Tach for '73.5 3.0 conversion

[BK911]

I've searched and most of the conversion info was on 3.2's.

Is there any way possible to get the existing tach to work, or do I definitely need one from an SC? Can I run a wire in parallel from the 'points'?

Thanks!
BK

[Elombard]

I have a 3.0 in a 73.5, I am pretty sure its the OE tach. I bought it converted. I think the wiring harness pin is different beacuse when I swapped engines I think I acutally ended up running a wire out of the harness from the engine to the tach wire up near the fire wall because I could never figure out how to get it through the "black box". I am sure it could be done....I need to revisit that.

If you search there is a nice wiring diagram for the SCs, make sure you have a signal from the tach at the harness and then figure out which wire goes up to the tach.

[}{arlequin]

3.0 w/ '73.5 here as well... stock tach. no issues. prior to swap it was wired throgh msd so there was no change necessary since then.

[304065]

Tachometer Compatibility Chart

[BK911]

Thanks guys. I am running the black/purple wire from the 6 pin CD box to my tach and it isn't working. The tach uses a different pin in the 14 pin connection. I swapped some wires around. I thought I had the wiring right, I guess my first step is verification.

Erik: BTW, I am NOT disappointed in the conversion. It's only seems a little quicker than the 2.4E, but I am just getting started! I see ITB's and EFI in the near future.

[BK911]

Hey John, excellent thread. However, I was pretty confused trying to read the technical jargon. From what I deciphered, my tach probably won’t work. However, I have one from a '74 that might work. Did I read that right?

[Elombard]

Thats right I remember something, I think you have to run the green white into the CDI and then out the bl/purple. The gr/wht is different spots on the pin connectors for the two years I think. Again I think the Tach will work, I went all through this, did not need a resistor for some reason.

If all else fails jumper it across into the CDI and make sure it wroks and then back trace?

Hey give me an update when you figure it out. I jumpered mine, got it working and then never went back and figured how to do it right.

If you get stuck I can shoot a picture of mine later.

Glad you are not disapointed, it is a strong combo!.

[304065]

BK, I would get an SC tach if you are using an SC six-pin CDI box. According to the data I gathered on the thread, the SC is a square wave between 1-6V and the 73.5 is an old 3-pin CDI with a small pullup resistor, so it's probably an 11v square wave. While it might work, I wouldn't trust it- I have a 74 tach running in my 911E for testing purposes and it is about 1000 rpm off from the digital tach signal displayed on both my shift light and a timing light hooked to the diz. So even small differences in voltage can affect the values.

Remember that the old tach is a "one-shot" or monostable multivibrator-- the voltage signal from the points triggers a transistor to fire off another transistor that sends a pulse of voltage to the meter coil, moving the needle. The more often this happens the higher the needle swings- sort of like hitting a volleyball- and the pulses are integrated by the mass of the needle and movement into a smooth display. If you change the voltage for whatever reason, it either won't trigger (as if you tried to use a pre-71 tach in a post-71 car) or could read erroneously.

Good luck!

[Elombard]

Ouch, I wonder if thats why my dist button rev limiter does not seem to kick in at same time as it should according to the tac? I think the 73.5s had 6 pin CDI?

[BK911]

Thanks John. I am currently looking for an SC tach. Even if I could get mine to work, I would rather have one I know would work and be correct.

The 73.5's were 3 pin CDI. At least mine was initially.

[BK911]

Ok, so I bought the tach out of the same car which I bought the engine. So it should work right? Wrong! This one doesn't work either.

I checked continuity from the 6 pin harness at the CD box up to the pu/bk wire at the tach and all is good. I checked for voltage at the pu/bk at the tach with the engine running and I didn't get any. So maybe the CD box isn't sending out a signal? Is there another place I can splice into to get a signal for the tach, either the SC or the old one?

Thanks.

[Elombard]

I think you can check for the tach signal at the green white before it goes into the 14 pin on the engine side.

[304065]

Well, how did you measure the singla at the 'TD' output of the Tachometer? With a scope, or with a voltmeter? I would think that a logic probe or scope would be necessary to see a 1.0-7.0 volt square wave. At idle rpm of 1000 (for example) there are three ignition events per revolution, 3000 divided by 60 = 50Hz signal. Trying to see that with an analog voltmeter probably isn't going to work.

Run a wire from the TD output directly to the RPM connection on the back of the tach, and make sure you have a good solid +12v and ground to the tach also.

6-pin SC/Turbo CDI unit repair documentation

Remember, the old tach won't work with the new six-pin box. Can you test the tach in somebody else's SC to make sure it works? Did it work before the car was parted? Nothing worse than not knowing.

[BK911]

I got it working. I removed the CD box and re-cleaned the 6-pin connections and used some dielectric grease. I connected and disconnected several times, then hooked everything back up. I did get a signal with an analog volt meter, so I hooked the tach up and IT WORKED!!

Thanks for the link. I searched under Tach and couldn't find anything helpful. Warren does it again.

[Elombard]

Great so did you need to mess with the 14 pins at all?

[BK911]

Initially I had to rearrange some of the pins for the tach signal. I think I switched from pin 12 to pin 8. But to get it to work this time I just had to clean the 6 pin connection. I swapped to an early fan, alt and volt reg so I had to mess with some wires there too.

I am pretty bad at keeping records and taking notes, but I have some scribble on which pins I moved and where I moved them. If you are interested I can email the list to you.

As soon as rush hour is over I am going to take her for a spin and see how she feels. Before I wouldn't rev her too high because I didn't know what the engine was doing. This time I can rev her up a little bit.