Tachometer and wiring

[SteveJ]

3 minutes ago, Dcreech0 said:

I will swap them when I get home in about and hour and report what I find via video on youtube

I did some testing with the wires swapped. While there should not be voltage at the coil while starting with the wires swapped, I did read a little over nine volts. That may be from slop in the contacts with a switch that is almost 50 years old.

I also pulled the negative off of the battery and checked resistance between the black/white for ignition and the positive post of the battery. At first it was just a couple of ohms with the key in ON and about 50 ohms with the key in START. However, I noticed some fluctuation in the readings with the resistance with the key in ON increasing when I released the key from START. That is why I think there could be some slop in the switch.  However, I was not seeing any continuity between the black/white wires no matter what position the key was in. I'll test some more later.

[cgsheen1]

5 hours ago, SteveJ said:

I will verify this evening. It will only take a few seconds with a meter. 
I still have my doubts about being able to start the car with the black/white wires swapped.

Ya, it'll run - because you've just wired the coil "+" directly to the IGN switch - but you've cut out the loop to the tach. Soooo, the tach won't work but the engine should run fine. (maybe that's what we should have told him in the first place -> just swap the B/W's...)

Edit: I watched the OP's videos and wondered in the second video if he verified that the tach also had a good GND connection, He said he verified B/W, G/W, (signal) and G (battery voltage to gauge) but didn't say anything about a ground.

(oh and, sorry I just skimmed todays posts, forgive if I missed something)

Edited August 21, 2021 by cgsheen1

[Dcreech0]

3 hours ago, SteveJ said:

I did some testing with the wires swapped. While there should not be voltage at the coil while starting with the wires swapped, I did read a little over nine volts. That may be from slop in the contacts with a switch that is almost 50 years old.

I also pulled the negative off of the battery and checked resistance between the black/white for ignition and the positive post of the battery. At first it was just a couple of ohms with the key in ON and about 50 ohms with the key in START. However, I noticed some fluctuation in the readings with the resistance with the key in ON increasing when I released the key from START. That is why I think there could be some slop in the switch.  However, I was not seeing any continuity between the black/white wires no matter what position the key was in. I'll test some more later.

 

[Zed Head]

6 minutes ago, cgsheen1 said:

Ya, it'll run - because you've just wired the coil "+" directly to the IGN switch - but you've cut out the loop to the tach. Soooo, the tach won't work but the engine should run fine. (maybe that's what we should have told him in the first place -> just swap the B/W's...)

 

The question isn't "will it run" it's "will it start".  The later switch diagrams, like the 72 diagram from EuroDat above, show separate power circuits to the coil.  A Start circuit and a Run/On circuit.

[cgsheen1]

If the tach is grounded, you have a bad tach

 

[cgsheen1]

And, yes, there is a "jumper" in the ign switch/coil circuit to bypass the ballast resistor IN THE START position. So at START the coil is getting full battery voltage and in the ON (run) IGN switch position power to the coil goes through the ballast resistor.

[cgsheen1]

8 minutes ago, Zed Head said:

The question isn't "will it run" it's "will it start".  The later switch diagrams, like the 72 diagram from EuroDat above, show separate power circuits to the coil.  A Start circuit and a Run/On circuit.

Sorry, my wording wasn't precise. It will both start and run with the B/W's swapped. If they are in the "incorrect positions" you will loose the tach but since the coil"+" is then wired directly to battery voltage from the IGN ON the "START jumper" isn't needed. You just lose the loop to the tach and power to the coil is not going through the ballast so it will always have full battery voltage.

 

[Zed Head]

Nice video.  Now I'm left wondering if a 72 will Start with the wires switched.  The switch diagram shows just one power source for the coil for either Start or On, there should be no power there at Start if the wires are switched.

The 71 diagram shows all 5 pins powered at Start though.  If we only had the 71 diagram there wouldn't really be a discussion.

Anyway, looks like a stuck or bad tachometer.

[cgsheen1]

Dcreech, I'd like to see a pic of the rear of the tach with special emphasis on the loop...

 

[Zed Head]

Thanks for the comments cgsheen1.  I'm just trying to figure out things using the wiring diagram.

[AnvilZ]

3 minutes ago, Zed Head said:

The 71 diagram shows all 5 pins powered at Start though.

@Zed Head I went back and found the 71 wiring diagram I used from the 1971 FSM Supplement and it seems to show the same thing as EuroDat's 72 diagram. If it is accurate I am still wondering how the car can start with the B/W to the Ballast being switched out of the circuit in START assuming you had that wire "swapped" (connected to Coil +)

Not to dwell on that issue too long, but I think this is what was confusing me back when I did the wiring on my car. 

[Zed Head]

It might just be that the parts changed but the diagrams did not.  There are other diagrams that don't keep up with the actual parts.  Or, the switch has been replaced along the way with a newer part.

Trust but verify, I guess.