84 928 Tachometer Non-functional

[GabrielS]

Hi to All!
After years of operating correctly, the tach suddenly stopped working. Checked all fuses and all appeared fine. I'm hoping the instrument panel doesn't have to be removed. Looking at the schematic in the factory repair manual is difficult making heads or tails where it leads from the distributor. Any clues of what might cause the issue and clarification on the tach wiring would be appreciated.

[JK McDonald]

Hey Gab - On "Miss Purdy" I've found that most gauge issues simply needed the edge connectors and/or the ribbon circuitry cleaned a bit. There are a couple of good Rennlist write-ups on "How To" clean the connector plugs/printed circuit traces for a 1984 instrument panel. Here's one -

https://rennlist.com/forums/928-forum/706384-instrument-cluster-and-central-warning-unit-with-color-wires.html

The tachometer signal originates from the Ignition Control Unit (passengers inner fender well - near radiator hose). Then to the sideways center black edge connector in the photo. Next it heads to the 3 brass terminal nuts (B+/Grd/Tach Input Signal - shown in the center of instrument panel). You might try very gently reaching up behind the instrument panel with a long screwdriver and "SLIGHTLY" moving the black connector plug. Make sure to disconnect the battery for safety and be careful not to pull the edge connector completely off. If you are a lucky duck, re-seating the connector plug will be enough to reestablish your tach signal. If this doesn't help - the instrument pod may have to come out to clean/tighten the 3 brass nuts a bit.

Good Luck- Michael

[GabrielS]

Hi Michael!
Thank you so much for your reply and the picture, they will aid me greatly. Will attempt after recovering from a mean sinus infection which greatly interfered with cognition of anything, especially the reading of a schematic.
Cheers!
Gabriel

[JK McDonald]

Hey Gab - In 1984 Porsche changed the 928 wiring schematic format to a confusing hodgepodge of seemingly unrelated and almost untrackable reference points. The previous year schematic format to 1983 are clearly laid out, concise, logical and each circuit is very easy to follow from page to page. The great part is that the 1983 schematics can be used for our 1984 cars...

If you don't have a copy of the "928 Maintenance Manuals" on CD's get a copy. Then print out a few sets of the 1983 wiring schematics in a large easy to read 14" x 8" format. (Keep one set back as an unused reference copy and at least one as a working copy). My working copy has all my notes, comments, Pelican Parts member's discussion info all over them and are about worn out.

Good Luck - Michael

[GabrielS]

Great info to know Michael, I'll look into it. Thanks Again!
Gabriel

[chriswpman]

last month took my '87 928 s4 into a Porsche specialist with a power steering fluid leak. they replaced the hoses and cleaned it up. the next day I noticed the tach wasn't working, and after a short drive also noticed that the odometer wasn't working also along with a couple of the warning lights on the tach side of the cluster. The Porsche guy is a long way to travel hoping they just knocked off a connection or 2. Any Ideas???

[honerboys]

hmmm....don't know what they'd be doing under the dash while fixing P/S leak - coincidence? - check fuses - not sure if the 87's still had those ribbon connectors to gauge cluster - check those also

[stepson]

Ribbon connectors from the harness to the instruments were used through all production years.
Latched connectors replaced the press on connectors to the foil in 1985.
Foil circuits were finally replaced with circuit boards beginning with 1989 model year.

[Danglerb]

What I have done from time to time when cluster stuff isn't working, release the lock and gently bonk the pod up and down to the limit of travel.

[GabrielS]

Originally Posted by Danglerb

What I have done from time to time when cluster stuff isn't working, release the lock and gently bonk the pod up and down to the limit of travel.

I'll give it a try. The easiest and simplest way of dealing with the issue, thank you!