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truckin 79 02-24-2020 04:52 PM

73-75 Gauge Tachometer

73-75: Amp gauge instead of volt gauge. This cluster will only work for these three years only and are NOT serviceable since they are completely sealed. Just plugging one of these clusters in a 76 and up truck will result in instant meltdown upon turning the key.
So I was reading the

73-87 Factory Tachometer Information
page and was wondering why a meltdown. I have a 73-75 Gauge cluster with a tachometer and wanted to see if it is possible to put in my 79. Would the meltdown happen because of AMP gauge instead of VOLT gauge in earlier models? I figure the tach has nothing to do with it, correct. So can I just change AMP to VOLT and be okay hooking up cluster to 79 truck?

ray_mcavoy 02-24-2020 05:42 PM

Re: 73-75 Gauge Tachometer
Yes, the meltdown being referenced there is due to the ammeter vs. the voltmeter and has nothing to do with the tachometer.

The coil inside the ammeter has very low resistance and will act like a short circuit if it is connected to the wiring that is configured for a voltmeter.

So yes, you can swap a voltmeter into the cluster to use it in your 79. If I remember correctly, the terminals on the back of the gauges are oriented differently for the ammeter vs. voltmeter so the voltmeter won't plug directly into the earlier cluster. You'll probably have to make up a small jumper lead to connect one of the voltmeter terminals to the cluster's printed circuit.

Also be aware that the cluster connector pin-outs are different for 73-75 vs. 78+ so you'll also have to re-pin the connector to use the 73-75 cluster in your 79. And you will also have to add an oil pressure line since the 73-75 cluster will have a mechanical oil pressure gauge instead of electric.

EDIT: If your 79 already has gauges (but no tach), a 1978 or newer tach cluster would be a direct plug-in swap. So it would probably be easier to see if you can sell or trade the 73-75 tach cluster for a 78 or newer one.

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