. Hence my question.

I take it you really don't know. But thanks anyway.