Bouncing tach with engine off?

[djbrand1]

Stumped the group?

[4495]

I had a problem like this in a Volkswagen years ago and it was a bad ground wire from the battery to the engine block.

[djbrand1]

Since I can't get any bad readings from the alternator, I'm going to replace the ignition switch electrical part and have the tumbler assembly serviced by a locksmith. Seems like it's easier to pull the whole thing out, rather then spend an hour trying to get the top screw of the electrical switch off the back from under the dash.

[djbrand1]

Originally Posted by djbrand1

Yeah, well the engine not stopping is a new issue unfortunately. I was able to get the engine off by cycling the key a couple times and cringing while the starter engaged a couple times.

Before the engine not turning off. I was testing a DIY heater relay that I built for the autoheat controller int he back. Plugged it in and the heater fans functioned like normal, but I pulled the key and the radio went off, but the dash warning lights were on and I was still able to operate the headlights. odd. I started it up and shut it off and everything went off like normal so I took it out for a quick drive. Ran fine, minus that tach doing it's weird thing, got back, removed key and engine was still running.

Got rid of the engine not stopping with the key off issue. I replaced the ignition electrical portion without fixing it. Pulled the "rigged" relay for the heater out and viola, car shuts right off. Didn't think those circuits were related in anyway but that did it.

So even with the new ignition electric switch I am still getting a crazy tach. Would fried alternators diodes cause overcharging too? I get a flat 14.2-14.5 volts with the engine running at different revs.

[86 911 Targa]

Originally Posted by 304065

Bouncing tach with key off = fried alternator diodes

Correct!

[djbrand1]

But I have no symptoms of overcharging..... Taking it to an electrical specialist for them to test. Tried a couple different meters and they all reported a steady 14.2 volts while the tach was doing its dance.

[86 911 Targa]

Originally Posted by djbrand1

But I have no symptoms of overcharging..... Taking it to an electrical specialist for them to test. Tried a couple different meters and they all reported a steady 14.2 volts while the tach was doing its dance.

The overcharging, as per the write-up is intermittent.

My bet is still on the VR diodes.

Good luck,

Gerry

[djbrand1]

Had the alternator checked out at a local shop. They couldn't find anything wrong. It is a Bosch Reman. Put everything back in the car and I am still getting the wacky tach. While I had the alternator out, I cleaned up all the connections behind it, and the wires looked fine.

Next up, pulling the gauge and sending to Hollywood for diagnosis? Or just replace the regulator on the alternator for the hell of it?

[chicago_82sc]

My $.02.

My tach on the 1982 was intermittent and jumping around. Mechanic said it was the voltage regulator, which was repaired. Problem was fixed.

[djbrand1]

Yeah, I ordered a new regulator. I was still leaning towards it as well, and I should be able to get the alternator out and back in a lot quicker this time around. I'll let everyone know.

[djbrand1]

Well, put in the new regulator in and the tach is still flying around about 50% of the time. The shift light indicator is working correctly it seems. I don't think its a grounding issue with the gauge, unless something else is wrong with it. How often do these tachs fail?

[djbrand1]

bump

[djbrand1]

Bueller?

[rick-l]

I don't have an answer but at least I read your posts before responding.

[calling911]

Im going with the flux capacitor on this one..

Ok I dont know if your tach is like mine but if it is:

Take out the tach and measure voltage.. an analog meter is ideal for fluctuations..

You should have a ground, a 12v and a signal I think.. If you have some power somewhere (almost certain) try and track it down.. what has capacitance in it on your car, Im not sure.. find out whats all on the fuse that you are seeing the power at.. I gotta think its getting something on the 12v line.. disconnect the alternator and see what happens.. I dont get how diodes would do this.. although there could be a magnetic build up in the alternator and there is the power....


this what i would do.. Im only jumping in because of lack of response.

After re-reading.. What the heck is going on with rigged relay? this must be sorted out first..

[rick-l]

Pin 2 on the tach comes from pin 21 of the DME.

1. When the ignition coil is charging this pin presents 510 ohms to ground.
   
2. When it is not charging the coil it looks like 2000 ohms to 12 volts. I would assume this is the state with key on engine off.

You could try to duplicate condition 2 without a DME.
And while you're in there check the condition of the brown wire.

[dshepp806]

Get the tach to NHS......remove any "rigged stuff"......

Did you get a different voltage reading with the new VR?

Doyle

[calling911]

Originally Posted by dshepp806

Get the tach to NHS......remove any "rigged stuff"......

Did you get a different voltage reading with the new VR?

Doyle

Send the tach for repair? I dont know that its the tach.. How do you know its the tach? Isnt he seeing it moving when power should not be applied?

[dshepp806]

Probably a bit premature,..sorry,....in re-reading, you are right,..he's seeing the action when the ignition switch is off (as you're contending). ...a new ignition switch, at that.....sure smells like some wiring could be the issue.....time to chase and trace....

In that case,...find where the (errant) power is coming from...

Best!

Doyle

[djbrand1]

Originally Posted by calling911

After re-reading.. What the heck is going on with rigged relay? this must be sorted out first..

The rigged relay is out of the picture now, replaced with a working used one. Thanks!