Tach not responding
 

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.

Was the white wire located in the engine bay or near the gauge? I don't recall seeing a white wire at either point.

Is the ignition unit stock (Bosch 6-pin) or aftermarket?

Jim

In the engine bay.

Stock 14 pin.

Jimmcc I am sorry I was thinking of something else, the ICU is Bosch 6 pin.

Tach not responding
 

I tested the tach wire at the back of it with the one at the harness connection to the ICU and no continuity. Where does the tach go from the ICU harness? Is it going by the side of the door then to the dashboard or where? Or is it better to run a new BLK/PUR wire from the harness to the tach?

Tach no power
 

Could I run a new wire from the tach to the engine harness of the ICU?

Yes.

Tach
 

Does anybody have the tach and speedometer back diagram that I could use to understand more about the connections?

Tach
 

I have hooked up the coil to my digital tester and no reading on my tester. Could the car still be working and driving if I had a bad coil?

Looking at the wiring diagram the white wire from the bosch 6 pin is the coil trigger which is why it gives a tach signal. Where did he tap in to your blk/purp wire? from there to the tach is good. To answer your email the blk/pur wire goes from the ign. to the tach directly thru the harness in the tunnel so look for a break between the white wire the mech. hooked up and your ignition box if it is in the harness then you can run a new wire to the tach and the box.

You can use the sc ele. dia. on pelican it is similar to what you should have.

It goes thru the 14 pin so I would start there.

I had to do that with the switched power to the ignition its not as clean but works fine.

Tach Problem
 

Do you have a diagram of the back of the tach and also the speedometer?
On the coil is the A the positive side and - negative? If I run a white wire from the Bosch 6 pin to the positive side of the coil, we will see what happen also I will check on the iginition side there should be a BLK/PUR wire correct?

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part1-2.jpg
Look at this.

Tach Problem
 

Thank you. This is a great schematic. I could try to work out of that. I let you know.

You could check continuity with an ohmmeter for the black/violet wire. If it fails, you could run new wire temporarily to determine which wire is not working and then install it permanently.

I think I understand. BLK/PUR wire goes from the 14 pin to the Bosch 6 pin then to the iginition then to the tack?

Hope these might help

Here are some pics and resistances

At the CDI connector with it unplugged
http://farm4.static.flickr.com/3115/...b394d7f9_o.jpg
Coax to dizzy (green cable) is 570 inner to outer wire.
Brown to White (coil primary) is .7

Coil secondary (coil HT to either + or -) is 649

At the CDI box
http://farm4.static.flickr.com/3241/...94346188_o.jpg
31/1 to 31d is 0 (closed circuit)
31d to 15 is 672
31d to TD will charge briefly and then go to infinity (open circuit)
31/1 to TD also charges briefly and then goes to infinity (open circuit)

My CDI is a 930.602.702.00
Bosch number 0 227 300 004