Odd Tachometer Problem - Jumps when Ignition Turned Off

[andrew15]

For some reason, the Tachometer in my 70E jumps / bounces to 2000 rpms then back to 0 rpm when I turn the ignition off. In various testes I have found:
- Jumps to 2k when the ignition switch is turned off.
- Does this regardless of if the engine was running or only accessories were on via igniton key.
- not a tach signal problem (does it even if the signal wire is disconnected and engine not cranked over)
- Not a ground problem (hooked up a temporary ground - still jumps)

Is this a voltage spike when key is turned off? I have to admit I don't really know how this would happen.

This is really driving me nuts so any help would be appreciated!

Regards,
Andrew M

[andrew15]

And.. Its not the Tach - I switched it out with a different one - Same thing happens

[RoninLB]

Only a guess.
I think I'd have the alternator and regulator tested after pulling the external regulator noise suppressor and trying it without.

[andrew15]

Thanks Ronin - I'll check it out

Any other ideas from the Sunday crowd?

AM

[RoninLB]

this is a '75 pic. #8 is the supressor. simply unplug the unit and plug the harness directly to the regulator. It's a "maybe" fix. That supressor has caused all kinds of elec prob on external regulator systems.

[andrew15]

Well - there is no noise supressor (#8 in diagram) to foul up, so I'm back to square one. Any other ideas out there before I remove the alternator and have it tested?

AM

[RoninLB]

Originally posted by andrew15

Any other ideas

What I'm searching for in your car is voltage noise or voltage spiking. So my latest bright idea before alternator removal is to completely remove the hot feed wire from the tach. Run a feed wire directly from the battery to the tach. This should remove an unwanted prob from the feed wiring. If you do this test? Check for AC ripple First. It should be done directly at the alt but try a battery test anyway. Hook up a meter. Set the meter to read less than 1 volt AC. A reading .2v AC, maybe .1v, is the limit your alternator should read.

I've never had this prob. I'm assuming we're searching for a non routine and oddball problem.. meaning maybe the ignition sw feed to the tach is dirty from the sw or another system is also attached to the tach feed causing problems. I'm hoping the battery feed would by-pass a dirty feed or buffer the prob. good luck.

ps: check AC ripple with engine operating

[andrew15]

Thanks Ronin - I was thinking of running a direct feed as well - looks like I have a project for the evenings this week!

AM

[widebody911]

Mine does this if the oil cooler fan is running at the time I shut off the ignition. It's always done it - even back in the 2.7L days.

[RoninLB]

Originally posted by widebody911
Mine does this if the oil cooler fan is running at the time I shut off the ignition. It's always done it - even back in the 2.7L days.

great info.. thanks Thom

[andrew15]

Well - I hooked up a temp lead from + terminal of battery directly to the tachometer and there is no needle jumping when disconnected. I suspect there is something funky going on with the ignition switch. I have a spare, so I'll swap it out this weekend and see if the problem is resolved.

Regards,
Andrew M

[RoninLB]

Originally posted by andrew15
I hooked up a temp lead from + terminal of battery directly to the tachometer and there is no needle jumping when disconnected.

I suspect there is something funky going on with the ignition switch.

great.

maybe check a wiring diagram to see what else is connected to the tach feed.. like an oil psi sender, etc. I think that disconnecting such a unit would be a wham bam attempt at a fix?

ps: you may be providing the Pelican Research Foundation with hard info if you're successful with a fix.

[andrew15]

yup - tach and the gauges to the left are all on the same circut I seem to remember - I'll check tonight

AM

[RoninLB]

Originally posted by andrew15
yup - tach and the gauges to the left are all on the same circut I seem to remember - I'll check tonight

there may be a split in the feed from the ign sw at a point that can't be seen from an inspection on top next to the gauges. So maybe a wiring diagram specific to your model year?