84- tach/instrument issues/Gremlins - I tried but now need advice

[GBG1000]

84 Carrera- recent purchase.

Instruments are driving me crazy. My Tach either does nothing (0), or hits about 1/2 what it should be, or goes berserk (0-redline).
There is a small but perceptible twitch to my Druck and oil level guages also, that I suspect is related.


I have read every post I can find, and have done the following-
-Rebuilt alternator, new belt, shroud. Voltage checks out.
-tried a battery swap
-new trans-body ground.
-new battery to ground cable
-new O2 sensor
-new CHT sensor
-check speed and position sensors in accordance with bently numbers
-ditto for the coil.
- cleaned, inspected, all grounds

The service records indicate $390 was spent to rebuild tach. I asked PO about this. He reported to the shop that it still didn't work and they basically blew him off (this was after he spent $4K with them).

Does anyone have any good suggestions?? Thanks in advance.

[Targa4now]

I feel your pain!! I have been attempting to track down similar gremlins and I think my issues may be related to the ignition switch.

What do the electrical gurus think? Could switch ignition issues result in the symptoms GBG1000 is observing???

[GBG1000]

Targa-

That thought has crossed my mind too. I notice that when I shut the motor off, the tach seems to move to about 1,000 rpm for that last split second.

Hopefully we will get some good opinions or eperience-backed advice on this.

[3.2 CAB]

Have you inspected the 14 pin connector? If not, do so, and clean all of the contacts on both sides, because the gauges receive their input feed through that big connector. It does sound like it is having some contact type issues. Good luck!! Tony.

[GBG1000]

Thanks Tony- I will check that tomorrow night.
I also forgot to mention on my original post that I have also replaced the DME relay.

[super_dave0069]

Seems like we have similar tach issues. Mine started last week. Tach just reads zero though. I was going to take the dash apart and check the connection and also put a meter on the wire to see if it has juice. If so, the interconnect and DME were the only other things I was told powered it. I probably won't be able to get into it till the weekend but I'm curious to see what you find too. Good luck. I'll keep you posted if I find anything too.

[GBG1000]

Super Dave and Targa4Now

Any progress??

I have not had any luck. I Have cleaned and secured all grounds and fuses and the 14 Pin connector.

[SC-targa]

Have you checked your system voltage? When the alternator/voltage regulator fails, the tach goes crazy.

Regards,

Jerry Kroeger

[Targa4now]

No luck on my part yet. I'm taking a break from the chaos. Last year I checked system voltages and replaced the voltage regulator. I'll be systematically checking grounds, cleaning connections, and looking for shorts.

[GBG1000]

I was rescued by Tom Butler. Tom set me up with a great used Tach for a very good price.

My Tach was bad, even though PO recently shelled out over $300 for a well known local shop to take it out and send it to another local outfit to be "rebuilt".

PO said it never worked right after the shop "fixed" it. They kept his money and told him something to the effect that all 911s are like that, and its just something you have to live with.

That's why I kept thinking it was something other than the tach.

my problem is solved. I hope you other guys have success with your gremlins too.

Based on the $ervice records I have seen from the PO and the sheer number of things that left that were left undone or poorly done from the this highly regarded local shop, I will avoid it like the plague.

Originally Posted by SC-targa

Have you checked your system voltage? When the alternator/voltage regulator fails, the tach goes crazy.

Regards,

Jerry Kroeger

[dimeified]

My tach is accurate for the most part but now and then it would go nuts and fly up and bounce all over the place. At the same time, i noticed sometimes my clock wouldnt work during the day. Sometimes at night the clock gauge lights wouldnt light. That was my clue that there is a grounding problem with the tach and clock. I replaced the ground from the clock to the tach. Problem was still there, so obviously the ground problem is at the tach to whatever grounds it. After moving the tach around in and out of the dash i havnt had a problem since. Im going to remove the big rectange plug from the back of the tach, and spray contact cleaner on the plug receptacles and make sure that the contacts are nice and tight. I would try the same if i were you.

I should mention, i had replaced the voltage regulator thinking it was that when the bouncy tach started.

[4flyboy]

Great discussion guys. Mine seems to read ok but it "sticks" at zero for awhile until I get the Rpms's up or it gets jostled. I'll try the 14 pin connector for fun. Where is it located please? Do you mean the big one under the drivers seat?

Thanks,

Scott

[spuggy]

Originally Posted by 4flyboy

Great discussion guys. Mine seems to read ok but it "sticks" at zero for awhile until I get the Rpms's up or it gets jostled. I'll try the 14 pin connector for fun. Where is it located please? Do you mean the big one under the drivers seat?

No, both the big harness that runs from the dash to the engine bay and the motor wiring harness meet at the 14 pin connector in the engine bay - at least on pre-Motronic cars.

The connector is mounted on a large metal plate/open box assembly (with relays and fuses) mounted on the left-hand fender wall behind the fuel accumulator/filter (and the CDI on an SC or earlier). There may be a detachable rain deflector plate (part #20) held on with a wing-nut over it.

The relay panel is part #1 on this diagram, that rectangular hole on the back/bottom edge is where the female side of the 14 pin connector is mounted. The male plug end is attached to a harness and plugs into that.




Several recent threads on cleaning this connector.

[Targa4now]

Update -

all electrical issues rectified! Shorted alternator produced erratic 18V surges that whacked EVERYTHING out. My suggestion would be to monitor your electrical system voltages using the cigarette voltmeter to see if erratic behavior and voltage issues correlate.