Tachometer wiring differences 2.7 and 3.0

[NeedSpace]

I have a 2.7 engine in my 1976 911s. However, the PO I believe had a 3.0 engine in the car. I thought tachs differ by year somewhat (or at least by engine). Would the owner have to rewire the tach wires to get the tach working in the older car? What would have to be done to return the wiring back? How would I know if it has been changed? Thanks in advance.

[SpeedracerIndy]

Pretty sure you will need one of these: Home

Developed and sold by a member of this board.

[304065]

Tachometer Compatibility Chart

Short answer is that the 3,0 uses a six-pin CD driving off the TD output (+5v) and the 2,7 uses a Three-pin CDI driving off the points (+11V). Both are square wave with duty cycle proportionate to dwell. (Yes dwell, measured at the trigger, not at the CDI (which has a pretty short Dwell!)

[SpeedracerIndy]

Originally Posted by 304065

Tachometer Compatibility Chart

Short answer is that the 3,0 uses a six-pin CD driving off the TD output (+5v) and the 2,7 uses a Three-pin CDI driving off the points (+11V). Both are square wave with duty cycle proportionate to dwell. (Yes dwell, measured at the trigger, not at the CDI (which has a pretty short Dwell!)

So if I understand that correctly: if you're intstalling 3.0+ tach in a 2.7 engined car, you would need to decrease the voltage to the tach down to 5v. And if you're installing a 2.7 tach in a 3.0+ engined car, you would need to increase the voltage of the signal to the tach to 11v. Is that correct? If so, is there a simple (i.e. Radio Shack) part that can be placed in series with the tach signal just before the tach to make it work correctly?

[silver_911]

Hmmh-I did an engine swap : 3.0 SC with 6 pin CDI instead of the leaking 2.7 with 3 pin CDI.
The only thing is that you need to connect your tach cable to the black/purple line (pin no.12) of the 14 pin connector (which is getting the TD signal from the 6 pin CDI via the engine harness). Works fine without any voltage conversion.
Warning: Be careful not to crack the 14 pin connector when opening - the available replacements are somehow smaller and its a PITA to fit all cables in...

jens

[NeedSpace]

Just to clarify, the PO had a 3.0 engine and a different tach in my 1976. MY car, had no engine, no CDI and no tach. The same wiring for the tach in the 3.0 is in my 2.7, that is, the black and purple line is the "tach" line and of course the hot line and ground.

So my hypothesis was true with a bit of a twist. While the tachs are different and get different voltage and have different dwell, the black and purple wire should still be connected to the same place on the tach and therefore, I shouldn't notice a difference in the wiring. The black and purple line goes to the same place regardless of 2.7 or 3.0 in the engine compartment and should not have been altered?

I am going to try the tach and the engine connecting directly to the connection points and see if that works, if could be a) my tach is broken, b) my wiring is out of wack, or c) always an option, I am just stupid.

[Wiese]

I had a similar problem. The original tach in my '73 wouldn't show anything although is was hooked up correctly. The car has a 2.7S motor. Getting a used tach from a 2.7 and just plugging it in worked great for me. You can also have the tach reworked at Hollywood Speedometer or Palo Alto Speedometer, or buy one of the adapters sold on this board (see post #2).

[NeedSpace]

OK, still not exactly sure where my purple and black wire goes in the engine bay. I have read that it goes to the 3-pin CDI, however, I see no black and purple wires here.

However, I have been mapping out the wires of my dash and I think I found my problem, thanks for the help. It appears I have the wrong tach for my purposes. You'll notice in between pos and neg is TD. I believe my tach is supposed to simply have a C according to this post Tachometer won't work



So, it appear my choices are to get the correct tach for my year or purchase the tach adapt below. Ah, choices.

