Tach issues

[Koizumi]

Intermittent Tach. I’ve taken it apart and cleaned up the guts with deoxit, scraped contacts clean etc.

My question is… If the shift arrow light works, does it mean the unit is receiving a working signal? I’ve fiddled with the black/purple wire but it doesn’t seem to make any difference. The Tachometer needle works sometimes and sometimes it’s dead.

If the unit is getting a signal due to shift light working, can I rule out external connection issues? Wouldn’t this lead to bad or crumbling solder joints? Or a wonky capacitor? Not sure since the tach needle sometimes works.

Signals and other functions work fine. Voltmeter indicates car runs at 14.2v-14.4v which rules out the VR. No light dimming etc.

[zuch]

There are two different wires going from the DME to the tach. The black/purple is for the tach needle. The black wire is for the upshift indicator. When the needle isn't working, tap on the glass on the front of the tach. Does it move now? Any chance you have an osilloscope? There are a few posts on here about what to look for. I believe the secret decoder is freq * 20 = RPM.

[wazzz]

Yep, seems correct for the ignition signal. Three ignition pulses per engine rotation.

And for the upshift signal, I think it should be Upshift signal freq * 60 = RPM

The upshift signal is derived from the injector drive signal. One injection pulse per engine rotation.

[mysocal911]

Originally Posted by Koizumi

Intermittent Tach. I’ve taken it apart and cleaned up the guts with deoxit, scraped contacts clean etc.

My question is… If the shift arrow light works, does it mean the unit is receiving a working signal? I’ve fiddled with the black/purple wire but it doesn’t seem to make any difference. The Tachometer needle works sometimes and sometimes it’s dead.

If the unit is getting a signal due to shift light working, can I rule out external connection issues? Wouldn’t this lead to bad or crumbling solder joints? Or a wonky capacitor? Not sure since the tach needle sometimes works.

Signals and other functions work fine. Voltmeter indicates car runs at 14.2v-14.4v which rules out the VR. No light dimming etc.

Remember, there's an analog (tach) and a separate digital (upshift) electronics section in the tach, either can fail not affecting the other.

[Koizumi]

Many thanks, gents. I appreciate the wise advice and troubleshooting methodologies. I don’t have a scope but always needed an excuse to buy one lol

I took the unit apart again and inspected the joints, can’t isolate any cold joints or continuity fractures.

The tach works for a minute then stops. It’s mostly not working.

I am not one to give up but this might be the one time I have to defer to an outside repair.

[Koizumi]

Can anyone recommend a rebuilder?

I live close to North Hollywood Speedometer but they’ve really gone up in price.

Cheers

[Koizumi]

After signal continuity testing…

At the Tach:
#2 pin black/violet wire
#6 pin brown wire
I get 8.6v at idle, slightly less with throttle

Conclusion: wiring intact and volts detected.

Must be capacitor(s) in tachometer!

Tachometer works upon start-up, for 2-3 minutes until it warms up, then shuts off.

Repair time. Any suggestions?

[GH85Carrera]

Unless you want an 100% original look, ditch the pointless shift light, and have a volt meter installed when you send it to a rebuilder.

Not only does it show you voltage, it is a very visual warning to a broken fan belt and no charging. The volt meter is just great to have right there in front of you. Super easy to wire up.

The silly shift light was one a federal mandated way to show drivers how to save fuel. I know when to shift, and that shift light was just stupid.

[Koizumi]

Wholly agree with your recommendation!

I have a voltmeter I installed a few years ago, it saved me from alternator failure.

I might have the tach modded as this was always the solution I wanted.

Cheers

[brighton911]

Koizumi, looks like you are handy so I suggest going this route, it is still working 100% for me. https://forums.pelicanparts.com/porsche-911-technical-forum/1058070-tach-repair-new-easy-way.html

[Correyyt]

The tach is getting signal, but the gauge itself has an internal issue. The most bang-for-buck fix is to reflow all the solder joints on the tach board, especially where the connector pins and big components mount.

[Koizumi]

I gave it another shot but failed miserably lol. Must be more than cold solder joints, etc. In the meantime, I slapped in a makeshift temporary cover. I quite like what I came up with

[mysocal911]

Originally Posted by Correyyt

The tach is getting signal, but the gauge itself has an internal issue. The most bang-for-buck fix is to reflow all the solder joints on the tach board, especially where the connector pins and big components mount.

Then test the IC & the transistors.

[Koizumi]

I did test as much as I could. Might be a board trace disruption that I can’t isolate?

Either way, the tach is in good hands currently. I got instrument surgeon JBell959 working on the patient.

[Duc Hunter]

I would send it to John Bell (jbell959). He did these for me. Amazing job. My Tach needle was a little slow and he explained why, what would be required to fix it etc. Very good price, amazing quality, amazing work, and very fast. Also communicated the whole way though. As you can see he also did a wonderful job on my custom graphics request and my quad gauge (built with all Porsche gauges).

[Koizumi]

Looks great!

[evan9eleven]

+1 for John Bell (jbell on this forum.) He is the best and in SoCal.