Tach Not Working

[porsche0nut]

Hey All,

Long story short, I replaced the battery in my '86 coupe because the alternator was over-charging it and essentially destroyed it (alternator has also since been replaced). As soon as the new battery was installed, my tachometer stopped working. The needle doesn't move at all. The eco-shift light still comes on, as does the high-beam indicator, but no tach. I don't know if it's a direct result of replacing the battery (is there a wire near the battery area I could have pulled?).

I haven't had a chance to swap in a working tach from a friend yet, but wanted to check here and see if any of the more experienced owners had any insight into simple things I should check (ground wires?) before I dig deeper. I've checked the main fuse box and they all look good - are there any other fuses hiding in the car that may have gone?

Thank-you!
-Jonathan

PS: Since I'm a new poster, I figure a picture is in order...

[Porboynz]

I guess that its either a coincidence or that overcharging voltage worked its black magic on a capacitor inside the tach and once you got your voltage back in spec the tach stopped working. Be happy the high voltage did not take out anything else. If the lights are functioning in the tach, the earth is good. There is a single black purple wire that feeds the tach with impulses from the distributor on the older cars, but maybe its some of that new fangled electronics on an 86? The swap with a friend is a good idea.

[Dave Colangelo]

if you have access of an oscilloscope i would check the tach input before putting a known good tack in there. If the cause of the tach fry is still present you may burn out your friends tach as well. I think you should be looking for a 12V P-P square wave but im not sure on the frequency. I'm going to ball park it at some where less than 500 Hz this is only because i have bench tested my speedometer and know that it operates between 0 and about 320Hz. I say try and scope it and see what you get then you can determine if the tach is bad. That being said if you have access to a signal generator hit the tach with a signal and see if you get a reading on it.

Regards
Dave

[unclebilly]

10 bucks says bad ground on your gauges or the ground strap on your engine or the yellow (?) wire that connects to your coil.

Check in that order.

[SaltWaterDog]

Hello,
From reading PorscheOnut tach not working post, I have the identical problem,
Replaced the battery after the car had been sitting for a couple years with very little use. Now with the new battery, the tach stopping working.
What was the solution to the problem you had with your tach?

thanks

[rw229]

Any updates on this? I'm facing the same issue... replaced battery and now no tach. (but all the lights are working)

[theiceman]

Jonathans problem was the tach Itself. I am the guy who loaned him the other tach. As soon as he showed me the shift light came on right where it should I new it was the tach as it was obviously receiving signal.

His repaired tach fixed it.

Those over voltages are killers.

[danci1973]

Sorry to drag this old thread up, but I need some help with a non-working tachomter on a 1984 911 Targa (3.0 150kW).

So far I haven't seen the shift light, but without the tachometer I wasn't pushing the revs. Where is the RPM signal picked up on this model?

There is a little yellow / orange triangle bellow the center of the tachometer that comes up when there's little load on the engine - does this use the RPM signal?


Thank you,

Danilo

[darrin]

Years ago, my tach failed after I let an unidentified overcharging situation go on for too long. Overcharging manifest itself through "dimming" headlights (actually they were over-bright when the alternator/regulator was overcharging my battery) and random illumination of my seatbelt warning light. Apparently, the tach didn't like being fed the high voltage (over 15 v) being sent to it.

Had to send my tach off to north hollywood speedometer to have it rebuilt.

Danci1973 -- any signs/symptoms of overcharging?

[danci1973]

I don't think so, but I'm going to check that with a DVM later this afternoon...

If anything, the lights seem dim / yellow compared to my daily driver (it's not HID or LED ).

[cabmandone]

Originally Posted by danci1973

Sorry to drag this old thread up, but I need some help with a non-working tachomter on a 1984 911 Targa (3.0 150kW).

So far I haven't seen the shift light, but without the tachometer I wasn't pushing the revs. Where is the RPM signal picked up on this model?

There is a little yellow / orange triangle bellow the center of the tachometer that comes up when there's little load on the engine - does this use the RPM signal?


Thank you,

Danilo

I recently had a tach issue after dropping my engine. Turned out a plug connector in the trunk near the fill neck for the fuel was loose. It had a black/purple wire IIRC that sends the tach signal. Mine is a 85 with a 3.2. I think I recall reading that the 3.0 got tach signal from the coil.

