3.0 Upgrade from 2.7 Tach Issues

[RNajarian]

Here is the Issue:

The Tach in my 1975 S does not work after a engine swap to a 3.0 motor.



Here is the History:

After my 1983 SC Convertible was totaled by an uninsured motorist I decided to drop the well running 3.0 motor into my 1975 Targa S.

I had a friend, well versed with Porsches do the engine swap. The '75 now runs and drives after 18 years in storage.


The Problem,

The Tachometer is not working. When my friend swapped the engine he was able to bypass the DME. My understanding is that the DME regulates the O2 (lambda) sensor in the 3.0 liter SC motor. He used the 6 pin CD unit from the 1983 motor rather than the 3 pin CD unit from the 1975.

The original Tach for the 1975 had 3 wires going into it. A ground, +12 lead, and the Tach signal (stamped TD on the back)

The Tach from the 1983 has four wires. The ground, the +12 lead, and two other leads (both marked TD on the Tach)

The wiring diagrams I found on this forum do not quite match what I find in the 1975. They indicate four wires to the tach rather than the obvious three that were originally there)



I have verified that with the engine running there is a Red wire with black spots which has 13.8 volts.

I have verified that the ground works.

I have verified that the black/purple spot wire at the tach puts out 7.4 volts, which is the same as the black/purple wire located near the CD unit in the engine bay.


Does anyone have any thoughts or advice? I want to get the car up and running for the 2010 TRE Fun Run on January 1.

Thanks Gang

[RNajarian]

UPDATE:

I was PMed by a long time Pelican (who after checking the archives has answered this question several times) who suggested I check the + terminal of the coil for the TD (tach drive)

When hooking the wires to the original 2.7 tach IT WORKED!! Unfortunately this tach is calibrated different than the 3.0 tach.

The 2.7 tach indicates a rpm of 1500 at idle (which is wrong)

Any suggestions on how to get the 3.0 tach running? There are TWO TD leads on this tach and I am stumped as to what to do.

Thanks Gang

[Ricks911s]

I've already dealt with this issue recently and can help, assuming you have a 3.0 tach. I'm going to start form the beginning since I don't know the extent of your converation with the other pelican.

Look here first - Quick Tach SC Spade Connections

Essentially you have two options.

(1) Use existing wiring from the '75 set up to connect to your '83 engine. How do you do this? You'll have to bridge the connection from the 14 pin connector (ENGINE SIDE) to the tach signal on the engine side (this runs up to the wiring behind the tach).

Engine side is pin 12. Below shows what each pin does. This helped me. ALSO A WORD OF CAUTION -- the plastic covering on the 14 pin connector is brittle and breaks easily. Take your time opening it.

Pin # Colour Purpose
1. yellow starter solenoid
2. no wire connected
3. grn/blk oil temperature sensor
4. grn/red oil pressure sensor (for oil pressure indicator)
5. grn/yel Backup light switch
6. grey Backup light switch
7. no wire connected.
8. no wire connected
9. grn/wht oil pressure switch (for idiot light)
10. red/wht power for WUR, temperature valve, supplementary air valve
11. blu (x2) from” D+” terminals on regulator and generator
12. blk/vio from ignition box, to tach (tach signal)
13. yel/blk heater fan motor power
14. red generator output.

Okay, now that you've located the signal from the engine now you have to get it to the tach. (Just for reference the 12-pin comes from the 6 pin CDI).

The purple black wire runs from the tach to the back (next to your CDI box). On the '75 engine it connects to a two-spade connection. Okay, after you locate this you're almost there. You need some extra wire to soder the wire (or use a female connection) and BRIDGE the wire to the connection to the black/purple wire that is on the right side of the CDI. Here's the last step. Before connecting back up your tach MAKE SURE you are reading the etchings above the pins. The connections are DIFFERENT on the 2.7 and 3.0 tachs. I think the positive lead or the signal are swapped. Connect everything.

Now fire the car and you should have a working tach.

(2) Run a wire from the 12 pin on the 14-pin connector all the way to the tach for signal. The positive and negative leads you can just hook up.

This is a little confusing and hard to explain. I know. I'm pulling out my engine to work on an oil leak today, so feel free to PM me. I could also explain it by phone if we're both looking at our engines. It wouldn't take longer than 5 minutes.

[bluesky]

Rick, I don't want to hijack RNajarian's thread so I'm gonna PM you.

Will this be similar with a 3.0L wokring in a 1973 911T?

[Ricks911s]

PM'd you.

[RNajarian]

UPDATE:

I'll first post my question and then there will be a detailed description of my progress.

Does Anyone know the pin out for a 1983 Tach (four wires) or a 1986 Tach 7 wires. I want to see if I can use one of these Tachs in my 3.0 Transplant.

