w123 tach test - is this sound? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 08-28-2010, 08:52 AM

Hello,

Because my 82 300CD tach reads slow, I want to use an identical tach that I have from my 83 to determine where the issue is. My thought is this:

-attach via a fuse and alligator clips the +12V and ground for the tach body
-strip some small gauge wire to form the two leads for the signal to the tach
-take the stripped leads and place them in the holes for the tach amp, then lightly place the tach amp back in, allowing contact to be made without forcing
-connect the other stripped ends of the leads to the two signal connectors on the tach being used to test
-take reading and determine delta at idle

My issue with this is just that then the tach signal circuit would effectively be in parallel (the test tach in parallel with the instrument cluster tach). Would this be an issue? I figure this is the easiest way to get a side by side reading, and my gut feel is that the needle on my in-dash tach is just off to the side by a few hundred RPM.

This way I dont need to remove clusters to do the test, which is beneficial in terms of time and chances of damaging anything... I can then remove it if need be to fix something,

But is the wiring scheme and parallelization of tachs sound/OK?

Thanks!

funola · #2 08-28-2010, 09:20 AM

No! Won't work.

Swap tach amp outputs from the 2 (idling)cars to drive the other car's cluster. You'll have to add longer wires.

JHZR2 · #3 08-28-2010, 09:37 AM

huh? I dont have 2 cars to swap. I have one car and then one spare tach.

So there is only one signal, going to two tachs.

Otherwise how do I pull the signal without destroying the wire harness? Removing the cluser means making a big mess with the oil pressure line or disassembling the cluster so the oil line can stay attached while the engine is running so I can have the other tach attached in lieu of the cluster.

funola · #4 08-28-2010, 09:51 AM

Didn't know you do not have the 2 cars. Try it. The concern I had was extra loading which may cause neither tach to work, and wiring errors which may blow something.

Come to think of it, I think there is a disconnect between the tach amp and the cluster that is in the engine bay. Discconect that so you are driving only the new cluster tach you are testing.

techguy512 · #5 08-28-2010, 11:27 AM

I'd just swap the '83 tach into the '82 cluster, rather than try to run them side by side.

The oil pressure line really doesn't make a mess when you disconnect to pull the cluster.

If you really feel the need to run them side by side, pull the '82 cluster and splice two extension wires into the harness at the connector that mates to the tach. Then run these to the '83 tach (will need power and gnd too, obviously).

The circuit inside the amp that drives the tach has a relatively high source impedance (27kohms) so I would think it would not be a problem to drive two tachs simultaneously.

JHZR2 · #6 08-28-2010, 02:10 PM

Well I have to say I'm pretty dumb... Compared the tach in my 82 with the one from my wrecked 83, and the needles sat at different zero points. Tried to pull off the needle from my 82 tach to recenter it, but it wouldn't come off, and I didn't want to break it... However I was able to turn it on it's shaft by turning it to max and then adjusting. Sits at zero and idles at 750 now...

And we adjusted idle and the rack dampener too...