Tach not working on 914-6, Not my car unfortunately... Options

[Nogoodwithusernames]

I'm trying to be of some assistance to the guy in town who is getting his -6 put back together and troubleshooting the myriad of little issues after a restoration. We already figured out that his speedo drive got swapped with a 911 unit at some point so he has the correct one on the way.

The issue at hand is tachometer isn't working. All gauges were rebuilt. I can't remember who he said did them but I recognized the name as a reputable place.

The tach has the purple and black wire hooked up to it, but there does not seem to be a purple and black wire back at the coil. Just the main wire to the distributor, and one wire on each terminal.

Any ideas of what to look for and check? Are we missing something obvious?

[sixnotfour]

@ClayPerrine wrote on Pelican yrs ago,
The 911 tach was originally designed for a standard Points and Coil ignition. The 914/6 used the same tach as the 911. But in 69, Porsche added the CDI ignition to the 911 to prevent plug fouling. That same change was added to the 914/6. Problem is, the tach is looking for the relatively high (12V) signal from the standard point ignition, not the millivolt signal that is being run through the points in a CDI system. So a "tach adapter" or "tach amplifier" was added to the circuit."

I don't have a real picture, but I have a shot from the PET5 program.
it is #11. Part number 901 602 611 00. Official name is the "switch unit BHKZ". It mounts on the 914/6 relay board. "

[Nogoodwithusernames]

Will check for that and make sure it's in place, thanks for the tip! He is running a 123 distributor if that makes any difference.

[ClayPerrine]

The 70 914-6 used a tach adapter, same as the 911. In 71, they changed it so the tach adapter was no longer needed.


Are you running the original CD box with the 123 distributor? If you are, and you have a 70 914-6, you will need a tach amplifier. The trigger voltage to the CD box is too low for the tach in a 70. Summit racing has tach adapters that will work. The MSD one I have used before. You just have to wire it into the relay board.

Or find a 71 and later 911/914-6 tach. Then all you need is a jumper wire on the relay board for the tach wiring.

Hope that helps!

Clay

[Nogoodwithusernames]

I stopped by his house on the way home from work to borrow his 700 tech tips book, we poked around a bit more on the -6 while I was there. The tach adapter is in place on the relay board. I forgot to ask the year and if he's got CDI in place currently or not.

I think next step will be trace the tach wire and check resistance make sure that side of things is good. Is the routing for that wire the same as on a -4?

[ClayPerrine]

The tach adapters were just small coils. And they are prone to dying.

I have three of them, and none of them work. I converted the innards of the tach on our 70 six to a later tach and took the tach adapter apart and put a jumper wire inside so it LOOKS good, but does nothing.

[Nogoodwithusernames]

The tach adapters were just small coils. And they are prone to dying.

I have three of them, and none of them work. I converted the innards of the tach on our 70 six to a later tach and took the tach adapter apart and put a jumper wire inside so it LOOKS good, but does nothing.

Do you have specs on the tach adapter so we can test it and see if it is good? If bad possibly gut it and shove one from summit inside the housing.

[ClayPerrine]

The tach adapters were just small coils. And they are prone to dying.

I have three of them, and none of them work. I converted the innards of the tach on our 70 six to a later tach and took the tach adapter apart and put a jumper wire inside so it LOOKS good, but does nothing.

Do you have specs on the tach adapter so we can test it and see if it is good? If bad possibly gut it and shove one from summit inside the housing.

Nope.. I do not have the specs. And the tach adapters from summit racing are way too big to fit in the tach amplifier housing. It is only about the diameter of a quarter and about 3 inches long.

You are better off getting a 71 and later tach.

[930cabman]

Dumb luck

I sourced a 911 tach somewhere, runs great with a MSD 6A and Pertronix

No tach adapter necessary

[Nogoodwithusernames]

We will check the PN on the tach and see what year it is.