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Discussion Starter #1
Pardon the non-Chevyness, but I don't know of anywhere better to ask.

I have an impulse type tach that hasn't worked since I got it. It's on a 66 MGB which was originally a positive ground car, but has been converted to negative ground.

The conversion involves splicing into a couple of wires and reversing two solder connections within the tach itself. I'll try to post a link.

www.mgbexperience.com/service/neg-convert.html

Another link said that a common problem with Smith tachs is a blown capacitor. I was going to check the one that usually goes bad but don't know how. Is there a way to check a capacitor other than removing and replacing?

I also think it's missing a metal piece which goes over the plastic piece which holds the loop wire in the illustration. It'd be nice to see one that's working...

Thanks.
 

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Love the way they can't seem to figure out if it's called earth or ground.
No exact way without the proper equipment. A meter can tell you if it's not dead shorted or if it's not open. The needle will rise some with the ohm meter connected to it as the capacitor tries to charge. That's no guarantee that the capacitor is really good.
 

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Discussion Starter #3
Thanks John. I know what you mean about the boot/bonnet lingo. I'm not sure whether I find it pretentious, amusing, or both.

Here's a link about the possible shot cap with a schematic. Thought you might find it interesting.

www.classictiger.com/techtips/motach.html

Maybe that should be mofotach.

I switched the resistor and green wire this morning, but I can't swear that I didn't do it 10 years ago. It looked like there was some evidence of my soldering, so it's possible I reversed the previous reversal.

There's a semi-local guy that restores English cars for a living, but he's like me in that he's forgotten a lot. He's got about 100 parts cars and might have a virgin tach I can take a peek at.
 

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Discussion Starter #5
I just Googled and found this.

www.mgcars.org.uk/MG_Elec-Tech/early-tach.html

The link doesn't work, maybe because the underscore between MG and Elec gets cancelled.

This is the best thing I've found yet. It has good pictures, and it looks like I converted the inside of the tach before, and put it back to + ground today.

Thanks, SNFU. I may try that. I'll a little nervous because I don't know anything about electronics and these tach are hard to find. I may do more damage trying to fix it.
 

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Try this link instead. They host it in the US versus the UK link.
Figure I'll end up with extra spam in the inbox because I clicked. Usually do after going on to the Brit and German sites.
http://www.british-cars.org.uk/kimber/

Instead of Google try seaching using a Brit search engine like MSN.UK. There are others but I don't remember them.
http://www.msn.co.uk
 

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What kind of measuring equipment do you have?

Easy solution it to just replace it! .5uF is you can get at Radioshack. Heck, you can even get a non-polerized one and not have to worry about it.

That guys descriptions looks fishy. Unless that transformer has some unheard of DC resistance, I see 12V normally on Q1, not 6V.

Do you have a dwell meter to check the accuracy?

the pickup of that circuit is pretty crazy, but I guess that car was built shortly after schotky invented the diode... :D
 

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Discussion Starter #8
John, the last link I posted just goes to the homepage for some reason, and not the link with the pictures. I can't find the link from the homepage, but I did bookmark it. I'm pretty sure that the tach is missing a little u-bracket for the impulse loop connector which may be important because it looks like the u-bracket would contact a couple of spades which poke out from inside the tach. I have a feeling that the bracket plays a part in picking up the impulses, but I could be wrong. I might have to take a piece of copper flashing and make one, but I'd like to see one that's complete first so I don't short something out.

Last night my son and I fooled around and did get the dash lights to work which means that the tach is grounded, so that eliminates one possibility.

Mike, I have a multi-tester, but don't really know what to do with it other than very simple things. I read that these tachs use "wet" caps which can dry out over time. Do you think 37 years is enough time to dry out? I might try to find the one rated .25uF and change it with a non-polarized one as you suggested. I'd forgotten that some are supposed to be installed a certain way.

Thanks again for the help.
 

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Discussion Starter #9
Well, I checked Radio Shack and they didn't have a capacitor close to what i might need. I switched the wires and started it, a slight movement of the needle was all. I really think it needs the metal u-bracket to work. There's a company in New York,

www.Nisonger.com

that works on Smiths gauges, mostly for Cobras, and they might have the u-bracket and knurled nut with the British thread (almost 4-40) it needs. I'll call them tomorrow.

Thanks again.
 
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