Tach not working after engine drop an install
 

I got my engine back in my car and got it fired after replacing head studs but my tach isn't working and it worked before I dropped the engine. I had to miss a wire somewhere. The connector in the tunnel had a broken wire that I repaired. I'm going to go back through my intake runner grounds and make sure all are making contact and I already checked the ground strap from the transmission to chassis.

Any suggestions?

first suggestion is to state the year.....of the car

Sorry, it's an 85. Not sure how much difference that makes when it comes to the tach other than my tach wiring works differently than the early year cars. I guess I figured the 3.2L was a giveaway for 85 or newer. Didn't consider transplants but figured transplants would be using motoronic.

DME plug, 14 pin plug solidly connected?

Check it as timmy2 says. Plug under the seat, near the DME box. Tach wire runs from there up to the back of the tach.

Have to recheck my connection to the DME. I might not have it squarely connected. I'll let you know if a few minutes.

Thanks for the suggestions.

Just checked, connectors are all tight. Noticed when I fired it to check, when I shut the car down the tach jumped a little. Don't know if that helps or not.

Yes do both, reseat the LARGE DME connection as well as the 14 pin mentioned.

No joy. I unplugged the Main harness and plugged back in. I also cleaned all the grounds back at the intake runner. Tach still jumping when I shut the engine off. It seems like I have to be missing something obvious since it worked 3 weeks ago when the engine came out for the head stud replacement.

I'm going to check the connection on my speed sensor. If it checks good, I'm going to verify the correct gap again. I think the speed sensor is what tells the ECM how fast the engine is turning.

If speed sensor didn't work properly, AFIK engine would NEVER start or run!

Cabmando
Really interested in this because my '76 2.7L is doing EXACTLY same thing. Tach worked before engine drop, no work after engine install, Tach needle jumps a slight bit when I turn the engine off.....I've hear ground strap betweem tranny and body, I just havent gotten under there. My strap is really frayed.

I'm going to get under my car this evening and check that strap. I know I tightened it but I'm going to recheck. I was thinking there was nothing I unplugged that would effect the tach but there has to be some connector that sends signal back to the Motronic for it to process the signal. I'm going to go over the wiring schematics and recheck all my connectors. It has to be something simple.

Think about it for a moment, on earlier cars with CDI ignition, the tach signal is developed IN the CDI unit. Same for DME, as the "CDI" is contained within the DME box. The signal is not driven by any sensor on the engine.
The tach pulses are sent from the DME on DME connector pins #4 and #11. #4 blk/violet wire, #11 blk wire. These wires run direct to the rear off the tach. Pull the DME connecter loose and check their continuity with a multimeter to the rear of the tach.

Good suggestion, I'll give it a try if I get time tomorrow. Darn work is going to get in the way of me working on my car!

FWIW I got under the car and checked the ground strap at from the trans to chassis and all was good. I cleaned it and reconnected. Still no tach.

Pin 21 blk/violet wire is the ignition signal to the tach.
Pin 11 blk wire is the fuel injector pulse widths for the CASIS light.

As per Bentley 240-12 (Fuel Injection)

http://forums.pelicanparts.com/uploa...1205355264.jpg

Above post ^^^^^^^ is why the Bentley manual scares me; it suggests pin 11 and pin 21 are identical. That is far from true. More likely the authors didn't know which pin does what.

Rick has it right. Pin 21 puts out a pulse for each spark event. This is used by the tachometer to display RPM. Pin 11 carries the fuel pulse length. The duty cycle of the signal is evaluated in the tachometer to decide when to turn on the shift light.

Further, if you mix these wire up when connecting the tachometer you will inevitably destroy your DME. First the car will run lousy and next it will quit altogether with no fuel. The reason for this is that the signal on pin 11 comes directly from the fuel driver. And when you hook that wire up to the tachometer input the pull-up resistor of the tachometer will drive current into the DME and destroy the fuel driver. This is an expensive repair.

Ingo

Sorry for a bit of miss information I gave on pin assignments, but the pic above is not true for all Carrera years!

I was looking at ACTUAL wiring diagrams in the Bentley, 970-30 '88-'89 DME does use
pins 4 and 11 for the tach. I just looked at it and assumed all years of DME were the same.(my bad to assume) Pg 970-26 shows the '84-'85 Carrera diagram, and indeed it uses pins #21 and # 11 to the tach. As the OP has an '85, pins 21 and 11 it is.
Obviously the '84-'87 DMEs are different than the '88-'89, at least for the tach signal.
Didn't know that, or is the Bentley possibly in error?

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

Ingo, I agree that the picture used could lead to mis-wiring, however the wiring diagram on 970-26 shows the wire colors #21 blk/violet and #11 blk. The #21 blk/violet is the only one needed the run the tach. The black
wire #11 can be disconnected if you don't require the upshift light (anybody really need it?)

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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?


http://forums.pelicanparts.com/uploa...1434810018.jpg

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.

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

http://forums.pelicanparts.com/uploa...1434813766.jpg

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.

Oh just one can't hurt :D

http://forums.pelicanparts.com/uploa...1434818655.jpg

LMFT4U :D:):D

http://forums.pelicanparts.com/uploa...1434830001.jpg

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