'72 Engine Harness Connector/Wiring

[prs143]

Neither the oil pressure gauge nor temp gauge are working on the truck I just purchased.

Just double-checking - is the purpose of this connector to supply 12v to oil pressure sender, temp sender and coil? Coil is getting power as truck runs so just figuring out if this connector is to blame.

If so can I cut it out, reconnect the sender wires and give each one 12v?

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[Busboy]

No!, the wires going to the senders conduct variable ground signals to the instrument cluster. They don't operate with 12 volts at all. Pull the cluster out and see if the instrument cluster voltage regulator is functioning correctly. It should have a pulsing signal to the gauges. Do a search here on this site.

[prs143]

No!, the wires going to the senders conduct variable ground signals to the instrument cluster. They don't operate with 12 volts at all. Pull the cluster out and see if the instrument cluster voltage regulator is functioning correctly. It should have a pulsing signal to the gauges. Do a search here on this site.

Thanks for this. I was thinking of replacing the senders anyway as they look to be original and adding extra grounds (battery/block/chassis/firewall etc) to make sure the connection is good. Currently the temp gauge is Permanently all the way to the right and the oil pressure gauge all the way to the left

[prs143]

Can anyone advise the best way to test the Instrument Cluster Voltage Regulator?

[prs143]

Here is my Voltage Regulator - not sure if the visual is of any help in the diagnosis..?

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[sparky72]

Easiest way is to get a test light and touch it to one of the terminals of the voltage regulator (I don’t recall which). It should pulse on and off.

But, since one of your gauges is pegged at max, the voltage regulator is likely okay. The gauges work in the following manner. Pulsed power is applied to the gauge. The current flows through the gauge to the sender (temp, oil, or fuel). The sender acts as a variable resistor, controlling the amount of current flowing through the circuit. The current then flows to ground. Inside the gauge is a small wound-wire coil, wrapped around a thin metal strip, which itself is connected to the needle. When the coil heats, it expands the strip, which presses against the gauge housing and forces the needle to move. When more current flows through the gauge (i.e., when the sender resistance is lower), the needle moves further, and vice versa. See images below.





The gauge above is broken, it has since been fixed. Thread coming soon

So, if you have a gauge that is pegged when you turn the key, that gauges circuit is grounding somewhere after the gauge. If it’s pegged when the key is off, either you have power being applied to the gauge at all times (meaning something is connected where it shouldn’t be), or the gauge is broken and is permanently stuck in the pegged position (which I doubt).

To test the other gauge, ground the wire going to the sender when the key is on. The needle should slowly rise. If it doesn’t, you either have a bad connection between the gauge and the sender, a bad regulator, or a busted gauge.

Of course, the PCB on the rear of the cluster could be bad too. Check for breaks in the copper.

Hope this helps with your diagnosis.