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Join Date: Dec 2017
Location: Sevierville, TN
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Power for gauges

Now that I finally got my baby running, its time to fix other issues.

My gas, speedometer do not work. Where does the power come from to these gauges? I do not have the back/purple wire connected to my coil (negative side) for the tach because it kills the engine. Does this wire have something to do with these other gauges?

I going to take out the cluster this afternoon.
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Old 10-02-2019, 06:06 AM
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dr914 ***********
 
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the gauges themselves or the lighting for them? Power is provided by the red with white wire, lighing is provided by the black with blue wire connected to the rheostat on the headlight switch, the fourth fuse over from the right powers the tach and gas gauge it often loosens and does not make good connection especially when you hit the brake which is on the same fuse. you may want to get the book "tech tips 700" which dave recommends and sells at pelican
Old 10-02-2019, 07:36 AM
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Quote:
Originally Posted by dr914 View Post
the gauges themselves or the lighting for them? Power is provided by the red with white wire, lighing is provided by the black with blue wire connected to the rheostat on the headlight switch, the fourth fuse over from the right powers the tach and gas gauge it often loosens and does not make good connection especially when you hit the brake which is on the same fuse. you may want to get the book "tech tips 700" which dave recommends and sells at pelican
Thanks Doc! It is for the gauges. The lighting works fine.
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1972 914/4
1970 Toyota FJ40 Landcrusier
1999 Lamborghini Diablo (build)

Last edited by surfdogskier; 10-02-2019 at 07:56 AM..
Old 10-02-2019, 07:53 AM
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The speedometer is purely mechanical. It works off of a cable coming from the transmission that spins. Through some quasi-magical effects (magnet-induced eddy currents) it causes a metal cup to spin against a spring depending on how fast the cable turns. The metal cup is attached to the needle of the gauge.

If the speedo isn't working, then either the gauge is broken, the cable isn't connected, the cable is broken, the right-angle speedo drive is broken, or the speedo drive inside the rear transmission cover is broken. Usually it's a problem with the cable; less often it's a problem with the angle drive. Other issues are much more rare.

The black/purple tach wire killing the engine is odd but not unheard of. It's almost certainly the case that the wire is grounded somewhere. Some people did this (through a switch) as an anti-theft device. Try to chase down the black/purple wire and see if there are any switches or buttons on it. Also test the distributor end of the wire to see if it is actually connected to ground.

The gas gauge is powered by the #8 fuse, as the Doc mentioned. Check that by removing it and inspecting; these old fuses can pop but look OK in the holder; they fall into pieces when you remove them when they're popped. The gauge itself sends power to the fuel level sender in the top of the fuel tank. The sender there provides varying resistance depending on how high or low the float in the tank is; it also provides a short to ground of the "low fuel" light wire when the float level is low enough.

I hope these help you to sort out your gauge issues. It sounds like you have multiple issues, sad to say.

--DD
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Old 10-03-2019, 12:10 AM
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