Tach, White Loop

[theleisure]

Hi guys,

I just opened up a spare tachometer and did the steps to change it to negative ground (my car has an aftermarket autometer gauge that I am about to replace). I was surprised to find that the white wire formed a loop and that was all, no connection to anything. Here's my question, if i replace that looped section of wire with a short section of new 14 gauge wire will the tach still work ok? 14 is what I have sitting around so that's what I'd like to use. What's that wire actually doing and will I mess it up?

For what it's worth, I bought a battery powered handheld soldering iron for this job. It couldn't get hot enough to melt solder so I had to go to home depot today to get a plug in version.

sean

 

[SilentUnicorn]

Here's my question, if i replace that looped section of wire with a short section of new 14 gauge wire will the tach still work ok?

Doublful <div class="ubbcode-block"><div class="ubbcode-header">Quote:] What's that wire actually doing and will I mess it up?[/QUOTE]

it creates an electromagnetic field.

probably



mark

 

[theleisure]

O.K. then. The wire needs replaced-- it looks like it got burnt by a nearby wire at some point. What should I use?

 

[jvandyke]

Don't know what effect the gauge wire will have but I hear you on the soldering joys. I've got a 100w gun and it's still too slow for me. I use my propane torch if there's nothing to damage behind it.
I did a tach conversion this winter too, going to negative earth and now I decide to rewire the car, so I have to decide if I'm going to take the loop off and reloop with new harness or cut and splice.

 

[SilentUnicorn]

sean

Check ~This~ out.


mark

 

[Pythias]

Since ALL the current from the ignition to the COIL (dizzy coil, not little coil at tach) goes thru this wire, replace it with the same size that come out. . .I THINK it's 14 gauge. You'll know if you try to fit something bigger than will fit.. THIS could well be the most important wire of the whole car. Without it, NO GO. (unless you're hot wiring)

Don't forget, the direction of flow of current has to be reversed thru that loop in order for the tach to function properly when you change the cars polarity.

 

[dklawson]

This is an AutoMeter tach? It sounds more like a later Smiths RVI type tachometer.

The white loop of wire is an induction loop. As Bill said above, use the same gauge wire as is there now. What happens is each time the coil charges and discharges a magnetic pulse is created (like Mark said). The wire is looped around what is basically half of a transformer. Each pulse from the coil creates a pulse on the transformer which is in turn used by the tachometer circuit. This leaves the tachometer electrically isolated from the coil itself.

Early Smiths RVI type gauges had this loop on the outside of the gauge. It was part of a length of white ignition system wire that physically clamps onto the back of the tachometer.

 

[theleisure]

Thanks guys, very helpful.

Alright, so I'll replace the burned up loop with a new length of 14 gauge wire on the Smiths tach I've been working on. At that point the conversion will be done and I'll be able to replace the incorrect Autometer gauge that's in there now. I think the PO put that one in 'cause they didn't know where to look for info on pos/neg conversions. I love this forum.

Next decision, should I do a short length and splice that in to existing wiring harness or just replace the entire ignition-tach-coil length.

 

[Pythias]

I like to leave push connectors on both sides of the tach wire loop. It makes really easy to swap out the tach. That's just me.

 

[jvandyke]

I was looking at my new harness and its tach wire, I haven't spent too much time trying to figure it out but the harness has a long white lead with a spade terminal in the middle of it. I was going to (and probably still will) just cut it off and splice into the existing whites on the tach but I wonder what the intention was on the harness maker's end? Maybe provide for tachs that just have a connection on the back and an internal loop? but also give you some wire to play with if you need it?

 

[dklawson]

I obviously don't know the specifics of your wiring harness, but it could be they were trying to provide a universal harness that could support both the Smiths RVI and later RVC tachs.

Where RVI was fitted as standard, there would probably be a spade terminal white wire on the ignition switch. The other end of this wire would connect tot he tach. A second white wire would leave the tach and go to the ignition coil.

Where the RVC tach was fitted as standard, the white wire from the ignition switch would go straight to the coil. A second wire (white/black) would typically be connected to the points side of the coil and be routed back to the tachometer.

 

[MikeP]

Assuming it's like the harness I've installed the spade is the intended end for the ignition. The intention is that you pull it off the wire, do the loop through the back of the tach, then permanently attach the connector to the end of the wire, that way you don't have it unsolder or pry at it. Pushing it on the wire a ways keeps it from being lost during handling.

 

[theleisure]

The harness on my car is an original with extra wires added here and there by previous owners over the past 42 years.

Here's a picture of the back of the tachometer that's in the car now. As you see, the colors don't match what should be there. No big deal, I know what should be there and where they should go(thanks to this forum).

In this picture you can see a nicely looped length of white wire . Where is it going? where is it from? This is my weekend project!

These pictures were taken last summer when I bought the car. All of the burned up sections have been followed and replaced. Looking under the dash made me understand why the PO had a fire extinguisher in the cockpit.

***edited because i can't get my dang photos to link!***

 

[jvandyke]

I wouldn't be surprised if that's a tach to coil wire that didn't get used or was spliced onto at the coil side so there's extra hanging around.
I'll go check my harness, now that I'm home.

 

[jvandyke]

Assuming it's like the harness I've installed the spade is the intended end for the ignition. The intention is that you pull it off the wire, do the loop through the back of the tach, then permanently attach the connector to the end of the wire, that way you don't have it unsolder or pry at it. Pushing it on the wire a ways keeps it from being lost during handling.

Mine looks the same. The spade was crimped onto the wire so tight it looked like a connection at first. I could not pry it off without damage to the wire, ended up cutting it off with a dremel. I looped it through the plastic holder on the tach in a fashion I believe to be correct for a - ground car, slipped the spade insulator on the wire, soldered on a new female spade, slid the insulator over it and called it good. They should not crimp that connector on there like that.