Tach wiring - Automotive Wiring and Electrical


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Auto Repair Home>> Automotive Repair>> Automotive Wiring and Electrical

Fordguy
New User

Feb 1, 2008, 1:26 PM

Post #1 of 5 (1711 views)

Tach wiring	Sign In

I have a 72 Ford Ranchero GT and I installed a Pertronix ignition. In order to get 12V to the coil and not make major alterations to the wiring I installed a 12volt relay and ran the coil wire to the relay as a trigger with a full 12 volt return to the coil. When I did that I disrupted the series wiring to my tach with the relay and it quit working.

As a workaround can I reattach the resistor wire to the coil continue that wire from the coil to the relay and then bring the full 12 volt circuit back to the same + terminal on the coil without hurting anything?

DanD
Veteran / Moderator
DanD profile image

Feb 1, 2008, 3:24 PM

Post #2 of 5 (1706 views)

Re: Tach wiring	Sign In

I’m not familiar with the ignition system you installed but all of the tachometers I’ve ever dealt with use the coil negative for a reference signal for rpm. Source voltage (battery) was used to power the tachometers internal works.

Dan.

Canadian "EH"

Fordguy
New User

Feb 1, 2008, 4:28 PM

Post #3 of 5 (1703 views)

Re: Tach wiring	Sign In

Thanks for the response dan. Up until the early 70's all Ford's and some other manufacturers ran their factory tachs in series picking up the pulse from the positive side of the coil and that's what I have. I think Ford went to the negative terminal in 74 or 5 and every after market tach I've ever seen runs off the negative side.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Feb 2, 2008, 9:36 AM

Post #4 of 5 (1699 views)

Re: Tach wiring	Sign In

Same with Dan. All I've dealt with (not tons) used the neg coil post. Of interest the big three went EE - Crystler in 73, Ford in 74, GM in 75.

Noted over the years: 12v vehicles with points used a ballast resistor to reduce voltage to that of a 6v system for the coil's sake and make points last but were full battery voltage while cranking,

T

Fordguy
New User

Feb 2, 2008, 2:18 PM

Post #5 of 5 (1695 views)

Re: Tach wiring	Sign In

that's true Tom but Ford used a small pink section of wire in the feed coming from the ignition switch. It's buried in the dash wiring behind the cluster so t's hard to get to and it goes on to the coil. In the case of my Ranchero it's a factory in the dash tach so the circuit is hard wired in but the pink section of wire is there. I also have a 65 Mustang Convertible with the Accessory Rally Pak that attaches to the steering column as a fitted factory addon so there are connectors in the wiring in the dash where you plug it in at the pink section of wire. If you unplug the connector or the tach malfunctions the car or truck won't run because the series connection is broken. That's probably not the smartest thing Ford ever did but it certainly is how it is.

With my seat of the pants limited knowledge of electrical I don't think leaving the resisted connection on the + coil terminal with the full 12V line coming back to it will hurt anything since it comes from the same battery but I'm afraid to try it because I don't want to risk my tach. I was hoping someone with a more in depth knowledge could answer that.

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