930 porsche

On my 1978 930, my tach quit a few months ago and my mechanic try troubleshooting and could not find the problem. He then hooked up a white wire with the BLK/Purple wire and worked fine since but I do not know what the white wire is for and also i rather find the problem and get the BLK/Purple working properly without depending on connecting other wire to it. What are the steps for me to do in order of getting this tach working again. I need education of where it is getting its power and how does it respond. A wiring diagram from the acual back of the tach connections to the ICU will be really helpful.

mcguff73

what was the original problem with the tach? did it just stop working or did it get eratic and jump all over the place?

930 porsche

At first it was erratic then it quit.

fiit

You can download a complete 930 shop manual including wiring diagrams from http://www.savefile.com/projects/808709228

This may help you to figure out the problem

mcguff73

on my 86 930 I had an eratic tac that the shop couldn't fix and after some research on forums like this I found that an eratic tach can be coused by a dirty signal from the alternator. My alternator seemed to charge ok, but my battery was always left on a trickle charger so it was hard to tell that the alternator had a problem. Sure enough, the alternator was bad and after it was replaced, the tack was smooth. Your shop might have changed the wiring just to hide this problem-- maybe. I would have the alternator checked and if thats not the problem, then the rest is just basic troubleshooting. find both ends of the wires, disconnect them, check resistance, run twelve volts down the wires etc, till you find the one thats bad. Its cool that you know the tack is ok so you don't have to mess with that. Good luck

930 porsche

How do you troubleshoot the tach wire. Please give me the steps.

Ferrari1

I have an 89(may be different)...but mine had a tach disconnect plug in the rear of the car by the ignition box. Ended up the the wire had seperated from the connector on one side causing no signal to get to the tach. (everything appeared to visually be ok, but a conductivity test showed a break in the circuit)

This happened when I was doing some work in the back of the car....so if your mechanic disconnected this , and by chance broke it...this could be it.


I just dont know if the older 930s used this tach disconnect in the rear.

930 porsche

No on my car the tach is part of the harness of the the CDI control unit. The wire is cut but I reconnected it and still nothing.

Ferrari1

do you have conductivity between the cut point and the wire at the tach? When I tested mine...that is what shocked me. There was no conductivity and allowed me to find the break inside the connector.

If you find conductivity....you may consider borrowing a tach to make sure yours is bad.

930 porsche

I will try that from the BLK/PUR wire on the tach and the cut wire in the engine bay. How about testing the tach without going to the ICU. Is there a way of testing with a regular battery. I am open to ideas to not hurt the ICU. Last week I hooked up the white wire to the BLK/PUR wire and shorted the ICU and I had to buy a new one. I do not want to test it or hook it up to some wire we are not sure of. Those ICU are very expensive especially the Bosh. Help me figure this out. Thank you.

Ferrari1

place your ohm meter on the two ends and make sure you dont show and open circuit between the 2. If you show the circuit is closed then.......

you either have a bad tach.(do you have a replacement tach or one you can borrow?).....or.....

the cdi box is not sending a signal

930 porsche

I ran a new wire and connected to the BLK/PUR from the ICU to the tach and nothing. Is there a relay somewhere between the tach and the ICU? Do you have the diagram for the back connection of the tach? What am I doing wrong as far as testing? No I do not have another tach.

Ferrari1

If you ran a wire from the ecu to the tach and its not working, this tells you one of 2 things.

1)the ecu is not sending out a signal or 2) the tach is broken and not receiving the signal.

At this point I would call a tach repair shop and ask for help as I can not recall how to read the tach pulse from the ecu.

This board seems kinda lame. Rarely do I see anyone willing to help. I think your question would be better received on the 911 forum. (same tach)

930 porsche

Thank you for your help. You are right this is a very slow non involved forum.