[Flat6pac]

On a 2.7 the purple/black comes off the distributor.
On a 3.0 it comes off the CD box ond runs through the 14 prong connector.
When you put a 3.0 in a 2.7 body, you have to jump the tach wir out of the 14 connector and run it to a T, 2 spade connector under the CD and fuel filter.
I ve never had a problem getting the tach to work with any conversion.
Bruce

[NeedSpace]

Bruce, this could be good news, I have to figure out where my black and purple currently go to. Funny, just as you posted this, I found a listing an ebay for a 1974-1977 tachometer. Sure enough, on the back is listed TD (ok so I am thinking the seller doesn't know what he is talking about, but it is a company I have heard of and unlikely they have misposted). Then I check the part number 911-641-301-29, EXACTLY what is listed in the 1977 parts catalog.

OK, then I got thinking, what is my part number 911-641-301-03. Looks like it has a date of 12.81 on it (if that is indeed the date in blue). So what the heck, I decided to hook it up directly at the engine and see what I get.

ta-dah -


I hooked positive to power for one of three fuses, neg to the body and TD I hooked to the distributor. I was getting about 800 RPM, which is approx what I would expect. Then I took out my engine analyzer and it is coming out just under 1000 RPM (ok looks to be about 200 off but I can manually fix that). My tach is working fine, it is the black and purple wire that is screwed up. So now I'll check to make sure there is no break in the black and purple in the dash, however, how does this wire connect to the distributor? That is how can I trace the line back?

[NeedSpace]

When I trace the distributor wire back, I get to the wiring harness and I would like to avoid taking the covering off of this to trace it back. I assume there is a bundle of wires that go the length of the car. I found this connection in the left rear of the engine bay. There is one connection point that is missing a connection. Not sure if this is home made and perhaps how the PO might have connected the 3.0 lead to this engine. Is this standard? Is it possible this white/grayish wire could be what connects back to the purple/black wire? I suppose I could get a long wire from the purple and hook it up to my continuity connector in my multimeter and see...but I would prefer a better description from others if possible.

[timmy2]

That is your foglight connection for the rear foglight in europe. (Grey wire)
I would suggest you download the '78 wiring diagram and trace out your wires if you haven't done so already.
Pelican Parts - Porsche 911 Parts Listings & Diagrams

[NeedSpace]

Thanks Timmy, wishful thinking right? Unlikely the PO would use an original Porsche connector.

Ok so gathering what I can from this site, this posting and the wiring diagram, the 14 pin connection number 12 runs to the CDI unit Pin C on the 3-pin CDI, this in turn is connected to the distributor points. SO, if I put a jumper between the the distributor directly to pin 12 of the 14 pin connection, the black and purple wire should work and I can reinstall my tach. I suppose I would prefer if the wiring was working...

BTW, this wiring diagram is much better than the Bentley, at least, more what I am used to reading.

[timmy2]

Here's a picture of my working copy...
Easy to read....

[NeedSpace]

Is that an engine hoist in the background? Did you use that with the porsche or some other creation?

Originally Posted by timmy2

Here's a picture of my working copy...
Easy to read....

[NeedSpace]

I have some work to do before playing with my p-car, but I took this pic. I was going to try running the extra wire to pin 12 of the 14 pin connector straight to the distributor and see if that makes my tach work inside the car. However, if I could get this hookup up the proper way I would feel better about it. I noticed that the wire that currently goes to the distributor is stripped part of the way. Here you can see how there is some wiring exposed. At first I thought it was shielding but that doesn't feel right. Someone suggested this could be grounding wire that I need to ground. Any thoughts?

[timmy2]

Originally Posted by NeedSpace

Is that an engine hoist in the background? Did you use that with the porsche or some other creation?

Lol, yeah I did use it to lift my motor off of the jack and onto my engine stand last fall. (and back)

It's in that picture because I pulled my son's '88 Celica GT convertible motor and tranny for valve stem seals, re-gasket the motor and a new clutch install.
Took 3 weeks of evenings and weekends and was more work to R&R than the 911 was! Doesn't leak though...

Tucked back in the front part of garage for now, waiting to drop the big block 400 back into my '69 442 ragtop some day next spring!

[timmy2]

Thread Hi- jack over,
The shielded wire should always be grounded on at least one end to provide a path for those crazy stray electrons.

[Flat6pac]

You need a dual slide on pin connector at the distributor and thats where you are going to pick up the tach drive.
Bruce

[NeedSpace]

Awesome, thanks. I'll give it a shot!