Tach not responding

[NathanUK]

Two more shots of the CDI plug.

[NathanUK]

Here's a shot of behind the tach.

[930 porsche]

Nathan,

Are those numbers measure while disconnected in OHM or Voltage? When you say coil primary meaning the CDI unit? And the coil secondary the actual coil? When you measure the brown to the white side do you touch and connect the BLk and RED connection of your tester? Explain. Thank you. I am starting to understand with those pictures.

[NathanUK]

The measurments are all disconnected so they are Ohms. By coil primary I mean the low voltage side of the coil which has the small tags on them and may well be marked '+' and '-'. By coil secondary I mean use one of the previous connections (+ or -) and the high voltage side of the coil which is where the HT lead goes. So to answer your question, just the coil.

I just read my earlier post and remembered there is no need to disconnect the low voltage side of the actual coil itself if you unplug the CDI. Simply remove the HT lead from the coil and place your meter across the brown and white wires in the disconnected CDI connector, put one probe on brown and the other on white (it doesn't matter which colour you use from your meter when checking resistances/Ohms).

Please ensure your ignition is switched off before unplugging the CDI or anything else etc etc.

Please feel free to ask if there is anything that is not clear.

[gsmith660]

Did you get it working?

[NathanUK]

Originally Posted by 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.

So, if your tach started to work when your wrench connected the white wire at the CDI to the BLK/Purple wire (thereby feeding the tach with a signal from the white wire rather than a signal from the CDI) then this would mean to me that your CDI is on its way out. Of course check all connections first and check the alternator & regulator are working properly.

It could be time to upgrade your CDI by replacing it with an MSD6AL or the newer more efficient one.

[930 porsche]

Nathan,

I ask my mechanic and he cannot remember which wire he hooked up the BLK/PUR with. That is why I am educating myself to do all of these things myself and I appreciate your time and effort of explaining the process. I am starting to get it. It is just the verbage that I am trying to understand. I do not want to depend on my mechanic like all of you guys. If I understood correctly the primary is the CDI harness connector end? And the secondary is the + and - on the coil? Which one is the coax that you mentioned to test with the green wire? When I test on the CDI harness connector, on the brown and white I get fluctuation between 1.2 to 5 to 140. Is this normal? On my CDI harness connector I have 2 BLK/PUR wire from the exact same location, 1 is connected and the other one cut with bare wire showing.

[930 porsche]

Here are pictures of my 1978 930 as I see it in my car. I tested th 6 pin as Nathan told me and everything seems to register just fine. The coil also is reading on my tester fine. I guest we have to go by elimination and narrow it down. We will eventually get it. I need all of you guys to pitch in. I finally got all the pictures and diagrams but they are not helping me nailing this small problem. Here it goes:

The BLK/PUR Tach wire at the 6 pin is talking to the 14 pin, But where does it go from there? I thought it went to the ignition switch but I checked it and no purple wire, only reds and 1 white. Now there is a purple wire obviously behind the tach but where is it going? On my drawing to the left behind the fuel guage there is a small round yellowish harness which has 6 wires including a purple wire which goes to a square relay which is located behind the boost guage. Also there is a purple wire at the front fuse box. The purple wire behind the tach is not talking to any of them in the front or rear of the car then where is it going and what am I missing? I know there is an easy solution but where? Help me figure this out.

[gsmith660]

Originally Posted by 930 porsche

Here are pictures of my 1978 930 as I see it in my car. I tested th 6 pin as Nathan told me and everything seems to register just fine. The coil also is reading on my tester fine. I guest we have to go by elimination and narrow it down. We will eventually get it. I need all of you guys to pitch in. I finally got all the pictures and diagrams but they are not helping me nailing this small problem. Here it goes:

The BLK/PUR Tach wire at the 6 pin is talking to the 14 pin, But where does it go from there? I thought it went to the ignition switch but I checked it and no purple wire, only reds and 1 white. Now there is a purple wire obviously behind the tach but where is it going? On my drawing to the left behind the fuel guage there is a small round yellowish harness which has 6 wires including a purple wire which goes to a square relay which is located behind the boost guage. Also there is a purple wire at the front fuse box. The purple wire behind the tach is not talking to any of them in the front or rear of the car then where is it going and what am I missing? I know there is an easy solution but where? Help me figure this out.

Wheres your pic? You have continuity from the 6 pin to the 14 pin correct now check from the 14 pin female to the tach if you have continuity just to be sure check from the CDI plug to the tach 14 pin plugged in if you have continuity then you have either a bad CDI or a bad tach. If you have a friend that has a tach you can swap it out to eliminate that component.

