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240 Botched Small Tach Installation or Bad Gauge?

Khrrck said:
Interesting! My small tach appears to read fine, but drops from high to low readings much faster + has a little bounce/overshoot at the end if the needle moves fast enough...

Wonder if that's a design change or a similar worn-out cap.
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I'd bet its the capacitor. They serve as an electrical buffer, so bouncing and bad cap makes sense.
 
Well done! To get the bezel on I use a small round cylinder like a thick round punch. Lay that on the workbench and holding the bezel tightly against the gauge you press and slowly roll the edge of the bezel back around the edge of the gauge. Any damage showing metal use some satin or semi gloss black paint to touch it up.

Glad you tested it in the car. A function generator probably can't make the type of signal that the gauge would respond to. .The signal when looked at on a scope is a large voltage spike then a shelf. Not gonna happen on a function generator. This quick little video shows the signal.
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Update: The tach AND remote start worked flawlessly through some little errands around town and most of the half hour drive home. About half way back I saw the needle drop to 0. :grrr: I gave the dash a good smack and the needle came back to life for the rest of the ride home. Odd, but okay.

This morning I hopped in, and the tach worked fine for about half the trip. This time the needle dropped to 0 and never came back. Dash smacking didn't help. I don't think I fixed anything. Unfortunately, this problem reminds me of the speedometer issues I was having in my first 240 which turned out to be an open winding. Sigh.
 
dag nab it! Those things are pretty tough. But that might be the kind of thing that is happening. I've had ignition impulse coil work fine when cold but then they short when the get warm. Giving you no spark trigger. That's on the Bosch breakerless ignition used from 75-85 in n/a and turbo 240.

However, if you are going to check it out. I'd get a good strong magnifier and check for cracked solder joints. Make sure the flux you use is rosin flux. Not acid flux for plumbing.
 
I'll remove it later tonight and bring it into work again tomorrow. A cracked solder joint is a good possibility given the dash slapping fix. lol Hopefully I can find something under the magnifier.

I've been using rosin core solder with no additional flux. Is that good enough?
 
Home Depot sells flux, if you don't find any. It will significantly reduce the possibility of needing rework. It's how you get those really nice solder points that stick to the board and component in a cone shape. Anything would be better than the stock wave solder job, but I can guarantee you'll be more pleased with the results if you use a few globs of flux on the board.
Ideally you should remove as much of the old solder as you can, but the traces on that board look a little too delicate to take that much heat. I'd just melt some new solder in there and be glad with the amount of the original solder that I can remove in the process. If you have a bulb desoldering iron, you can probably get a pretty good amount off the board safely.
 
I just went through and re-flowed all the circuit board connections. It's not pretty, but I'm hopeful. One of the green wires was looking really ugly, so I chopped it back and reconnected it as well. I have a feeling that the gauge is either fixed for good or completely dead in the water now. :lol:

I actually requested a quote from an instrumentation restoration shop to get this professionally repaired. They said "no problem", but it will cost me about $300. I don't think that's unreasonable or anything, but a tachometer is not worth the cost to me.
 
Take three!

I reinstalled the tach during lunch. I fired the car up and things seemed to work even better this time. I'm trying not to get too excited, but the signal seems even more crisp and responsive than before. The bouncing is completely gone. We'll see how it acts long term, but I'll be sure to report back in a few days whether it's operating properly or not. If all is good, I'll prepare an article.
 
It's still working. Case closed!
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And time consuming! :lol:
 
Regardless what you think of your soldering job, that needle looks beautiful in action. Very smooth. You saved it from the trash can. Nice work.
 
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