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Erratic Tach

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ObiRichKanobi

ObiRichKanobi

Jedi Knight
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While driving the Spider tonight, I noticed that the tach has gotten erratic. For awhile now, while idling, it will be idling smoothly, but the tach would be bouncing in about a 200 rpm range. Tonight while driving on the freeway, I noticed about a 400rpm bounce at 2500rpm. What could be causing this? The engine was obviously not idling or running roughly. It's got the Delco ignition system. Thanks!
 
Hi Rich!
Can't be much help I fear, but for what it's worth my tach has never reacted as soon as the engine starts. It usually kicks in by the time I'm out of the driveway, but if I'm impatient a tap on the dash over the tach will get an instant response. My guess is a faulty ground in the printed circuit. A jumpy signal from the coil I think would also affect the fuel injection as I think it's the same circuit. Good Luck and let us know if you come up with anything.
 
voltage regulator is one place to start...the TR7 tach isn't mechanical (or is it?)
 
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my tach has never reacted as soon as the engine starts. It usually kicks in by the time I'm out of the driveway, but if I'm impatient a tap on the dash over the tach will get an instant response.

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Mine has never given an instant response either, although the time delay is only 5-10 seconds. I also noticed that my ammeter on the dash has a delay, about the same as the tach. Maybe it's time to tear the dash out and do some serious contact cleaning.
 
You're correct in that the 7's tach is electric. There is but one wire to it from the coil. I don't think the voltage regulator is involved, but it is British, so anything is possible. Rich's problem could be in the meter itself, but it would be best to check out all the extraneous stuff first and make sure that all connections are still connecting etc.
 
True to the Prince of Darkness' reputation, the culprit seems to have been a bad ground on the coil. Checked the wiring diagram and found out which wire was the ground. Found the bolt securing the ground wire and the coil was slightly loose, so tightened it up and went for a ride. Tach was rock steady at cruising speed, and seemed to be less erractic at idle. Thanks for the reminder to "check the simple stuff first1"
 
Hey that's Neat!
Will have to check the coil ground on mine now. Thanks for the reminder that there is never just "one" wire involved in a DC circuit as there must always be a return to "earth".
 
The old axium is "if its real wierd, check for grounding"
My best story(all true) is the brake lamps lighting up the head lamps instead.........
MD(mad dog)
 
If the tach is electic, you could have a leaking (old) capicitor. I recently replaced the .25 microfared capicitor in my 64 Healey tach, that had been misbehaving. The tach read very high and would jump all over the place. In the Healey tach (a Smith's) the .25 uF cap is a black tubular device about 5/8 inch long with leads coming out each end. A new cap at Radio Shack (I used a .25 uF 50 volt replacement) only costs $1.79 (actually you get two at that price). They are green and look similar to a tooth. Very easy to solder in the new cap if you are careful. Otherwise, you can send the tach to Nosingers or MoMa Industries for repair at about $125.
 
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