OK HERE IS THE HISTORY

With the help of a few Pelicans I've made some definite progress.

I was able to get a tach signal directly from the + terminal on the coil. I was able to use this Tach Drive signal hook up the +12 lead, the ground lead and the original 2.7 tach WORKED beautifully. It responded to revving and seemed to work well. Unfortunately the 2.7 tach is calibrated differently than the 3.0, at idle the 2.7 tach indicates 1500 rpm (which is wrong.)

I am unable to get the 3.0 tach to respond. The tach has two TD leads and no combination seems to get this going.

I tried to get a Tach Drive signal from the 14 pin connector and at a pigtail near the CDI. NEITHER of which provided a signal which would run the 2.7 tach. I may just run a wire from the coil to the instrument cluster to get the tach drive from the coil. When my friend did the upgrade he removed the Lambda sensor and some other mods to make the engine run. (Mods to CIS, fuel distributor etc)

I've got the wiring diagrams for the 1975 and the 1983 but cannot make heads or tails of them. Any insight on how to wire up either the 1983 or 1986 would much appreciated.

[Ricks911s]

See this picture. Above where I've labeled 1a 2a 3a what is etched above the spade connection. Above my 2.7 tach it says TD. Look closely. Report back.

[RNajarian]

That looks very similar to my 1975 tach. Interesting that I also have a plastic housing which has three blade connectors, however my blank spot is number four where it appears to be the first slot in yours.

My tach indicates from left to right
TD +12 - and the blank spot

WOW you pulled your Tach to take this photo?!! I appreciate your enthusiasm. Thanks

[RNajarian]

Here is a Photo of my 2.7 tach

Left to right

TD +12 - blank

[RNajarian]

Here is an update of my problem.

After spending hours looking over schematics for the '75 and the '83 911 I could not figure out how to get the tach from the '83 SC to work. I know that the tach worked before the swap. I also tried a tach from an '86 which did not work either. However, the original tach to the 1975 2.7 liter DID work.

I pulled the tach drive directly from the + terminal of the coil.

The problem was that the 2.7 tach was calibrated differently. At idle it indicated a RPM of 1200.

Here is what I did. Using existing threads on the forum I opened up the '75 tach and changed the front face for the one in the 1986 tach. I put everything back together and everything works. The "new" tach indicates 850 RPM at idle (which is normal for this motor).

Now on to the next problem . . .

[RNajarian]

Attached are a couple of photos of my fix. I think I finally have the courage to repair my speedometer. First Photo is of the 1975 tach. The second is of the same tach with the 1986 face.

[tonykara]

I have the same issue except I would still like to keep my original 1975 tach.

I also had an engine swap from my 1975 2.7 to a 1983 3.0
What do I need to do to get my old tach working with the new engine?

[Flat6pac]

All you need to do is remove the purple/black wire out of the 14 prong connector in the 3.0 harness. Close the connector up and run a jump wire to the T shaped, 2 prong, white connector under the CD box on the longitudinal and connect it to the purple/black chassis harness tach drive wire in that connector.
Bruce

[Walt Fricke]

Bruce

When I swapped an '82 3.0 into my '77, I swapped in the 3.0's CD (which came with it). The word on the street at the time was that, because the 3.0 used a reluctor instead of points in the distributor, a 3.0 tach was needed. So I bought one. Wiring changes (some needed anyway for the swap at the 14 pin) simple.

Now you (and others) are saying the older 2.7 tach reading the points will actually work fine if driven from the same pin of the 3.0 CD box?

[Flat6pac]

Walter, I cant remember ever swapping out a tach because of the engine change. The SC tach has a RPM shut down built inside as I have been told. Having done 3.2 and 3.0 into early cars, all I ever do is jump the tach drive wire, either in the engine bay for a 3.0 or under the hood for the 3.2.
Bruce

[tadel001]

Flat6pac, I am not sure I understand your post. I have a 76 and just put in a 78 3.0 euro. Are you saying to remove the purple/black wire from the wiring harness at the 14 connector. Then have that wire connect to the T connector for the tach? Or are you saying cap off that purple/black wire from the harness and take the purple/black wire from the car (not engine harness) and connect that directly to the T harness?

I am trying to get my tach to work so any help is appreciated!

[tadel001]

Flat6pac, I am not sure I understand your post. I have a 76 and just put in a 78 3.0 euro. Are you saying to remove the purple/black wire from the wiring harness at the 14 connector. Then have that wire connect to the T connector for the tach? Or are you saying cap off that purple/black wire from the harness and take the purple/black wire from the car (not engine harness) and connect that directly to the T harness?

I am trying to get my tach to work so any help is appreciated!