Tach wiring question

[sballard]

My tach died a few weeks ago and I got the unit repaired by Palo Alto Speedometer. I hooked up the wiring according to my notes from removing my old unit. Unfortunately, the rebuilt tach is not behaving right. The needle movement is erratic. At idle, the needle is off scale at the low end and the needle is jumping around when I rev the engine. The engine stumbles and misses as well. I suppose the first thing to do is suspect that I got the wiring mixed up, but I was very careful when I took my notes. Here is a drawing of back of the tach showing how I connected the wires. The car is a 1972 911T. As far as I know, the car is stock.



The first color is the wire color while the second color is the band (ie., White/Blue is a white wire with blue banding).

Can anyone tell me if the wiring schematic would be correct for this year car?

Thanks

Steve

[Flat6pac]

If you believe you're wired correctly, which your drawing makes sense, I would check the dwell on the points and the wire on the side of the distributor for good contact.
Bruce

[timmy2]

The individual black wire bottom right shows as brown in the factory schematics.
Normally a brown Ground for all gauges in a daisy chain.
A Black wire could be power to the clock.

[sballard]

Thanks for the replies.

After looking at the Porsche wiring schematics, a friend of mine, who knows motorcycle electronics, agrees with timmy2 that the black wire on the ground plug normally should be a brown ground wire. My friend suggested using a brown wire on that plug and grounding it to the metal dashboard. I will try this tomorrow.

I am unsure what exactly the black wire is connected to. Perhaps the PO did some jury-rigging to the wiring, and I may have grabbed the wrong black wire.

[Flat6pac]

Ohm the black wire to ground to see if it goes there.
Bruce

[sballard]

Thanks Bruce. Good suggestion.

[sballard]

The black wire was not grounded.

I managed to get a wire screwed to the sheetmetal inside the dashboard and ran it to the ground connector on the tach. Same issue persists - bouncing tach needle that doesn't match the obvious RPMs. I am sure that the wire is grounded as I see continuity with a bare metal spot in the door jam.

I also see that one of the previous owners re-wired most of the wires under the dash at some point. All the wires that I can see have been spliced with crimp connectors. The crimp job is pretty poor as nearly every one I touch falls apart. Amazing that I have not had an obvious electrical problem with this until the tach went out.

I suppose my next step is to check the +12 volt and signal wires to make sure that they are at least connected at the splice points then check all the other crimp connections I can get to. What a mess.

[sballard]

I did find a black wire in the dash that is grounded - it was spliced onto a brown wire. It was probably on the tach before when everything was working.

The big problem is that the Red/Black wire to the tach (which should read +12 volts) reads 0 volts. I've checked the Red/Black wires at all other gauges and I get 0 volts with continuity to ground.

I've looked at the electrical schematic for '72 911T and it appears that Red/Black wires should connect to fuel gauge, oil temp gauge, rear window defogger switch, safety belt warning light, auxiliary starting relay, shut-off valve solenoid, and ignition starter switch. I only found Red/Black wires from the fusebox to the gauges and between the gauges.

I suppose this means that the Red/Black wires are grounded somewhere. I spent most of my day chasing this down with no success. Anyone have a suggestion(s)?