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Happiest when Tinkering
 
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Wires good tach is bad or cd unit is bad.
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Old 08-23-2009, 05:35 PM
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930porsche
 
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Tach Problem

Gsmith660,

Then you agree the tach is bad. It is not the CDI since I just bought this one brand new. You mentioned earlier about a tool that would make it easy to open it up. Where can I get this tool and where can I buy the parts for the tach?
Old 08-23-2009, 05:48 PM
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You got the wrong person I dont agree it is the tach it could be either just cause it's new means it works I have bought way too many things lately that were brand new and didn't work and I have no idea what you are talking about a tool to open it up. Good luck.
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" Porsche there is no substitute" I always liked that saying. Air cooled is the only way to go!
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76 Blazer also restored by me
Old 08-23-2009, 05:58 PM
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930porsche
 
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Tach problem

Gsmith660,

I went to Vertex and bought a new CDI unit and they tested all 6 pin including the Tach and everything came out fine. It probably was not you that mentioned about that special tool, I may have seen them talking about it in other threads. I do not have a spare tach to plug in to see if it is bad. What else can we test or do. Is the alternator and regulator have anything to do with the tach at all?
Old 08-24-2009, 04:53 AM
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I started this conversation with you several weeks ago then my time got diverted and I'm now catching up.

You say you bought a new CDI. Is it a stock Bosch CDI or another brand?

From your description it appears the tach drive circuit has failed in the Bosch CDI. That would be rare but it could happen.

You say your mechanic hooked up the black/purple wire to something else and it ran fine. Did it fail again or were you interested in correctly repairing the problem and remove what he did and now neither you or he can remember the repair solution? I'm confused here.

Hooking the white wire from the CDI unit (pin A) to the tach would not be advisable as that is the 400 volt or so pulse to the coil which could destroy the tach. There might be some possibility of the tach working if pin 7 were connected to the black/purple wire but I'm not sure what that would do to the CDI (run or not).

I am confused about your testing the black/purple wire. You should show continuity using and ohm meter or continuity tester from the tach (wire disconnected) to the CDI harness (also disconnected). If there is a problem a test should be done between the tach and pin 12 of the 14 pin connector in the engine bay and from that pin to the CDI harness plug.

You can test the tach in the following way. The tach responds to a negative going transistion. Here is what you do. Connect a short wire or clip to a suitable ground point. Connect the ground and power wires to the tach but leave the black/purple disconnected. With the ignition on rapidly stoke the ground wire you connected to the tach pin where the black/purple wire connects. If the tach is ok you should see the tach needle jump around.

If the tach works and the black/purple wire shows continuity from the tach to the CDI plug then you should assume the CDI is bad. It can be repaired but requires someone who is knowledgable abou the unit.

Sorry to be gone.

Jim
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Old 08-24-2009, 05:38 AM
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930porsche
 
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Jimmcc,

It is stock CDI.
Old 08-24-2009, 05:42 AM
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Jimmcc,

To make sure I understand: behind the tach, I disconnect the BLK/PUR wire and hook up a power and ground wire together (can I take a existing red 12Volt from the tach?) and turn the ignition on (Am I suppose to crank the engine and leave it on or just turn the ignition on only?) then with those 2 wires together (power and ground) stroke it to the tach location.
Old 08-24-2009, 05:54 AM
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Jimmcc,

I was just re-organizing all of my wires in the engine department and disconnected the connected wire from my mechanic's hook up and I forgot which wire it was so I guessed it was the white one and hook it up to the BLK/PUR wire and I heard a clip, then I knew I most likely blew my Bosh CDI unit and maybe even the tach in which it did not start the car anymore. The following day I went and bought a new Bosh CDI unit and the car started just fine but no tach. That is my story in detail so I am pretty sure that the new CDI unit tach connector works. I did all the test with the engine of and engine running from the 6 pin to 14 pin female, and 14 pin male to the tach. I have continuity and reading so the wire is OK. I will try your new suggestion but I need you to explain to me in more detail. Thank you. I am glad you are back. There is hope to find this simple problem I am sure.
Old 08-24-2009, 06:06 AM
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Ok. Now I'm following. The post got to scattered and I couldn't follow the actual circumstances.

From your description I would quess as I stated in my last post that the tach drive circuit failed in the original CDI. That is difficult to understand as that circuit is relatively simple but things happen.

If you connected the white wire to the black/purple then I would belive the tach was damaged but I would think the CDI would be ok but not having tried this and not wanting to risk damaging mine I'll pass on judgment.

If you installed a new CDI and the car runs and the continuity on the wire is good then my assumption is that the tach has failed.

Let me know what clarification you need on testing the tach itself.

Jim
Old 08-24-2009, 06:39 AM
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Jim,

Please re-explain in detail about the power together with the ground and turning on the ignition and stroking the tach location. Can I use one of the red connection from the tach as power to connect it to a ground? Explain to me step by step as if I was 2 years old. I just do not want to proceed the test doing it the wrong way and blow something else. Yes the previous CDI was definitely shot from me hooking up that wite wire to the tach wire.

