Intermittent Tach Issue

[Mechaniker]

Hi there, I'm new to pelican and Porsches but not to owning old cars. I recently purchased a '85 Carrera 3.2 that has an intermittent issue with the tachometer, and I'm looking for some input from folks who might have had a similar problem, or might know what the problem is. The car has about 95k miles.

Symptoms:
The tach usually will not work when I first start the car, the needle just rests at zero, though the shift light appears to work normally. After a minute or two it will usually start to work normally, but periodically and randomly it will stop working again over the course of a drive, sometimes for just a minute, but other times for several minutes. Again, the needle drops to zero but the shift light continues to work. One other observation is that when the tach acts up, it will appear to rest at zero at lower/cruising RPM 1-3k rpm, but if accelerating at higher rpm, maybe around 4-6k rpm, I will notice it lift slightly above 0 rpm, almost as if it's severely out of calibration.

Diagnostics so far:
Since the shift light still works continuously, it doesn't seem to be a major connection issue. I had a couple of the gauges pulled out the other day when I was tightening my wiper motor down and I checked for loose connections anyway, but nothing seemed out of order. The car has an Autothority chip, which I suspected might be the issue so I swapped in the stock chip, but the problem has persisted.

Questions:
Has anyone else had similar problem? I've read about "bouncing tach" issues attributed to failing alternators, but that doesn't seem to be the symptoms I have. What should I check next? Could the tach itself be at fault?

Thanks in advance!

[Bob Kontak]

Do double check the voltage going into your battery. It's the v-reg that usually causes the problem. If you can't check the voltage, idle your car at night and watch the headlights. If they waver in intensity, that would probably be the v-reg.

Another suggestion I have, which is worth what you are paying is to push the tach out and remove/refit the wire connectors. Could be a poor connection issue but I do not have enough experience to state if this could cause your symptoms

[Walt Fricke]

A bad battery ground will cause tach issues also.

[Mechaniker]

Ok, I'll check the voltage and grounds next.

[Mechaniker]

I checked the voltage at the battery when the tach was working and when it was acting up. In both cases, it was a pretty stable ~13.95v. The battery ground looked fine as well.

Any other suggestions? I think I'll pull the tach to take a closer look at the connections. Is there a trick for getting the tach out? I've got a fishbone to get it started but it's tight between the top of the dash pad and the steering column, and I can only get the tach out about a half an inch or so before it gets wedged.

[DRACO A5OG]

+1 check the connectors behind the tach especially the ground ( brown wire connector ). clean and polish for extra measure. tighten it needed.

Two options: push from the trunk or pull the clock or fuel gauge then stick your hand/arm in and push from behind ( it looks like you got to this part ) then grasp the tach with both hands and pull the tach out.

The rubber ring will get stuck under the dash lip's edge, push up the dash inner edge and it will slip right out, take care after it pulls out not to take the tach out too much as not to disconnect the wiring.

Also for safety, disconnect the battery negative side.

Oh yeah, welcome aboard!

[porwolf]

There was another thread here recently dealing with an intermittent tach. What happened there is that the tach was working/not-working when gears were shifted. Turned out to be an intermittent tach wire connector issue near the shifter coupling under the tunnel access panel behind the seats.

[Mechaniker]

I checked the connectors and grounds, and everything looked fine. When I pulled the tach, I heard something rolling around inside it when I was turning it around to get at the connectors. Perhaps unrelated, but I assume that's not normal...? It's looking more like I need a new tach.

[Mechaniker]

Solution: Tach rebuild.

I finally got my hands on a loaner, which worked perfectly when installed. I sent mine out to North Hollywood Speedo for rebuild and the problem has been solved.

[tirwin]

By the way, good idea to always disconnect the negative on the battery. A bunch of people have accidently shorted stuff messing with gauges. Never let the magic smoke out...

[Bob Kontak]

Originally Posted by tirwin

Never let the magic smoke out...

It's easily replaced. Ask any Englishman.



Mechaniker, thanks for closing this out. Good news.

[Nico911]

I had the very same problem with my '85 911. My mechanic sort it out : it was a small cracked weld in the tach. He had to open the tach like a can.

I hope it helps.