Intermittent tach '84 3.2

[josephvman]

This morning I was driving on a particularly bumpy road and over some railroad tracks, and my tach went dead. It came back to life a few seconds later and indicates correctly, but is now dropping out intermittently. Car starts, idles, and runs perfectly, so I'm assuming the bumpy road caused a connection issue somewhere. A few weeks ago I pulled the bulbs out of the climate panel, as they had pulled out of their respective tubes and I didn't want them heating up any wires back their and have been dreading the process of getting under their to sort it out. If any of those wires were grounding out can that cause a tach issue?

Any suggestions on where to start, or just work my way back from the battery?

[0172XCarrera]

Chances are it had nothing to do with changing the lighting or a ground. There are plenty of articles or posts here on Pelican indicating that this is a relatively common or "normal" thing that occurs with the Tach's over time. I had the exact same issue in the summer of 2018 with my 84 3.2 at just under 70K miles. My mechanic confirmed what I had read here that it is relatively common and the plastic gears inside the tach become brittle over the years and can break. He just happened to have a couple of re-built tach's at his disposal because he gets a few every season. I swapped it out with a rebuilt / reconditioned one he provided and problem is gone. No re-occurance in about 1,500 miles. If you feel confident enough to do it yourself - its pretty easy and there are a couple of very reputable shops on the west coast that rebuild them. Otherwise, if you have a mechanic that works on air cooled 911's regularly, he or she should be able to hook you up. Good luck!

[josephvman]

If the gears break wouldn't it just die completely? It's dropping out maybe 10% of the time, the rest it's reading engine speed perfectly, not bouncing at all.

Originally Posted by 0172XCarrera

Chances are it had nothing to do with changing the lighting or a ground. There are plenty of articles or posts here on Pelican indicating that this is a relatively common or "normal" thing that occurs with the Tach's over time. I had the exact same issue in the summer of 2018 with my 84 3.2 at just under 70K miles. My mechanic confirmed what I had read here that it is relatively common and the plastic gears inside the tach become brittle over the years and can break. He just happened to have a couple of re-built tach's at his disposal because he gets a few every season. I swapped it out with a rebuilt / reconditioned one he provided and problem is gone. No re-occurance in about 1,500 miles. If you feel confident enough to do it yourself - its pretty easy and there are a couple of very reputable shops on the west coast that rebuild them. Otherwise, if you have a mechanic that works on air cooled 911's regularly, he or she should be able to hook you up. Good luck!

[mysocal911]

Originally Posted by 0172XCarrera

My mechanic confirmed what I had read here that it is relatively common and the plastic gears inside the tach become brittle over the years and can break.

There're no plastic gears in any Porsche (air cooled) tachs.

[jbell959]

Originally Posted by josephvman

If the gears break wouldn't it just die completely? It's dropping out maybe 10% of the time, the rest it's reading engine speed perfectly, not bouncing at all.

The plastic gear he's referring to is in the speedometer and it only effects the odometer not the speedometer.

I would guess you're first inclination of a lose connection is probably right. There are only 3 wires on the back of the tach that matter. 12V, ground and the tach signal. You can check the 12V and ground with a meter and if those are good confirm a good connection of the tach signal. On a 3.2 the tach signal comes directly from the DME.

[mysocal911]

Originally Posted by jbell959

The plastic gear he's referring to is in the speedometer and it only effects the odometer not the speedometer.

Not really! It's explicit in this statement that he's referring to a tach;

"Originally Posted by 0172XCarrera View Post, My mechanic confirmed what I had read here that it is relatively common
and the plastic gears inside the tach become brittle over the years and can break."

Unfortunately his mechanic doesn't know much about Porsche tachs.

[Keithg]

Our 84 just had same issue. $130 repair at No Hollywood shop.

[josephvman]

Are there any fusebox wires I need to check? I'm working my way backwards from the battery. Positive battery cable doesn't fit down on the terminal very far (Odyssey PC925 terminals are kind of odd) and I need to make sure that connection is solid and rule it out as the issue.

[Pedro '84 Coupe]

I sent the tach off for repair when I got mine too. It was intermittent until it finally died one day.

[Pedro '84 Coupe]

Originally Posted by josephvman

Are there any fusebox wires I need to check? I'm working my way backwards from the battery. Positive battery cable doesn't fit down on the terminal very far (Odyssey PC925 terminals are kind of odd) and I need to make sure that connection is solid and rule it out as the issue.

The tach signal comes from the DME (black and purple wire), pin 21. Sounds to me like the gauge itself needs to be looked at. When I sent mine to NHS for repair, they didn't say what was wrong with it but I suspect a bad solder joint. Do you know of anyone close by that could loan you a working 3.2 tach?

[jbell959]

Originally Posted by Pedro '84 Coupe

I sent the tach off for repair when I got mine too. It was intermittent until it finally died one day.

I would just confirm a good 12V and ground at the tach. You will spend all day starting at the battery, unless you are trying to track down other issues. If you have a good 12V and ground at the tach then just double check the black/purple wire to make sure you have a good connection. If you don't find anything external, I too would suspect a bad internal connection. Borrowing a known good tach is a good idea.

[ClickClickBoom]

Check your running voltage. Wonky alternator will crisp up your tach among other things.

[Discseven]

Originally Posted by clickclickboom

check your running voltage. Wonky alternator will crisp up your tach among other things.

+1

[0172XCarrera]

My apologies for my earlier post where my recollection of information about there being plastic gears was incorrect. That said - the changing of the tach for a rebuilt unit resulted in the problem going away. I will refrain from providing comments in the future when I don't know what I'm talking about. Sorry for any confusion I may have created.