Last edited by 930 porsche; 08-24-2009 at 06:52 AM..
Old 08-24-2009, 06:47 AM
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Jim,

On your test: I understand about connecting a ground with a 12volt power together but is it also connected with the BLK/PUR wire?
Old 08-24-2009, 06:57 AM
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Quote:
Originally Posted by 930 porsche View Post
Jim,

On your test: I understand about connecting a ground with a 12volt power together but is it also connected with the BLK/PUR wire?
No.

Do not connect the BLK/PUR wire to the tach.

Let me lay it out in steps as that's easier for me to explainthan an long sentenance.

1. Connect piece of wire to a ground point.
2. Connect the ground and 12 volt power to the tach
3. Do not connect the blk/pur wire
4. Turn ignition switch on but do not start the engine
5. Take the short grounded wire and repeatedly and very rapidly tap the wire to the connection terminal on the tach where the blk/pur wire normally goes.
6. If the tach is functioning you should see the tach needlejump around at the low end of the scale.

This technique only applies to SC and 930 era tachs where the tach is driven from the CDI. The 964 and beyond use a more conventional pulse to drive the tach.

Jim
Old 08-24-2009, 07:08 AM
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930porsche
 
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Jim,

On #2 is it a seperate ground from #1 ground wire together with 12volt to the connection terminal behind the tach? Which means we have 2 ground for the test?

Last edited by 930 porsche; 08-24-2009 at 07:33 AM..
Old 08-24-2009, 07:17 AM
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I think basically you just need to pull out your tach. Remove the blk/violet wire, switch on the ignition and then touch the connection where the blk/violet wire went with an earthed cable. If your needle moves then it is ok, if not then it is damaged.

Please confirm if I am right or wrong Jim.
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Old 08-24-2009, 08:28 AM
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Quote:
Originally Posted by NathanUK View Post
I think basically you just need to pull out your tach. Remove the blk/violet wire, switch on the ignition and then touch the connection where the blk/violet wire went with an earthed cable. If your needle moves then it is ok, if not then it is damaged.

Please confirm if I am right or wrong Jim.
NathanUK

I'll send all my explanations to you from now on. You explained it much simplier. The one thing I will include is that you have to repeatedly and rapidly touch the grounded wire (earthed) to the terminal on the tach to simulate the input from the CDI. The needle will bounce just a bit but will let you know the tach is functioning.

Jim
Old 08-24-2009, 09:03 AM
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Ok I got it. I will take out the BLK/PUR wire and turn my ignition on and with a earth ground wire I will rapidly touch the connector on the tach.
Old 08-24-2009, 09:22 AM
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what is the voltage range that one should see at the terminal on a functioning tach as it idles thru to 6800rpm?
I assume the voltage changes from one value to another at both ends of the rpm spectrum in order to move the gage needle to show the various rpms?
For example at idle it is 4.5 volts and drops to 3 volts at 7000rpm.
Or does it function some other way?
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Old 08-24-2009, 09:27 AM
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Quote:
Originally Posted by fredmeister View Post
what is the voltage range that one should see at the terminal on a functioning tach as it idles Thur to 6800rpm?
I assume the voltage changes from one value to another at both ends of the rpm spectrum in order to move the gage needle to show the various rpms?
For example at idle it is 4.5 volts and drops to 3 volts at 7000rpm.
Or does it function some other way?
It's not actually a voltage from the CDI to the tach such as the output of a MSD or DME provides. It is a negative transition from what could be best described as a neutral state to negative. It simulates the action of an old point based system.

The tach provides a pull up voltage of about 6 volts which is then pulled to ground (low state) when the CDi triggers. Watch it on a scope with the tach connected and it appears to be a positive pulse. Disconnect the tach and the scope gets interesting. Then you spend time with the schematic and go ah. The first time I saw it on a scope it was weird to say the least and not what I was expecting. I have the scope images somewhere and will post if I can find them.

That's why pulsing the tach input terminal to ground will semi-simulate a working CDI.

Jim
Old 08-24-2009, 12:19 PM
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Jim and Nathan,

I took the tach out, unplug the BLK/PUR wire and turn the ignition on and took a grounded wire and stroke the connector of the tach location and nothing happen. Any other test we can do or can we conclude that the tach is dead? If it is has anybody taken a tach apart?
Old 08-24-2009, 03:46 PM
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Quote:
Originally Posted by 930 Porsche View Post
Jim and Nathan,

I took the tach out, unplug the BLK/PUR wire and turn the ignition on and took a grounded wire and stroke the connector of the tach location and nothing happen. Any other test we can do or can we conclude that the tach is dead? If it is has anybody taken a tach apart?

From what you have said I would believe the tach is dead. Did you rapidly stoke the grounded wire to the tach? That would be the only other thing to check for. Connecting it to the white wire from the CDI probably damaged it.

There are repair shops that specialize in Porsche instrument repair but I don't know if they can repair a tach. I don't believe I've ever seen a schematic of the tach but I believe it uses a single vintage chip from the 70s which could be difficult to find.

Are you in Napes Italy or Naples Florida?

Good luck and sorry.

Jim
Old 08-24-2009, 05:03 PM
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