tach dropping out intermittently

jmohn · jmohn's Garage

I have a '78 SC. Just recently (last couple of days) my tach has been dropping out (needle falls to zero) on occasion. It's generally only for an instant, but on a couple instances it lasted for several seconds. It's not bouncing eratically, which would indicate voltage fluctuations. My first guess is I have a marginal connection in the tach signal wire somewhere between the CDI unit and the tach. I intend to explore this possiblity this evening. If anyone has other or better suggestions I'd appreciate their input.

I don't know if there's any relationship, but, for as long as I've owned it (7 years) if the dash and interior is hot (car sitting in the sun for several hours) quite often the tach won't function until the car has been moving for several minutes and the interior has cooled somewhat.

Thanks,

Jerry M
'78 SC

dimeified · 06-11-2009, 07:16 AM

now and then my tach bounces around, but most recently drops out now and then. It isnt the voltage regulator either, i have swapped the alternator and regulator out for a different one and it still happens. I need to check my grounds and my connections at the tach. i would check ground connections if i were you.

Hetmann · 06-11-2009, 07:22 AM

I have a 78 SC and it does the exact same thing. Tach cuts out when hot. Works again when cool. Sometimes the needle sticks against lens. A tap will get it going sometimes, sometimes it won't. It's been doing this for the 13 years I've owned the car.

Need to fix that someday.

PCA7GGR · 06-11-2009, 07:31 AM

I'm having the same issues with my 10K tach. I believe that its driven by the Electromotive crank-fire module. I checked the connection at the Electromotive and need to check the connection at the tach to insure that its tight and that there is no corrosion.

Paulporsche · 06-11-2009, 07:57 AM

Mine did that a couple of days ago. I had loosened up a connection near the CD box.

I agree w/ Jerry that sometimes these things expand and stick when hot. If not a connection, I would look into that.

jmohn · jmohn's Garage

Thanks, I'll check connections at the tach and CD.

Jerry M
'78 SC

Paul Crowther · 06-11-2009, 12:05 PM

Regulator. Trust me.
How old is your alternator? If it has internal regulator you may have to replace the whole setup.

Paul

84_Carrera · 06-11-2009, 03:21 PM

Might be the VR, then again might not be. My 84's tach was doing the same thing. It became more & more common. I sent the tach out to NH Speedo, they did a custom face on it & went through it physically.

Came back, was doing the same thing. They told me must be VR, though I had done a new Alt & battery a couple of years ago. So, I replaced the VR again, even though the voltage at the batt was a-ok. Still did it. After that, I sent the tach back to them, they replaced the board or something inside, have not had a problem with it since (knock on wood).

I had them rotate the dial for me so I didn't have to look at the directional indicators at 6:00 & 12:00. Not a bad idea if you're going through the motions. I didn't want the shift point at 12:00 to be redline, but rather just before.

dimeified · 06-11-2009, 03:33 PM

84 if you dont mind me asking how much did they charge you and what was the turn around time not counting having to send it back and all.

cmonref · 06-11-2009, 03:51 PM

John Cramer's thread on tachometer compatabilities contained a reference to earlier threads that established that the SC and 84-89 Carrera tachometers read the frequency of 12v pulses and convert that to a reading on the gauge. I suspect that the exact voltage is not critical because the tach is looking for how often a pulse is detected. The problem is either in a connection at either end (or a "new" connection in the middle created by frayed wiring) or electronics in either the CDI/Motronic or the tach itself. Given the ravages of heat on electronics, the CDI/Motronic might be a good bet after physical wiring is eliminated as a cause.