What's Grounding Here?

[Gary in NJ]

Sunday evening while I was doing some exploratory work on the water-temperature sensor I grounded the yellow wire to determine if the gauge or the sensor were defective. At first it appeared as though the gauge was the problem, but after trying a few different grounds, I was able to get a full scale reading on the gauge. I attributed the poor ground to the condition of the connector, which is corroded.

Monday afternoon I decided to go out for a drive. The car wouldn't start, which is unusual since this engine starts easily. I figured I must have accidentally disconnected something near the distributor while looking for a good ground the night before. A quick check showed everything connected.

Then I remembered that I had removed the harness that contains the yellow water-temp wire, and a wire to the distributor (the tach wire?), from the clip on the drivers side wheel well (the same clip also holds the wire from the coil to the cap). I pulled the wire from clip and moved it about a inch, then reinserted it back into the clip. The good news is the car started. The bad news is something kept it from starting.

So there must be a few chafed wires somewhere in that harness. I'll have to remove the harness casing and look for damaged wires. What other wires are in that harness? Could the tach wire disable the ignition system? I tried to get that from the wiring diagram in the BE section of the FSM, but had little luck.

Edited June 1, 2010 by Gary in NJ

[MikeW]

It's likely the tachometer wire as you've surmised. It must be connected properly for the engine to run.

[prilofx]

It's likely the tachometer wire as you've surmised. It must be connected properly for the engine to run.

The car shouldn't run if the tach wire is faulty? I'm asking because mine is completely disconnected and I'm still able to start and run my 240. In fact, I'm trying to find a new tach wire. Mine had been connected with a nest of copper taped on to a length of speaker wire.

[Arne]

Depends on the year and which style tach you have. The early 240Zs with the so-called "4-wire" tach will not run (without a jumper wire) if the tach is disconnected. I don't believe that is true for the cars with the "3-wire" tachs.

[ChrisZ]

I don't believe that is true for the cars with the "3-wire" tachs.

Witch most -73 models came with, I wonder when they changed to 3 wire.

Chris

[prilofx]

Depends on the year and which style tach you have. The early 240Zs with the so-called "4-wire" tach will not run (without a jumper wire) if the tach is disconnected. I don't believe that is true for the cars with the "3-wire" tachs.

Good to know. Thanks. With all the backwoods mods done to my car I'm never sure what's up.

[Arne]

Witch most -73 models came with, I wonder when they changed to 3 wire.

There's been other threads on that question here, the answer has been mixed. Sometime in late '72 or so?

[MikeW]

Arne - thanks for the clarification. I wasn't aware of the later change and would never intentionally spread misinformation. Gary's HLS30-91415 is clearly a mid 1972 model which makes it a candidate for the earlier tach based on your "late '72" guess.

[Gary in NJ]

Gary's HLS30-91415 is clearly a mid 1972 model which makes it a candidate for the earlier tach based on your "late '72" guess.

Good eye, its has a build date of 7/72. I guess that the water temp wire (yellow) and the tach wire (blue, I think) are touching. A couple of connectors and some heat shrink and this problem should be solved.

If the wires look brittle, I'll just go ahead a replace 'em. Thanks for confirming my suspicion.

[ChrisZ]

There's been other threads on that question here, the answer has been mixed. Sometime in late '72 or so?

Arne, member lpraun, have a -73 with a 4 wire tach and you say the 3 wire came in -72?

Chris

[ArnieTX]

Do you mean "Start" or "Crank"? Would the starter kick over when y ou tried to start the car?

I'm troubleshooting a 72 with an automatic. I can't get it to crank.

Thanks

[Gary in NJ]

It would crank away, but not fire.

No crank; battery, battery cables or the starter. Also check the switch to make sure you are getting a ground closure at the solenoid when you engage the starter.