74 engine tach issue in a 78 chassis - wiring assistance

[yama1]

I have a 74 engine in a 78 chassis and am in process of changing to carbs. Currently, the engine runs and most pin outs on the 14 pin connector are correct. The vehicle runs, but no signal to the tach. Upon inspection of the PO's wiring, I noticed the signal blk/pur from the distributor did not match the chassis side connector. I have switched this with no luck. I have tried both a 78 and 69 tach. Is the 74 wiring schematic different than the 73 cis on pelican? I have found some non-legible 74 diagrams in black and white line type. Does anyone have a color coded or correct 74 diagram of the tach, cdi, and alternator wiring? I have also heard the 74 tach is different from the 78, but thought would be similar to the 69 off points. Will attach a pic of the side panel when I get to my computer and not this iPad
Thanks in advance for any support

[Walter_Middie]

Yama1,

The black/purple wire is the tach signal and does not go through the 14 pin connector - it comes directly off the 3 pin CDI unit used on mid-years. Your 1978 SC had a 6 pin CDI unit. Here's some info from a previous post:

1974-1977 Tachs. I know these were driven off the CDI box, but I don't know when the CDI box changed from an "001" to an "008" or when the cutover from a 33 ohm pullup resistor to a 120 ohm occurred. With 120 ohms the input current is only 116mA so the tach must have changed somewhere along the line to match. If you send your tach to NH speedo to be recalibrated they want to know what CDI box you have, so this must affect the input value to the tach.

1978-1983 Tachs. These are driven from the "TD" tachometer drive output from the six-pin CDI box because the points were eliminated in favor of a magnetic reluctor. The SC tach signal is the same as the later Motronic:SC.

The 1969 tachs were driven by an "11 volt square wave" (no CDI - just points and coil). Up until around 1971 the tachs were driven from an "intermediate unit" which was a silver can with Bosch part number 0 227 990 001.

What are you using for a distributor? And what CDI unit are you using?

[yama1]

Thanks that will help. The blk/purple wire I have on the 74 engine harness routes from the distributor to the 14 pin connector. I need to verify that the wire is not split and feeding from the cdi at the connector. I plan on taking the protector off, so I can see exactly what wires come off the cdi and where they go. This will help. Thanks

[yama1]

Well - this is a pic of the left engine control panel. You can see 2 relays which are not related and the only relavent item is the 3 pin CDI.



My motorola alt requires a voltage regulator that appears missing. Will a VR on a 69 work for this? Is the rev sensor unit required to for this as shown in the wiring?
The 2 pin connector red/brown is just using the red and connected to the center terminal on the CDI to the fuse panel.
Has anyone modified their harness to integrate the VR and other?? back into a newer harness like the 78? I confirmed the blk/purple lead comes off the distributor and runs through the 14 pin connector as a stock 74 engine harness and not from the CDI. Only the black shielded wire connects to the cdi from the same point on the distributor as the blk/pur signal wire.

What is the correct tach for the 74 model? Any special resistors or missing modules that may be required?

The engine does run - just no tach signal or VR provisions for the wiring on the chassis side. This would explain no charge from the alt and the light staying on.

thanks

[Flat6pac]

On the SC engine the tach drive is in the 14 prong connector as is the chassis harness.
The purple/black of the 74 is derived from a 2nd wire on the distributor connection for the points.
you need a jumper from the distributor base into the 14 prong connector to pick up the chassis tach wire.
Bruce

[yama1]

Wel. I matched up all the wiring between the two harnesses and all is connected properly. I am using the 30 ohm 001 cdi and was able to get my matching 74 gauge back in. Well fired it up and the 74 tach was working....but hold on... After about 3 minutes of trying to dial in my idle rpm on the carbs, the tach dropped off. I verified voltage, and signal wire and even ran a direct wire to e gauge with nada. So either this is a coincident that the gauge fried or the signal to the gauge was altered. Tried my other tachs for kicks and no luck. I don't have a way to verify the actual signal input coming in. Any ideas - can't really set my carbs up by sound, although they are pretty darn close.
Thanks

[tdskip]

Do a search on "tach adaptor" - lots of good info there and a Pelican member who build a kit.

[Bob Ashlock]

Hi, I sent you a private email on this subject yesterday. I'm the guy referred to in the previous post. I find it curious that the tach worked for a few minutes, then died. Can you confirm that your distributor points are properly gapped? The point gap is not very critical for triggering the CDI unit, but if the gap is almost closed, they may not be providing the correct duty-cycle square wave the tach expects to see. I would verify your point gap before moving forward. Perhaps this is all you need to get your tach to come back to life. If this doesn't work, then you may need to use the TACH-ADAPT unit I make. It locks to your points signal and regenerates a clean tach signal that all the Porsche tachs like to see.