Tach not working after engine drop an install

[uwanna]

Originally Posted by ischmitz

All NA 84 - 89 use the exact same pin-out for the DME. Pin 11 for shift light and pin 21 for RPM. Pin 4 is the starter (to turn on the fuel pump via the DME relay).

The turbo is completely different, it doesn't use a DME.

Ingo

EUREKA!! Bentley strikes again!! If you have the Bentley, look at the DME pin out on the '88-'89 Engine Mgmt wiring diagram page 970-30. Clearly shows pins 4 and 11 going to the tach!
I suspected it may be wrong, thanks for verifying it.

[JJ 911SC]

Originally Posted by uwanna

... If you have the Bentley... the '88-'89 Engine Mgmt wiring diagram page 970-30. Clearly shows pins 4 and 11 going to the tach!...

It does but for the 84-85 (pg 970-26) and 86-87 (pg 970-28) show pin 21 & 11 going to the tach, very strange.

[cabmandone]

I guess the question remains, what could I have unhooked in the engine compartment that would effect tach signal? Anything? It' seems odd that my tach stopped working when it worked when I pulled it in the shop 3 weeks ago to drop the engine.

[JJ 911SC]

Did you do a continuity check from DME connector pin 21 to the back of the the tach?

[cabmandone]

Originally Posted by JJ 911SC

Did you do a continuity check from DME connector pin 21 to the back of the the tach?

I'm about to jump in it, go get a coffee and then do some digging. When I get back I'm going to pull the tach and do some checks. I'll let you know what I find. I just have a feeling I'm chasing my tail since it stopped working when I pulled the engine and nothing inside the car itself was touched.

[Flat6pac]

The tach connects on a flat 4 wire connector.in the trunk above the gas tank , Doesn't necessarily have 4 wires. 2 small black wires, 1 black, purple stripe tach drive wire and a yellow wire.
There is nothing to drive the tach in the engine compartment of a Carreras.
Bruce

[cabmandone]

UPDATE:
No continuity between Pin 21 black/purple wire at tach
Pin 11 has continuity to what appears to be a black wire next to the black purple in the 6 pin that goes to the back of the tach.

[cabmandone]

Originally Posted by Flat6pac

The tach connects on a flat 4 wire connector.in the trunk above the gas tank , Doesn't necessarily have 4 wires. 2 small black wires, 1 black, purple stripe tach drive wire and a yellow wire.
There is nothing to drive the tach in the engine compartment of a Carreras.
Bruce

I'm not doubting you but it seems to me there has to be some wire in the engine compartment that sends a signal from the engine to the DME. How does the DME know what the engine is doing if there's no signal from the engine itself?
It would seem there would have to be a wire from the coil or the injectors telling the DME what rate they are firing at.

[JJ 911SC]

So you got continuity between pin 11 & the BLK on the back of tack which is good.

Since you got no continuity between pin 21 & the BLK/VIO (purple), you will need to trace that wire.

If you can find the connector that Flat6pac mentioned, re-do the continuity check from that connector to the back of the tach one way and to pin 21 the other way.

[uwanna]

Originally Posted by cabmando

I'm not doubting you but it seems to me there has to be some wire in the engine compartment that sends a signal from the engine to the DME. How does the DME know what the engine is doing if there's no signal from the engine itself?
It would seem there would have to be a wire from the coil or the injectors telling the DME what rate they are firing at.

The high voltage pulses (several hundred volts) that are fed to and drive the coil are developed INSIDE the DME using speed sensor/index sensor etc. The tach pulses are derived from that circuit and sent from the DME directly to the tach. Obviously, if the car runs, the speed and index sensor work AOK. Older cars with conventional ignition use the points or an output from the CDI unit to drive the tach.

[Flat6pac]

The black wire on the 4 wire connector I mentioned, there is 2. One runs to the up shift light and the other runs to the fuel pump relay under the seat. I think the yellow wire shuts down the A/C under hard acceleration.
Bruce

[uwanna]

Originally Posted by Flat6pac

The black wire on the 4 wire connector I mentioned, there is 2. One runs to the up shift light and the other runs to the fuel pump relay under the seat. I think the yellow wire shuts down the A/C under hard acceleration.
Bruce

Actually the yellow wire compensates the idle speed up a bit when AC compressor is energized so the engine runs smooth at idle.

[cabmandone]

I tested this harness to the DME and got continuity on the larger black/violet wire but not from the smaller to the tach. It would seem I have a bad wire. So should I run a small from the larger black/violet up to the terminal and the tach and test?

[cabmandone]

Oh good grief, now it shows continuity when I plug the connector in. I'm going to fire and test.

Aaaaaaaaaaaaaand we have tach again. How flaky.

[JJ 911SC]

Good. its 12:00 somewhere in the world so have one or two...

[Flat6pac]

We've all learned some things today..
Bruce

[cabmandone]

Originally Posted by Flat6pac

We've all learned some things today..
Bruce

Thanks for your help Bruce, JJ and others. I really appreciate it. I'm on to a beer or 3 but before I do I have to pull the valve covers on the right side. I have a clicking noise coming from the #6 rocker on the top side (used a stethoscope and could hear louder click at that rocker location on the valve cover) I'm going to verify the valve is adjusted right.

[cabmandone]

Oh just one can't hurt

[JJ 911SC]

Originally Posted by cabmando

Oh just one can't hurt ...

LMFT4U

[cabmandone]

Originally Posted by JJ 911SC

LMFT4U

See, I should have known better. Things were out of balance in my pic. Thanks for fixing that!