[930 porsche]

It is not taking any pictures and diagrams. They are all Jpeg. What am I doing wrong?

[930 porsche]

Gsmith660, I will do the testing from the 14 pin female to the Tach. If nothing what is next?

[gsmith660]

Originally Posted by 930 porsche

Gsmith660, I will do the testing from the 14 pin female to the Tach. If nothing what is next?

If you dont get continuity from the 14 pin to the tach then your problem is solved the wire is open you can order more blk/purp wire from eagle day or just get some wire from the local auto parts store and run you a new wire from the 14 pin to the tach thru the tunnel use a wire fish it will help.

[930 porsche]

Gsmith660,

I hope it is the case, I let you know.

Thank you.

[930 porsche]

Gsmith660,

How do you test the 14 pin female if not connected to the 14 pin male, or do you test it while it is connected through the wire? Are we checking the voltage?

[gsmith660]

Originally Posted by 930 porsche

Gsmith660,

How do you test the 14 pin female if not connected to the 14 pin male, or do you test it while it is connected through the wire? Are we checking the voltage?

Go ahead and plug the 14 pin back together and check continuity from the 6 pin to the tach you are checking the resistance of the wire it should read nearly 0 ohms as it is just a wire if it is open then the reading on your multimeter wont change or if you have a continuity checker use that. If it is an open wire then run a new wire.

[NathanUK]

Originally Posted by 930 porsche

Nathan,

I ask my mechanic and he cannot remember which wire he hooked up the BLK/PUR with. That is why I am educating myself to do all of these things myself and I appreciate your time and effort of explaining the process. I am starting to get it. It is just the verbage that I am trying to understand. I do not want to depend on my mechanic like all of you guys.
I agree, most of the so called P-car specialists don't know as much as all the people on these boards. Two heads are better than one as they say.

If I understood correctly the primary is the CDI harness connector end? Yes And the secondary is the + and - on the coil? No
To check the primary of the coil you don't actually need to unplug the coil itself (just remove the HT lead), because if you unplug the connector at the CDI you can then check the coil AND the wires connecting the coil to the CDI all in one operation.

The secondary is when you check one of the terminals you tested above (doesn't matter which) but this time place your other meter probe where the HT lead plugs in.

Which one is the coax that you mentioned to test with the green wire?
If you look a the pics of my CDI harness connector it is the green wire and then the black one below it. The black one is the shield so it is wrapped black to protect it.

When I test on the CDI harness connector, on the brown and white I get fluctuation between 1.2 to 5 to 140. Is this normal?
Is this with the connector unplugged from the CDI? If so then mine read 0.7 ohms. If yours does not read this then maybe someone else wouldn't mind taking a reading for us?

On my CDI harness connector I have 2 BLK/PUR wire from the exact same location, 1 is connected and the other one cut with bare wire showing.
Not sure why yours is like that, get the bare one taped up ASAP and only take your readings from the connector itself..

Answers in red.

[NathanUK]

Originally Posted by gsmith660

Go ahead and plug the 14 pin back together and check continuity from the 6 pin to the tach you are checking the resistance of the wire it should read nearly 0 ohms as it is just a wire if it is open then the reading on your multimeter wont change or if you have a continuity checker use that. If it is an open wire then run a new wire.

Yes, this should prove that you blk/purple wire is fine.

To add confusion to all of this Porsche used blk/purple for the wipers. This wire comes from the fusebox and goes to the wiper control stalk as well as other places.

The blk/purple wire from the CDI on a pre '86 930 should only go to the tach.

Actually, the factory call this wire black/violet to be exact.

[930 porsche]

I do get a reading of .18 Volt testing the tach, 6 pin and 14 pin while the engine is running. My strong suspicion is that the tach is dead. It has been mentioned earlier that there is a special tool to open the tach without damaging it. What is it and where can I buy one? Also where can I get the inside parts for the rebuilt?

[gsmith660]

Originally Posted by 930 porsche

I do get a reading of .18 Volt testing the tach, 6 pin and 14 pin while the engine is running. My strong suspicion is that the tach is dead. It has been mentioned earlier that there is a special tool to open the tach without damaging it. What is it and where can I buy one? Also where can I get the inside parts for the rebuilt?

What is the ohm (resistance) reading on the wire from the 6 pin to the tach connector?

[930 porsche]

I believe 0 OHM.