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I've installed a '66 gauge cluster in my '65. I installed a oil press sending unit for the gauge and reconnected everything correctly (to my knowledge) but the gauge pegs when I turn ignition on. I installed the constant voltage regulator and confirmed the 5 - 6 volt average. Gauge cluster was swap meet fodder. Any ideas. Am I missing anything besides the gauge being bad? It only has two connections (sender and power) and I ran a ground wire to the back of the cluster. Any help would be appreciated.
 

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I am not an expert here but everyone here suggest running a ground wire when diagnosising guages. I suspect the ground wire is pegging the guage causing the reading. The best for this one is MIDLIFE. That Smarta$$ knows it all!! We all love ya Mid!!!
 

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If you have an ohmmeter, measure the resistance from the senders terminal on the gauge to ground. If it's near zero, your gauge will peg I believe. You should see the resistance of the sending unit if all is proper in your wiring, which is normally in the double digits I think. Yhe other side of the gauge should have the 5-6 volts from the regulator.
 

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I'm not familiar with the '66 panel so I'll make some assumptions. The guage itself penetrates the backshell of the cluster and is held in place with 4 insulating washers and 2 nuts. Harness wires attaches to the same two posts.

First check is to remove the wire at the guage that leads to the sender. Turn key on. If guage still pegs, the problem is with the guage. See #1 below.

1. The most likely fault is for the guage to have shifted in the cluster and one of the posts is touching (shorted) to the back shell. In this case, it must be the side connected to the wire leading to the sender unit. Remove your cluster, remove the nuts, center the posts and fiber washers and then replace the nuts and tighten enough to make sure the posts don't slip around.

If removing the wire to the guage results in the guage reading zero with the key on then do test #2 below.

2. The next most likely fault is the wire from the sender to the guage is shorted. It is chafed has a bare spot or a sheet metal screw screwed into the harness causing the short. Disconnect the wire at both ends and use an ohmmeter or electrical test light connected between the wire and the car frame. If the ohmmeter reads near zero or the test light illuminates with both ends of the wire free, then you have a shorted wire to fix. OTOH, if the ohmmeter reads infinity or the test light doesn't illuminate, then the wire is likely good.

If you still haven't fixed it, do #3 below:

3. The next most likely fault is the sender. If the sender is shorted at the upper terminal, you'd have the fault you see. Use the ohmmeter to test the sender with the engine off. Connect the ohmmeter between the sender post and the engine block. It should read infinity or a reading higher than 200 ohms. If it reads lower, tell me what the reading is.

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if you got a sender for a oil LIGHT, it would peg the guage with the key on and the engine not running and when you ran the engine you'd get zero oil pressure. Maybe this is the prob.
 

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I agree with sharkmouth. When you bought the new sending unit, what year car did you tell them you had? I'd be willing to bet you have the sending unit for an idiot light, rather than the one for a gauge. The one for an idiot light is either on or off (hence the name, switch), whereas the one for a gauge varries with the oil pressure, and is usually called out as a sensor or sending unit.
 
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