Factory tach with aftermarket pickup? - PeachParts Mercedes-Benz Forum

ngarover · #1 10-06-2011, 09:35 AM

On this 85 300D I'm putting back together, I discovered another fubar. The 84 300SD donor engine trans adapter plate does not have the mounting hole for the "I believe" the tach pickup that reads the flywheel. Now, this would have been a simple swap from my original engine but now that everything is in, I'm not about to take it all out again to correct.

So, has anyone ever driven the factor tach with and after market pickup?

Stretch · #2 10-06-2011, 09:59 AM

I have read that the 85 has a flywheel pick up for the tach - the others have a tachometer pick up on the other end of the engine close to the crankshaft pulley. The tach amp is mounted on the driver's side inner wing (fender).

http://www.peachparts.com/shopforum/diesel-discussion/285719-help-identifying-diesel-tachometer-reqd-want-fit-my-car-2.html

The older system might work with a bit of rewiring to make it fit the 85 tachometer display unit in the dash... I'm not sure but I think they all work on the Hall effect so in principle they do the same thing...

DeliveryValve · #3 10-06-2011, 10:11 AM

Wow that's not good fortune. However, I did mention to you that this was an issue earlier.

http://www.peachparts.com/shopforum/2793735-post2.html

Anyhow, you can use the front pulley sensor and have a working tach.
But the problem you will encounter is you may not have A/C function, for that is important for later '85 computer controlled units with the RPM sensor, OVP and kilma relay setup.

.

winmutt · #4 10-06-2011, 10:23 AM

I have also mentioned it as well

Best bet is to swap out old tach and not have ac OR just deal with it

You could try to drill and tap a new hole for the pick up but thats be a big guessing game

charmalu · #5 10-06-2011, 11:37 AM

It wouldn`t be possible to drill and tap. the 85 adaptor plaet has a flat spot and hole where it connects to. the plate he has now is round, and if a hole was drilled. the sensor would be farther from the FW.

would it be possible to use the front crank for this sensor? and have it work properly? I would say it wouldn`t since the FW is a larger diameter than the crank pully etc... so the sensor must count the teeth or maybe a magnetic thing?

If I screwed up like that, Iam probably just stubborn enough to pull it back out and swap the plates.

Charlie

DeliveryValve · #6 10-06-2011, 11:56 AM

Quote:

Originally Posted by charmalu

It wouldn`t be possible to drill and tap. the 85 adaptor plaet has a flat spot and hole where it connects to. the plate he has now is round, and if a hole was drilled. the sensor would be farther from the FW.

would it be possible to use the front crank for this sensor? and have it work properly? I would say it wouldn`t since the FW is a larger diameter than the crank pully etc... so the sensor must count the teeth or maybe a magnetic thing?

If I screwed up like that, Iam probably just stubborn enough to pull it back out and swap the plates.

Charlie

Charlie you're right, the front sensor will not work. You'll need the magnetic pickup to count the flywheel teeth.

Pulling the motor and swapping out the intermediate plate is the only option.

.

winmutt · #7 10-06-2011, 12:05 PM

Quote:

Originally Posted by DeliveryValve

Charlie you're right, the front sensor will not work. You'll need the magnetic pickup to count the flywheel teeth.

Pulling the motor and swapping out the intermediate plate is the only option.

.

He can use the front sensor with an older tach and amp setup. Alot of work for not much gain. He would also probably lose ACC and probably CC.

sixto · #8 10-06-2011, 01:31 PM

You can regain AC by bypassing the Klima relay.

Maybe getting cruise to work is as 'simple' as adapting an earlier tach amp and possibly an earlier cruise amp.

Sixto
87 300D

Stretch · #9 10-06-2011, 01:49 PM

Good point it is counting teeth and not a single pin - so stick a flywheel on the front too!!!!!!! (JOKE)

SirNik84 · #10 10-06-2011, 02:23 PM

I'm pretty sure 84 also has the flywheel pick up. at least my 84 SD had the flywheel pick up. My car now has an 82 engine and a manual trans. the only part of the drive train original to my car is that adapter plate, just for that reason. Changing the adapter plate can be a Horror. the flywheel bolts for the automatic flywheel round off so easily that you will want to quit after the 3rd head rounds off. I had to weld a large nuts onto the head of the rounded bolts to get them out.

All that being said I think the flywheel system is superior to the balancer pickup from a reliability stand point. and retro fitting to older system would not only be work, but a step backwards.

locating the correct adapter plate and installing is what i would do... or should I say, what i did.

ngarover · #11 10-06-2011, 03:03 PM

Quote:

Originally Posted by sixto

You can regain AC by bypassing the Klima relay.

Maybe getting cruise to work is as 'simple' as adapting an earlier tach amp and possibly an earlier cruise amp.

Sixto
87 300D

Cruise control... I have yet to own one of these car where that worked anyways lol. The Tach didn't work in the car before I swapped the engine, and really what good is it in an automatic anyways. Kilma bypass is stupid simple to do for the AC, so no issue there.

So, maybe adapt the older 2 prong tach, since I have to take the pod out to fix the speedo cable anyways. All depends on what I find around here as far as parts go

winmutt · #12 10-07-2011, 09:09 AM

Quote:

Originally Posted by SirNik84

I'm pretty sure 84 also has the flywheel pick up. at least my 84 SD had the flywheel pick up. My car now has an 82 engine and a manual trans. the only part of the drive train original to my car is that adapter plate, just for that reason. Changing the adapter plate can be a Horror. the flywheel bolts for the automatic flywheel round off so easily that you will want to quit after the 3rd head rounds off. I had to weld a large nuts onto the head of the rounded bolts to get them out.

All that being said I think the flywheel system is superior to the balancer pickup from a reliability stand point. and retro fitting to older system would not only be work, but a step backwards.

locating the correct adapter plate and installing is what i would do... or should I say, what i did.

84 calis had the tach pickup on the flywheel. In modern terms these are called crank position sensors. I've never had problems with the flywheel bolts auto or mechanic and I have done that job probably 12 times now. It is still a PITA.

Quote:

Originally Posted by ngarover

Cruise control... I have yet to own one of these car where that worked anyways lol. The Tach didn't work in the car before I swapped the engine, and really what good is it in an automatic anyways. Kilma bypass is stupid simple to do for the AC, so no issue there.

So, maybe adapt the older 2 prong tach, since I have to take the pod out to fix the speedo cable anyways. All depends on what I find around here as far as parts go

It all depends on how stock you want to keep it. The 85 EGR system works with the klima, the tach etc etc. I stripped all of this out of my car.

The easiest solution would be to make your own tach amp.

charmalu · #13 10-07-2011, 11:29 AM

Looked in Fast Lane Parts, and is listed as RPM Sensor / Inductive Sensor.

It shows the 84 SD/D and 85 SD/D as having it.

Charlie