![]() |
|
|
|
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
Tach issues
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. Last edited by Koizumi; 07-31-2025 at 07:43 PM.. |
||
![]() |
|
Registered
|
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.
|
||
![]() |
|
Registered
|
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.
__________________
Gilles RoW 88 Carrera coupé |
||
![]() |
|
Registered
Join Date: Feb 2014
Location: Lomita, CA
Posts: 2,695
|
Quote:
__________________
Dave |
||
![]() |
|
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
Tach issues
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. |
||
![]() |
|
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
Can anyone recommend a rebuilder?
I live close to North Hollywood Speedometer but they’ve really gone up in price. Cheers |
||
![]() |
|
![]() |
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
Tach issues
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? |
||
![]() |
|
Get off my lawn!
|
![]() 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.
__________________
Glen 49 Year member of the Porsche Club of America 1985 911 Carrera; 2017 Macan 1986 El Camino with Fuel Injected 350 Crate Engine My Motto: I will never be too old to have a happy childhood! |
||
![]() |
|
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
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 |
||
![]() |
|
Registered
|
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
__________________
1986 Targa Guards Red 2021 MT09 SP |
||
![]() |
|
Registered
Join Date: Aug 2025
Posts: 32
|
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.
|
||
![]() |
|
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
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
![]() ![]() |
||
![]() |
|
Registered
Join Date: Feb 2014
Location: Lomita, CA
Posts: 2,695
|
Then test the IC & the transistors.
__________________
Dave |
||
![]() |
|
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
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. |
||
![]() |
|
Registered
|
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).
![]()
__________________
Chris - Insta @chrisjbolton 1975 911s Insta: @911ratrod steel wide body, 3.6 conversion 1989 911 Carrera 25th Anniversary Ed (5th from the last car to ever leave the original Porsche factory assembly line) 2001 996 Turbo - ~54k miles Last edited by Duc Hunter; 08-20-2025 at 07:50 AM.. |
||
![]() |
|
Burnin' Rubber
Join Date: May 2011
Location: Los Angeles
Posts: 1,040
|
Looks great!
|
||
![]() |
|
Registered
|
+1 for John Bell (jbell on this forum.) He is the best and in SoCal.
__________________
1981 911SC restomod "Minerva" 2004 Boxster S 2021 Cayman GTS 4.0 manual "Olive" 2014 Cayenne GTS V8 (wife's lover) The slope is not slippery; in fact it is entirely frictionless. |
||
![]() |
|