The Stovebolt Site Forums The Shop Area The Electrical Bay Tachometer

I'm trying to slip this in sort of like a 2nd or 3rd cousin. Since some GMC trucks had Pontiac engines in them and this is a Pontiac tach I thought I might get by without being bounced. My son is restoring a 69 GTO and it has this tach mounted on the hood. The car sat outside in the elements for 23 years and the tach filled up with water a number of times. Since I am recuperating from hip surgery and needed something to do he brought me the tach to see if I could do anything with it. I got it taken apart OK down to the movement. It is heavily corroded but the movement is not frozen. I am wondering how I can clean it up and test it. It gets it signal from the distributor. It looks like the needle moves by some magnetic pulse. I think the movement is called a "D'arsonval movement. Any of you electric gurus know the principal of operation and how I can clean it up. Hoping Alan won't delete .

Weeds

http://picasaweb.google.com/lh/photo/D

Your a masochist. 8^) Most of the laws and principles were covered in the first semester of Electronics (DC Theory) I took many years ago. D'Arsonval movements were not the easy part of the course. The text we used was "Electrical Principles of Electronics" by Gillie.

I'm thinking an easier approach would be to find a Tachometer kit, it should provide a schematic. That would provide a good starting point to decipher the Pontiac tach. I did a search on that auction site and found this;

http://cgi.ebay.com/ebaymotors/VINT...000QQptZSnowmobileQ5fPartsQ5fAccessories

For what those tachs are selling for on ebay some effort to salvage that one is worth it. I have some doubts about what the water may have done to the little coils.

Larry

LGriffin, I figure I can't hurt it so I am going to keep going.

Weeds

Good Luck!

I'm presently reverse engineering a 86 Class A RV.

Larry

If you can find an old voltmeter with that type of meter movement, or any other galvanometer movement, you may be able to disassemble the tach movement and replace the tach coil and axle with that from the voltmeter/galvanometer. You have to check the coil resistance to get something pretty close, but I think you can find one and make it work. The risk that remains is that the bearings will be badly corroded, so you may not get smooth needle movement, even by replacing the coil and axles. But the existing axles may be hard enough that they can be cleaned up. If you are good at soldering and have steady hands, you can probably make it work.

Harvester

You came up with the name "D'arsonval Movement", so you already have a pretty good idea of what you have there. There is a permanent magnet in there and when current flows through the coil winding, the two magnetic fields push against each other and the armature turns and the needle with it.
I expect that there is a tiny delicate coil spring on the shaft or even one on both ends. A clock repair person might have an easier time working on it.
I wonder if a sonic cleaner would clean it up without damage.

I have emailed you the wiring diagrams of the corvair tachs. I would guess that they shared the same circuitry. The Corvair used a 1 milAmp meter movement.

Richard, I have received the Corvair schematic. I have printed it and will save it in case I decide to go that route. Thanks

Weeds