[danci1973]

Originally Posted by darrin

Years ago, my tach failed after I let an unidentified overcharging situation go on for too long. Overcharging manifest itself through "dimming" headlights (actually they were over-bright when the alternator/regulator was overcharging my battery) and random illumination of my seatbelt warning light. Apparently, the tach didn't like being fed the high voltage (over 15 v) being sent to it.

Had to send my tach off to north hollywood speedometer to have it rebuilt.

Danci1973 -- any signs/symptoms of overcharging?

I checked the charging voltage and it was 13.4-13.5V. I left the engine run for a while to get the battery topped up in case it only plays up when it's full, but it was rock-solid at 13.4-13.5V - even with higher RPM...

Isn't 13.4 a bit low (at least in modern cars it's usually higher - ~13.8V).

Originally Posted by cabmando

I recently had a tach issue after dropping my engine. Turned out a plug connector in the trunk near the fill neck for the fuel was loose. It had a black/purple wire IIRC that sends the tach signal. Mine is a 85 with a 3.2. I think I recall reading that the 3.0 got tach signal from the coil.

That's good info as I've been told the coil has been removed (or replaced or something) recently.

Thank you.

[Raypratojr]

84 Carrera. To make a long story short, my car died on me earlier this year. Alternator wasn't charging. I pulled the alternator out to find a reputable Porsche shop had shaved the voltage regulator to fit the white cone (incorrectly) on the back of the alternator. I cleaned everything up and put it back together correctly. The alternator now feeds 13.8v to the battery.

I don't recall if my tach was acting up before or after the break down. The tach began dropping to 0 then bouncing back and forth until it ultimately just stopped working. Any ideas would be appreciated.

[stuart chase]

I have joined the tach club. My alternator was overcharging, Relaced the alternator with new, tach stopped just after that. Sent Tach to North Hollywood speedometer. They did something, you never know what, but works on their bench, Reinstalled still not working, tried my ECU in another 88, ECU was good in another car, back in my car, no tach.
As a side note, North Hollywood added a volt meter also on bottom of tach face

Out of check this, check that, anyone found the fix??

[Walt Fricke]

I can't help on the tach problem directly, but Stuart seems to be the first on this discussion to note that on the 3.2s the tach signal comes from the ECU, not from the distributor (up through 1977) or the CD box (SCs), but from the ECU. Because it is the ECU, not the distributor, which fires the spark plugs on these cars. That may not make diagnosis easier, but you have to take it into account.

It is a good idea to have a manual. I have the Bentley (from the wife had an '84), and it shows that pin 11 of the ECU connects with a black wire to pin 1 on the tach, and pin 21 connects with the traditional tach signal colors of black/violet to pin 2 on the tach. It also looks like these two wires get through the fire wall via one of those under dash round plugs. So you have two wires, with three connection points each, to check out. Cabmando noted his multi-pin plug somehow got disconnected. Surely the ECU connects with a multi-pin plug which isn't apt to have broken wires. Maybe reversing the black and black/violet wires causes a problem? But continuity itself between ECU and tach should be easy enough to check.

It would be good to post what you found, as this seems to be a recurring issue.

Having a volt meter is really the key to preventing overvoltage, and hence this issue. Adding one in the tach or other stock instrument is neat, but all you need is one which plugs into the cigarette lighter. Inexpensive.

[jgurnari]

mine is a 1985 911
the issue is, when cold the tachometer does not move, sometimes stays lower than 500 RPM, just sits still. after driving for maybe 20 mins it starts working as it should. any ideas? thanks

[Walt Fricke]

This is interesting, since the tach, unlike the drive train components, isn't something you think of as warming up due to running for a while.

Your tach gets its signal from the ECU (which fires the spark plugs, and thus knows what the engine RPM is). My not likely to be helpful suggestion is to burnish all the connectors on the tach, as well as the grounds at the ends of all the brown wires on the tach, and maybe the main battery ground to the chassis as well, including its connection to the chassis. The tach itself is basically a voltmeter, with the needle's torque balanced by a spiral spring against the average voltage generated by the likely square waves the ECU puts out. Any extra resistance in any of the wire connections would result in a lower voltage, so lower reading.

If that doesn't work, well this is what speedometer shops are for. Somehow the tiny bearings which support the needle shaft might develop some stick? I know shops can deal with a needle which bounces too much by tightening up something (just what is a mystery), so maybe they have a way of cleaning, if not lubricating. Maybe just the vibrations of driving some loosen up the shaft?

All guesses. Emphasis on the "any" part of your request.

[jrwilson]

Cold solder joints.