Help if you can please.

I have the 3.2 ECU and a tach from a similar 911 for my conversion. I can pin out all the ECU wires. I know the tach will need to use the black/violet for the signal. I plan to splice into the existing black violet already in the original loom heading for the old tach. My question concerns the other black wire coming from the ECU to the tach. If it is there to provide +12v, then I can just plug in the original +12V and ground wires already under the dash for the original tach. But perhaps this tach or the ECU really wants to see that particular black wire from the ECU. In that case I need to run it up to the dash and tape off the 12V wire for the old tach.

I would like to make this simple, and to me reusing original wires makes sense where possible. I have read the threads but not seen an answer to this. My approach to wiring this is more integrative than most people's approaches, so I am not just making cross over 14 pin swaps. In my car the relay shelf is gone.

Thanks much, and I hope you can help.

-Warren

This pic shows the wires coming from the DME harness.
You may want to just use the wires from the org DME harness and fish them up to the tach.

I believe the Tach has to read both #11 (Black Wire) and #21 (Black/Purple) from the DME. How these wires are handled within the DME would be the question to consider if you are considering using the original 914 tack harness, etc. I think it could create problems for you.

I hope I am understanding the question correctly.

The enclosed schematic above is a very good one and I have found it to be accurate on everything except #1 pin which should be #2 pin. I have used this diagram years ago and I think that is the only item with a problem. Please correct me if I am in error.

I just extended the black/violet and the black through the tunnel, zip tied it to the existing wiring harness, and connected directly to the tach. I didn't want to cut the existing harness just in case somebody someday wanted to revert it back to a 4. That made it clean....dme direct to tach.

Thanks. As far as the black violet, it is simply a question of whether to add a little piece of new wire to extend the ecu harness into the instrument cluster or use the matching color wire previously used by the old tach as my extension wire. It runs up in the main harness right to the tach position, no longer is driven by points and already has the spade connector on it. I would splice into it at the harness, making one less wire to feed through the snorkel. The 911 wiring diagram suggests that the only thing else needed by the tach is power and ground, which is what the previous tach also needed, and is provided for at the dash. There is certainly no difficulty extending a wire, but I like to know what the components are doing, and thought it might be known.

One thing I am definitely trying to avoid is running wires out into the engine bay from the ecu in the cabin, make a splice near the old relay shelf and then run the wire back into the cabin. I find that this simple approach brings its own complications though, as some have pointed out.

Maybe you can help. I have a tach that I am pretty sure came from the donor car - perhaps an '85. Gauge says 11.85, so maybe an '86. Problem is there are only 3 tach pins to wire to. #2, 5, and 6.
Click to view attachment
That should be DME pin #21 to pin #2 on the tach, and then 12 v red to #5 and brown ground wire to pin #6. Or that matches the circuit diagram at least. However since there were a lot of interconnections and some cars came with upshift lights, my tach doesn't match most people's instructions. My biggest worry is what to do with pin 11 on the DME. Do I tuck it away, and use the existing red wire for +12v? Or is that wire supposed to be the power supply? Does it matter? My preference is to use the black and violet from the original harness spliced to its counterpart on the DME harness, and then the power and ground already in place. But with electronics you kind of want it right the first time.

Please help if'n you can!

Pin 11 - Black Wire - Upshift indicator: Use only if you don't know how to drive.

Thanks!! So... I am supposed to get one of them racing lights aimed at my eye and hook up #11 to that. In truth, I could not figure out from the diagrams what the wire actually was supposed to be doing but saw some diagrams apparently integrating the speedo, ECU and tach to come up with a light. Of course the tach I have never had the light.

This was actually holding me back, so thanks again.

My 1994 Escort Wagon that I use as my winter daily driver has a 5 speed with a shift light, but no tach. My wife's Honda van is an automatic, has a giant tach and an "eco" light that comes on when you let off the gas. I am hoping both of them are vacuum switches.

And they wonder why we like the old cars