My recently acquired 1987 240 DL wagon did not come with a tach. I recently found a 1988 240 GL at the local parts yard so I pulled the instrument panel and 2-inch clock. While at the yard I didn't have any way to record which wires went where. However, when I pulled my panel I documented carefully so that I'm pretty sure everything is hooked up the same.

The parts yard panel was pretty dirty, so I removed the clock and transferred the tach to my panel. The white/red wire was easily located and connected to one of the two spades at the bottom of the tach, and the clock connector from the parts donor re-attached at the top of the tach. I also confirmed there is a white/red wire at the "1" terminal of the ignition coil.

The problem now is that when I turn the key to "on" before starting the car, the "new" tach shows 1500 RPM. When I start the car the tach slowly climbs to 2000 and stays there regardless of actual engine speed. So, I'm wondering have I done something stupid to break the tach, or is it just DOA? The donor car was hit from behind, so I can't imagine what could have toasted the electronics.

Any suggestions would be welcome.
Thank you and Best Regards,
-- Jeff Buckles
-- Aloha, OR, USA
-- 1987 240 DL wagon

Well, it seems silly to reply to my own message so soon, but...

In the hour I spent searching google and these boards I had apparently left the ignition in the "on" position. When I went back out to the shop after posting the question I found it was reading zero with the key still on. And when I started it then it worked! So, I guess maybe there's a capacitor that needed time to charge up, or something. Anyway, it works now.

Best Regards,
-- Jeff in Aloha

I installed a tach from a 1988 240 into my 1989 240. At first it worked fine, then like yours it would immediately go to 2000 RPM and sit there, then slowly creep back down to 0 (while driving). I took the cluster back out, redid the connections and checked if anything was loose, reinstalled. It worked fine....for a few days. Now once in a while it will randomly go 2000 RPM, stay there, and slowly climb back down to 0. The really odd thing is that even with the red/white wire disconnected, the tach needle would move. This indicates some sort of voltage going through the cluster to the tach that shouldn't be. I haven't sorted it out yet, and it works ok most of the time - has yours been working correctly?

Zack
1980 245DL M46 290k
1988 745T+ M46 233k
1989 245DL M47 149k

Same thing here. It works fine most of the time. But maybe twice in the week I've had it installed it has done exactly what you described. No good ideas yet. But before I complain too much I need to follow the advice from another thread and clean all the ground connections between the engine and body, and between the ignition and body.

Best Regards,

J.

Another thought is that if there are any electrolytic capacitors in the tach itself (which I would expect there are since it's filtering and integrating the pulse train from the coil primary), they can age and dry out and start causing odd behavior. Given that it's twenty years old, this is something that should be checked out when I have time.

Best Regards,

J.