Tachometer Issue

[thunderroad]

Hello all: I have a 1974 Porsche 911 with a 1983 911sc engine in it. Purchased car about a year ago and have been restoring it. A month ago, the tachometer spontaneously stopped working. I took car to a very reliable Porsche mechanic who told me tach itself was shot. I bought a replacement tach and installed it this evening and the new tach doesn't work either! Turn signals and everything else on the tach work just not the RPM gauge. Anyone have any insight on this?

I appreciate your help.

[timmy2]

Do you know the replacement tach works?

Wiring to adapt the '83 6 pin Bosch or other CDI system possibly messed up in the engine compartment? It is the purple/black wire that carries the tach signal from the rear fuse panel area to the tach

Without knowing what you have driving the tach, (CDI type) it is difficult to say.
A number of wires need to be moved around to adapt an '83 engine harness to a '74 body harness. Was it done correctly? Has something come loose?

Pictures of what you have in the engine bay at the rear fuse panel area would help.

[thunderroad]

Pictures of what you have in the engine bay at the rear fuse panel area would help.[/QUOTE]






Thank you Timmy. I'm not the sharpest knife in the drawer, but I don't seem to see a purple/black wire thought there definitely was a purple/black wire connecting the tach inside the dash.

[timmy2]

Almost looks like a purple wire coming out of the grommet beside the 3 fuses.
Is it connected to anything?
Purple black on a '83 engine harness normally runs directly from the 6 pin CDI connector to pin 12 of the female 14 pin female connector terminal. (on end of fuse panel) it should have been intercepted and connnected to the purple/black in the fuse panel wiring.

[timmy2]

'74 wiring diagrams.
https://www.dropbox.com/sh/s292jtsyvl6llw2/AABF-ofgMfoMltGa99Asphr5a?dl=0
The purple/black to the tach is in the rear male 14 pin connector (14b) of rear fuse panel, but may be on a different pin than the '83 female connector is. I'm thinking pin 9, but not positive.
You should trace it out from back of the tach purple/black, to the rear most male 14 pin connector. (Battery disconnected) then it may just be a need to move the pins in the connectors to suit.

[thunderroad]

Cheers. I will keep you posted.

[thunderroad]

Another thought: I now have two tach's. Slightly different from each other. When I connected each of them, turn signals, high beams, etc. were all working. Everything except the RPM gauge. Does it matter which tach I use or should both work as long as I correct the wiring issue?

Thanks,

Nick

[timmy2]

I would make sure either or both of the Tachs work first.
It may not be a wiring issue if both have failed.
Only need to confirm you have continuity from the 6 pin CDI connector purple/black wire to the back of the tach.
If you do, then it is likely a problem with the tach.

[ericbash]

You seem to have a Permatune CDI box:

1 = System ground, brown P-T wire.
2 = Tachometer signal, white P-T wire.
3 = Signal ground (distributor pin B) Green wire shield, black P-T wire.
4 = Coil + Hot (coil – to ground),green.
5 = Power, red P-T wire.
6 = Signal (distributor pin A) Green wire center conductor, yellow P-T wire.

The wire colors may not match your car.



If you connect a long wire jumper from the #2 terminal directly to your tach you will eliminate any issues about broken or mistaken wiring. This may make your troubleshooting easier.

Some tachs will not work well with certain CDI systems.

Pelican sells a $67 box called a Tach-Adapt which solves this problem. I installed one in my 1975 911 and it works great. Congrats to Bob Ashlock for this great invention.

More info here:

Tachometer Adapter

[thunderroad]

Thanks all. The saga continues. I sent my tach to North Hollywood. They repaired it. Charged me $250.00. Sent back tonight. I reconnected. Still nothing doing. Seems to be a wire or connector issue? Question: I have found the No. 2 terminal in the engine. Where do I connect this wire to on the tach? Just going to wire directly and see if it works per your suggestions.

Cheers,

Nick

[ericbash]

Here are some good descriptions of the various tachometers from over the years:

Tachometer Compatibility Chart

Tachometer won't work

Complicated right?

So now that you have an expensive restored tach, maybe a protocol might be:

1. Do your troubleshooting with a $20 ebay/auto parts house tach to make sure your CDi box is throwing a good signal and to verify your wiring to the front of the car, and then:

2. Use a $67 Tach-Adapt to ensure the signal you have will drive the tach you want to use. Bob Ashlock - the engineer of the Tach-Adapt will help you if you own his box and can't get a known good tach to work.

You might want to post the model # of the tach you had repaired, so others might chime in.

[Bob Ashlock]

The Bosch 6-pin CDI TD 8V (Tach Drive) signal needs a pull-up resistor. This is normally provided within the SC tach. The earlier 74 tach does not provide this, and also expects to see a 12V signal from the points, so, it usually doesn't work mixing then together. The product I offer resolves this incompatibility.

[thunderroad]

Bob: thanks for you chat today. After our conversation, we tested the tach signal wire for continunity directly with the Tach. It showed all was well. Previously, we bypassed the CDI altogether and connected the coil directly to the Tach. Still the tach is not working.

At this point, am I left to conclude that it's an issue with the Partsklassic CDI?

Cheers,

[thunderroad]

A follow up and a kudos to my friend Bob! Bob spent 30 minutes with us tonight on the phone troubleshooting. Ultimately, per his advice, we removed the ************ CDI and put in the original Bosch CDI, which I was advised to replace last year. Seconds later, the tach sprang to life! What a moment! Thanks Bob for your help! Will contact ************ on Monday and hopefully they will honor the warranty on the CDI. Thanks all!