Tach issues with change to original CDI

missoula911 · 12-04-2017, 12:42 PM

I have a 1971 911 E which is giving me tach issues. I recently changed the ignition system from a Permatune system back to the original Bosch CDI system. While this took care of my stalling issues, I have not been able to figure out the Tach problem I am having. I hooked the original CDI unit up and wired it correctly but my tach is not working correctly. It registers a RMP but it floats and flutters and is seemingly gives an incorrect reading. The Permatune system had a white wire which was connected directly into the tach wire just below the CD box. This connection bypassed the RPM Transducer and other wiring in the fuse box area in the engine compartment. I have my father's car to look at and it seems that all the original wiring is hooked up right. I took the RPM Transducer out of my car and put it in his to see if this was causing me the issue but it worked just fine in his car. I have traced all the wiring through the back harness and it seems to be working correctly. And with the Permatune system in place, the tach worked perfectly well. I took the tach out to see if there seemed to be something out of place there but everything appears to be fine based on the wiring diagram. Any help with this issue would be greatly appreciated. At this point I have disconnected the wire for the tach that runs off the coil so I do not damage the tach. Thank you in advance for any help or thoughts any of you might have!

Trackrash · Trackrash's Garage

'71 and tach issues oh my.

I would guess that your tach is not original to the car or it was modified to work WITHOUT the RPM transducer.

Since you have access to another car with the same tach try switching tachs between cars.

Fred Winterburn · 12-05-2017, 06:44 AM

Why not try hooking the tach signal wire directly to the points. If your tach was upgraded to more modern circuitry at some point it could be that the Bosch signal conditioner (I assume that is what your are calling the rpm transducer?) won't provide the correct pulse. It could be that it will drive from a weak 12V square wave instead. Fred

Quote:

Originally Posted by missoula911

I have a 1971 911 E which is giving me tach issues. I recently changed the ignition system from a Permatune system back to the original Bosch CDI system. While this took care of my stalling issues, I have not been able to figure out the Tach problem I am having. I hooked the original CDI unit up and wired it correctly but my tach is not working correctly. It registers a RMP but it floats and flutters and is seemingly gives an incorrect reading. The Permatune system had a white wire which was connected directly into the tach wire just below the CD box. This connection bypassed the RPM Transducer and other wiring in the fuse box area in the engine compartment. I have my father's car to look at and it seems that all the original wiring is hooked up right. I took the RPM Transducer out of my car and put it in his to see if this was causing me the issue but it worked just fine in his car. I have traced all the wiring through the back harness and it seems to be working correctly. And with the Permatune system in place, the tach worked perfectly well. I took the tach out to see if there seemed to be something out of place there but everything appears to be fine based on the wiring diagram. Any help with this issue would be greatly appreciated. At this point I have disconnected the wire for the tach that runs off the coil so I do not damage the tach. Thank you in advance for any help or thoughts any of you might have!

Bob Ashlock · 12-05-2017, 07:27 AM

Around August of the '71 production year, Porsche changed the tach to work directly from the points. Before that, the tach was triggered from a 'filtered' signal derived directly from the high voltage discharge A-lead of the CDI/coil. The 'filter' was the round-can "Intermediate Unit". Like Fred suggests, perhaps your tach was modified at some point, or it simply could be a later '71 version.

Jonny H · 12-05-2017, 01:42 PM

^. Good advice here. I'll just add that the tacho should have a date stamp on the back.

It amuses me how the early cars run the CDI 400V line into the back of the tacho. I don't think we'd get away with that today. It'll give you a nasty shock if you touch it so be aware.

The only time I've ever got 'buzzed' by a CDI box was taking an old one apart, capacitor was still charged! Won't make that mistake again!

ericbash · ericbash's Garage

Adding a Tach-Adapt (available from our host for $70) will solve almost any tach compatibility problem. Highly recommended.

timmy2 · timmy2's Garage

Sounds like you may have the early ‘71 wiring that is the same as the ‘70?
2 yellow/blue wires to the coil?
If your tach is able to run as mentioned without the “filter” you will need to connect the purple and black wires to each other in the rear fuse panel wiring.
Using the 1970 diagram, Item 10 (intermediate unit) terminal C needs to connect to item 11 (speed switch) terminal 4. Use the 1970 SL35 wiring diagram. (Compare to SL 39 1971 on)
Later 71’s have them connected to each other. (SL 39)
I have done this mod to a few early fuse panels for cars with newer tachs.

missoula911 · 12-20-2017, 03:44 PM

Thank you all for the information you all have provided. There could have been a tach change when the prior owner changed the CDI to a permatune system but I have taken the tach out and it appears to be original to the car. My 71 was made early in the production year late in 1970. I will have to do some additional research on this issue but I just can't figure this thing out. At least the issues with the car stalling seems to be figured out with the change back to the original CDI system. Glad to have that problem solved it seems. Thanks again for the input and if anyone thinks of other ideas for me to try I am all ears.