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Discussion Starter · #1 ·
72 24Z, Working to get my tach working (along with most of the electrics). I found a black w/white stripe wire up by the coil which I put on the neg. post and now it actually seems to be working while the engine cranks when starting. As soon as she starts, it goes dead. It's got an MSD 5 on it, wondering if I need a fix for that.

Anybody got a wiring diagram of the connections I need up there? There are several loose wires from the original config. Green/white and a black are still loose. When I hooked up the black w/white stripe, it at least works while cranking. Is there a simple reason why it stops when the engine starts?

-Kurt
 

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Were you able to start and run the engine WITHOUT the tach being hooked up into the circuit? If so, then you have the black / white wires going to and from the tach and ignition switch reversed on the coil.

The way the wiring schematic has the wiring for the tach, it is an INTEGRAL part of the coil / dist wiring. Without it in place the engine will crank and "start" but will die as soon as you release the key from the starter position. So to avoid that, people reverse the black /white wire going to the tach. That allows you to crank AND run.

Look at the wires going to the tach, and be absolutely sure that you have the POLARITY of these wires correct. The only way I found to do it is to do a continuity check on one of them, identifying which one is which, and making sure that I connected it to the proper pole on the coil. Check which wire goes to which pole on the coil via the schematic.

The POSITIVE pole of the coil gets hooked up to the wire that leads to the TACH. The return wire from the tach, connects with the GREEN/WHITE wire that goes to the resistor that is right beneath your coil. By the way LOOSE THE RESISTOR. You don't need it any longer with the electronic distributor. The Green/White wire gets connected to the Black/White wire that connects to the other side of the resistor which in turn hooks up to the ignition switch.

To summarize;

1 Loose the resistor.

2 The Green/White wire on one side of the resistor gets hooked up directly to the Black/White wire that was on the other side of the resistor. If you can't figure out which Black/White is the one that connects to the ignition switch, get a circuit tester and connect to the black /white wire at the ignition switch, and then check the wires at the coil area until you get a light. That black/white wire is the one to hook up to the green/white.

3 The OTHER black/white is the one that connects directly to the tach, (to check do a circuit check here if you want) hook this wire to the POSITIVE side of the coil. (look at the coil top for identification of the poles.

This should do it.

If you have an Automatic, lose the Red/Black and the Black coming from the Thermo Relay. This won't be needed any longer now, since you don't have the dual point distributor.

That's it. If you're still having problems let me know. I have a 71 240 AT with a 78 dizzy with electronic ignition and the original 240 tach. It works, that's why I'm able to tell you
what you've got here.
 

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Discussion Starter · #3 ·
Now I am really cornfused (Iowa humor, sorry). I don't have a wire on each end of my capacitor. The capacitor goes to negative. I could unhook it and it'd be dead. The MSD 5 has 3 wires: Orange goes to - on coil; Red goes to + on coil, and White goes to distributor. The original black wire was cut from harness to dist. and now the MSD white goes to the disributor.
The following wires are now loose, some may need to be hooked up or not. Green/white and black are loose (a second black from the one to the dist. that was cut) Also, a black w/white stripe are loose.
There is one black w/white stripe going to positive already, if I unhook it, she won't start. The other loose black w/white stripe makes the tach read while cranking only if I hook it up to the - terminal on the coil, but then the tach goes dead when the engine starts. Ohhh this is fun, why the f**k do I have multiple black/white and black wires???
 

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Discussion Starter · #4 ·
Thanks for the help Enrique. Followed your procedure and the tach fires up now. Next problem is that it is erratic and not reading accurately. Reads 3k at a slow idle. Maybe I need a 'tach kit' or ballast resistor for my MSD 5?

-Kurt
 

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I e-mailed this to Kurt, but I'll post the heart of the message here for others to read / comment / correct.

The tachometer does have what appears to be an adjusting rheostat, however, I would caution you VERY STRONGLY do not use a metal screwdriver to do this. If you do, I'll guarantee you will burn out one of the resistors inside and the tach will simply not work.

You MUST use those plastic screwdrivers for adjusting flat rheostats common in the television repair industry.

If you look at the back of the tachometer, on the upper part between the two holes for the dash illumination light bulbs, you'll see an opening. It may have scotch tape, or the remains of it. Remove the tape, and look inside. Way deep you'll see NOT a screw, but rather a slotted opening in a brass circle. This is the flat rheostat. I haven't done this, so I can't tell you if you adjust clockwise or counter clockwise, but it is IMPERATIVE that you do not use a metal screwdriver. I'm telling you this, because I did in fact blow my first tach by doing just that. Plainly put, I screwed up, and I'm trying to warn you so you
don't.

I would suggest that you hook up the tach to it's proper circuit, you won't need the lights for illumination or turn signals, but you do have to have the 4 wire connector plugged in. Then with a plastic screwdriver (available from Radio Shack, tell the guy you need a NON-CONDUCTING Screwdriver ) insert it GENTLY into the hole. Start the car, if possible have another method for checking the RPM. In the old days we would use a Dwell meter and that would do it for us. Since you now have an electronic dizzy not sure how you would do it. Then calibrate to that other tach. Remember, these aren't going to be the most accurate tachs. They're only supposed to tell you your approximate RPM, and not an exact count.

Well, hope this helps.
 

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Discussion Starter · #6 ·
Thanks, I'll give it a shot. I am running a Mallory Mark II coil, btw. (Can't find any info on that or the MSD 5, so they must be pretty old.

-Kurt
 

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I PLAN TO HOOK IT UP TOMORROW,SO NOT SURE YET,BUT I'VE BEEN TOLD THAT YOU NEED A MSD TACH ADAPTER,PART NO.8920 WHEN CONVERTING YOUR IGNITION.I'LL BE USING A GM RACING UNIT/COIL/REV LIMITER THAT IS SUPPOSED TO BE MADE BY MSD AND IS IDENTICAL TO THEIR "BLUE BOX" FOR MY '78 280.A FRIEND RECENTLY HAD THE LOCAL Z SHOP INSTALL THE MSD 6 (RED BOX) IN HIS '70 240 & THEY HAD TO INSTALL THE TACH ADAPTER.
 
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