Tachometer cutting out...

[markmalin]

I did a search on the forum and found a few threads, but most were sort of general "test your alternator" kind of things. Here's the situation. My tach started cutting out intermittently (drops to zero or jumps). I put a volt meter across my battery while running the car and it's a solid 14.1 V, so I'm going to assume the alternator and voltage regulator are fine. Here's something interesting, though. If I tap on the glass it will come back. I figured it was a ground issue, so I removed the tach but I don't see anything suspect. Question:

- is it worth opening up the tach? (I've done that with my speedometer and don't want to mess up the trim ring if I don't have to.

Thanks
Mark.

[markmalin]

A little more information. I just cleaned off the contacts, re-installed the tach and started the car. Dead, but when I tapped on the outer case it started working. I wiggled the wires, the connector, etc, and nothing seemed to phase it - but if I banged (tapped) the outer shell of the tach, it would sometimes go off, and sometimes come back on. So it must be some loose connection inside. I'm going to reinstall it and I'll have to leave that for my next weekend project... (cause I just got new tires and want to drive it!)

[86 911 Targa]

Originally Posted by markmalin

A little more information. I just cleaned off the contacts, re-installed the tach and started the car. Dead, but when I tapped on the outer case it started working. I wiggled the wires, the connector, etc, and nothing seemed to phase it - but if I banged (tapped) the outer shell of the tach, it would sometimes go off, and sometimes come back on. So it must be some loose connection inside. I'm going to reinstall it and I'll have to leave that for my next weekend project... (cause I just got new tires and want to drive it!)

Here are the "inners" to a tach indicating a component failure.

You may want to re-flow the solder joints.

Hope this helps.

Gerry

[jjemg]

How did you get the bezel off without damaging it Gerry?
Is there a "how to guide" on gauge disassembly here?

Jimmy

[markmalin]

Thanks, Gerry!

Jimmy -

Regarding the bezel, if you look up info on replacing the odometer gears, there are a few good writeups. They go through how to get the bezel off. It's kind of a hair raising experience because you have to gently pry the edges up and to do it well it takes a lot of time and patience because for the longest time it won't look like it's moving.

The other scary thing (that I am not looking forward to) is pulling the needle off. I don't remember how I did that on my speedometer, but I will search for that writeup on the odo gears again for reference

Mark.

[86 911 Targa]

Originally Posted by jjemg

How did you get the bezel off without damaging it Gerry?
Is there a "how to guide" on gauge disassembly here?

Jimmy

Jimmy,

These are pics from my reference files FYI only.

I would suggest a repair shop.

Good luck,

Gerry

[86 911 Targa]

Originally Posted by markmalin

Thanks, Gerry!

Jimmy -

Regarding the bezel, if you look up info on replacing the odometer gears, there are a few good writeups. They go through how to get the bezel off. It's kind of a hair raising experience because you have to gently pry the edges up and to do it well it takes a lot of time and patience because for the longest time it won't look like it's moving.

The other scary thing (that I am not looking forward to) is pulling the needle off. I don't remember how I did that on my speedometer, but I will search for that writeup on the odo gears again for reference

Mark.

I would not try the needle removal.

It's a risk/cost decison.

For decades, here's the shop who has done work for me,
and, also for Pelican:

Paul's Speedometer Service - Redondo Beach - Redondo Beach, CA

[markmalin]

So I pulled my tach apart, but don't really see any visible signs of any issues. I did re-flow the solder. Haven't re-installed it yet to see if that helped.

For the record, here are some internal pictures of a Carrera tach.

Mark.

[86 911 Targa]

Originally Posted by markmalin

So I pulled my tach apart, but don't really see any visible signs of any issues. I did re-flow the solder. Haven't re-installed it yet to see if that helped.

For the record, here are some internal pictures of a Carrera tach.

Mark.

My second guess.

Hope this helps:

Along with cold solder joints which can appear intact,
the CKT capacitors are the "Usual" suspects.

And, most likely, your DVM will not detect voltage spikes.

Good luck,

Gerry

[rick-l]

WOW

That 40 pin IC must be the computer part of the Computer Aided Shift Indicating System (CASIS).

[86 911 Targa]

Originally Posted by jjemg

How did you get the bezel off without damaging it Gerry?


Jimmy

Not my unit.

[markmalin]

Originally Posted by 86 911 Targa

Not my unit.

I half considered documenting my effort getting that off. I've had to do this before to get at the ODO gears on the speedo.

There was a writeup on the forum somewhere (this was nearly 2 years ago), but basically you go around the edge with something very sharp and sturdy and find a place where you can insert the knife or whatever. Then with TONS of patience, you just carefully work your way around the inner lip bending it it every so slightly -- making very tiny adjustments. For the longest time you'll think you aren't moving it, but don't rush it. As it starts to open up, eventually you can insert a tiny screwdriver in there and continue going around widening the gaps. After about an hour or so (seriously) you'll have it to where the lip on the trim ring is opened up enough to nearly come off. Then you end up destroying your fingers trying to push it off, but that's part of the fun. I wound up wedging a tiny screwdriver between the edge of the can and the inside of the trim ring while pushing with my fingers. It's a lot like getting a tire off a bicycle, but requires a lot more patience and care because you don't want to scratch any of the visible surface of the ring.

Putting it back on, you can slip it into place and kind of re-crimp the edge (again without scratching anything!)

[jmohn]

I had the same issue - did the usual trouble-shooting with no help. Finally I sent it to North Hollywood, they rebuilt/refurbished it and replaced/shimmed the bezel - no problems since. Cost about $150 - well worth it in my opinion.

Jerry M
'78 SC

[markmalin]

Here's another followup....fwiw

I dismantled the tach (see pic's above) and re-flowed all the solder joints. One thing I noticed was the connector block -- the way it mounts to the board is using pins on the back that slide into receptacles. I used a dental tool to bend the tabs of the receiving ends (on the board) and tighten them up. Was thinking, a friction contact like that may have issues over the course of nearly 30 years. Anyhow, I think that may have done the trick because I re-assembled and re-installed the tach and so far it's working. Need more driving time to confirm, but I'm optimistic...

Mark.