Printable Version of Topic

Click here to view this topic in its original format

914World.com _ 914World Garage _ 914-6 tach wiring question

Posted by: Root_Werks Apr 19 2005, 11:13 AM

Someone here mentioned this situation before. That a piece was missing? I searched, couldn't find the thread:

Have a 3 pin stock CD box and one tach from a 70 911. The wiring is pretty darn simple, but the tach doesn't work. Tried another tach I had around and even another CD box. I am pretty sure all the units are working. Plug in a late 914-4 tach and it works, just off. Plug in an early 914-4 tach and it doesn't? Hmm? I am thinking I am missing something? Some sort of facorty resistor or something?

Attached thumbnail(s)

Posted by: Eddie914 Apr 19 2005, 11:31 AM

Dan,

I'll look this evening. My setup is similar other than a Permatune CD box.

What is the redline on your 911 tach?

Mine must be an early 911s tach - 7200 redline.

Eddie

ps: great job on the exhaust!

Posted by: Root_Werks Apr 19 2005, 11:36 AM

It's an 8k tach, I have two of them and I am pretty sure they both work. Redline is just below 7k on both. 911E tachs I believe?

Thanks man. I love doing work like that. Fun stuff!

Posted by: ArtechnikA Apr 19 2005, 01:05 PM

your diagram does not show the tach ballast coil that is used in the 914.6 and 911E ...

Posted by: Root_Werks Apr 19 2005, 02:07 PM

QUOTE (ArtechnikA @ Apr 19 2005, 11:05 AM)
your diagram does not show the tach ballast coil that is used in the 914.6 and 911E ...

Oo, Oo, Oo, what's that? I want one!

Seriuosly, the what? I can't believe I haven't heard of it before. I have a complete relay panel that the CD box sat on from the 911 this engine was pulled from. Would it be on there?

Posted by: ArtechnikA Apr 19 2005, 02:27 PM

901.602.601.00

911 parts list calls it "inline resistor"; 914.6 book calls it "switch unit".

it's a tach adapter. a loading coil designed to increase the current draw at the points so there will be a big enough signal for the early tachs to sense.

yes, it should be on the 911 electrical console from the 911. round, IIRC 3 pins at the bottom. i gave mine to Rusty since the one in his 914.6 died. (my AutoMeter tach in the 911 doesn't need it...) i can't recall which two pins are used - one's a spare. dofferent ohmmeter readings between them.

although it has a resistance, i suspect it's acting more like an inductor than a resistor (IOW - a plain ol' carbon resistor may not work, but a properly-sized transformer coil -which is all the MSD "tach adapters" are - probably would.

it's shown on both the 911E and 914.6 wiring diagrams; you have those, right ?

Posted by: Root_Werks Apr 19 2005, 02:52 PM

I do have the wirring diagrams at home. I am shocked I have never run into this one before. I haven't ever had an original six and probably haven't ever had a 911E tach then either.

Too cool! Thanks a bunch! I have the part number in hand and will check what I have when I get home tonight. A working tach, how cool is that!?!?

Posted by: dimitri Apr 19 2005, 04:37 PM

QUOTE (Root_Werks @ Apr 19 2005, 12:52 PM)
I do have the wirring diagrams at home. I am shocked I have never run into this one before. I haven't ever had an original six and probably haven't ever had a 911E tach then either.

Too cool! Thanks a bunch! I have the part number in hand and will check what I have when I get home tonight. A working tach, how cool is that!?!? :lol:

The original 6 I have has a special driver plug in unit for the tach.
When doing the conversion on the 4 cylinder with a 2.7 S motor, I used the 3 prong early porsche CD unit. To get the tach from a 911 going, I spliced a wire into the blue wire on the CD plug, connected a diode in line blocking the signal and attached the tach to this
diode on the other end. This blocks the signal when charging and the back emf drives the tach.
Dimitri

Posted by: Cap'n Krusty Apr 19 2005, 05:43 PM

One thing not yet mentioned: You MUST get the signal from the connector on the side of the distributor (just like it shows in the picture), or the wire to the CDI box, and NOT from the box itself (and not from the coil post). The Cap'n

Posted by: Root_Werks Apr 20 2005, 08:33 AM

I didn't have a chance to try it, but my 911 relay board has all the goodies on it and there is a couple little resistor looking things still on it. I will check part numbers tonight. I got busy with someone else's project last night. But I will post my findings here. This will be a good thread to search on for 914-6 tach troubles hopefully. I will also modify and re-post my wiring pic from above.

Thanks everyone for the help on this. I am still shocked after like 7 six conversions, I haven't run into it before.

Posted by: Root_Werks Apr 21 2005, 08:16 AM

The "switch" or "resistor" wasn't on the relay board I have. But I did figure something out last night. I have actually used a 911E tach in the past and it has worked. The reason is I had a pertronix unit installed. The signal must be strong enough for that tach to read? This engine still has points.

So I can either purchase a pertronix unit or looks like the MSD tach adapter #8910 will also work. I will post what I do and what worked here. The Porsche part number listed above is good, I just can't find it. I will try a few other suppliers this morning though.

Posted by: 914Sixer Apr 21 2005, 08:55 AM

Are you using a 914-6 relay board and wiring harness? The 6 relay board has a plug in for the ballast resitor on it. Last month I inquired about one from Porsche they said NLA.

Posted by: Root_Werks Apr 21 2005, 09:21 AM

QUOTE (914Sixer @ Apr 21 2005, 06:55 AM)
Are you using a 914-6 relay board and wiring harness? The 6 relay board has a plug in for the ballast resitor on it. Last month I inquired about one from Porsche they said NLA.

No relay board at all. 914 is a 73' with board removed. I called the local parts store. They can have the MSD 8910 tach resistor tomorrow for $27.99. Good enough for me. If it works, I will post it here. It would be nice to know were you can get a NLA part replacement, not stock, but at least a replacement.