Tach troubleshooting... at a roadblock and need suggestions

[LeeH]

We're working on DonDavis's '72 that has an '89 3.2 installed. The tach has never worked.

Facts:
The DME has been tried on other cars and their tach worked.
We've got a known, good tach.
We jumpered the tach's signal wire directly from the DME to the back of the tach.
12V and ground are good at the tach.
O-scope signal is inconclusive. Doesn't really look like we'd expect, but, as stated above, Don's DME made the tachs work on two different cars.

We're in the garage and open to suggestions!

[911quest]

looking at the diagram the wiring form the tach generator goes to the speedo then to the tach...I would make sure the wiring is correct on the speedo or for that mater the correct speedo...not sure if this helps.

[LeeH]

What is the "tachogenerator" and where is it located? Is it the Speed sensor on the flywheel?

[DonDavis]

Here a few pics from my Bentley page 970-82.
I don't know what a Tachogenerator is or where to find it.

The green wire goes from the speedometer to the Tachogenerator then on to the Tach at pin 3. Any thoughts?




This is the Tachometer connections.

[charleskieffner]

funny you mention this. i am going to add a volt gauge to my 87 tach. but i cant use my existing 87 3.2 tach, must use 930 tach or so i have been told. maybe that will steer you correct????????

[LeeH]

Don - Did Jeff's car have the illusive green wire going between the speedo and the tach?

[charleskieffner]

leeh congrats on your autocross 1st place. just saw it in going places. right on!

[LeeH]

Originally Posted by charleskieffner

leeh congrats on your autocross 1st place. just saw it in going places. right on!

Yep! 1st in my class and, well "only" in my class!

[911quest]

Lee does this car have the stock 72' transmission?? Cause another diagram has shows it as the speed sensor for the speedometer.

I think it is used to process the shift light.

I would try to make some jumper wires and run directy from the black/violet and black wires of the DME to the back of the tach and see if it isn't a wiring problem up to the Insturment panel you will also need a ground.

[DonDavis]

Car has a stock 72 transmission.

[DonDavis]

Anyone???

[Lorenfb]

The DME ECM provides a 0-12 volt signal which is too low for older tachs.

Bottom line: Incompatible tach which must be modified for a low level signal.

[DonDavis]

Thanks Loren, however, this engine, dme, ecu, tach, fuel pump and harness all came out of a '77 911. It was removed to go "Turbo wide body" and sold to me as a package deal. I'm convinced there's an issue with my wiring.

Pisses me off becasue I've gone over it about 19,000 times. Seriously, 19k

edit... This '89 engine, et al, was previously installed in a '77 911. Everything worked fine then.

[Lorenfb]

"this engine, dme, ecu, tach, fuel pump and harness all came out of a '77 911."

Still HAVEN'T verified that the tach received is really a low signal tach that a DME ECM can drive. Use a test light to check for +12 and ground at the tach.

A simple test is to take a coarse file which is grounded and have a jumper wire connected
to the tach input. That wire is moved on the file rapidly which simulates the ECM tach signal.

[LeeH]

Loren,

We borrowed a known working tach from an '87. All results were the same as with Don's tach.

The scope was not giving the expected results on the signal wire - despite having tested Don's DME in another car. Don put the black wire (that goes to the shift light) on the scope where we saw a squre sine wave. Out of curiosity we put the black wire to the tach in place of the signal wire. Both tachs did respond in the same way. The tachs seemed to work, but on a different scale. At idle the needle barely moved. When the engine was revved to 3-4000 RPM the need indicated a few hundred RPM.

Resistance on the signal wire from the DME to the tach was negligable, but just in case we ran a jumper wire from the output of the DME to the tach. The scope showed the same signal on the real wire and the jumper.

[ljowdy]

When I did a 3.2 conversion into my 1973 911, I sent my original 7,200 tach to North Hollywood Speedo and they converted the innards to accept the signal.

[Lorenfb]

"Don put the black wire (that goes to the shift light) on the scope where we saw a squre sine wave."

That's the upshift signal which is NOT the tach signal
and provides fuel consumption rate info.
It comes from pin 11. The tach signal comes from pin 21.

Note: If pin 11 is connected incorrectly, the DME ECM is damaged.

[DonDavis]

It's a long story how it got to this but I'll be brief.

When I first installed the engine, it would not start and I could not get spark. After much t-shooting and hand-wringing, I found what I thought was the answer. In 1988-89 the tachometer wire leaving the ECU was yellow and later changes to the normal black/violet. If anyone has the factory manuals or a Bentley check it for yourself. That was where I went wrong. Every time I went to Jim Patrick he said the same thing but I was over thinking it and couldn't get past the wire discrepancy. I had all wires going to the right places according to the '88-89 schematics.

Finally, I gave up and took the car to Patrick Motorsports and the tech began work. We looked at the schematics together and he was very confused to see the color change. My ride showed up and I left and they kept working. Later, the tech told me Jim came over and folded the schematics away and said "Just wire it the way we always do. Forget the schematics".
In the end, Jim was right and my tach works fine. Ugh!

Here's the connections for the wires running forward.

Yellow- goes to the ignition terminal #50

Black/violet- pin 2 on Tach

Black- .75 mm wire, needs terminal #15 switched 12v

Black- 1.00 mm wire, pin #3 on the Tach, it's the "upshift light" and not needed unless you've also swapped transmission and speedometer from the donor car. Don't worry about it.

Red/green- feeds the fuel pump via fuse panel 25Amp fuse #3

Red- 4.0 mm wire, positive battery terminal

Black- 2.5 mm wire, needs switched 12v via terminal #15

Green/yellow- A/C, not used

Refer to the factory or Bentley schematics to find the "# terminal" locations if needed. I chased this same issue FOREVER! The tach signal comes from pin 21 on the brain. Put an O-scope on there and you should see a varying duty cycle square wave.

[dshepp806]

Originally Posted by Lorenfb

"Don put the black wire (that goes to the shift light) on the scope where we saw a squre sine wave."

That's the upshift signal which is NOT the tach signal
and provides fuel consumption rate info.
It comes from pin 11. The tach signal comes from pin 21.

Note: If pin 11 is connected incorrectly, the DME ECM is damaged.

Good, valuable info,..kick arse!


Best,

[88911coupe]

So, where is the “tachogenerator” located?