Gauges or Warning Lights…does it matter?

Most classic car owners know if their car has factory gauges or original warning (aka “idiot”) lights. What they may not know is why this is an important factor in selecting the correct wiring harness and sending unit.

Wire isn’t picky. It will send information to the back of your gauges no matter what. The usefulness of that information all comes down to the way sending unit makes its signal. There are two basic types of sending unit, and both rely on grounds:

Warning Light:

Warning light senders create an “open” in the circuit, so they are essentially an on/off switch. Contacts inside the switch are open (no current can flow) during normal operation, but close at a specific value- be it coolant temperature or oil pressure. When the critical temperature or pressure range hits, the circuit is completed and the dash light turns on.


This one is resistance to ground. Most gauge sending units control the dash signal by decreasing the amount of resistance in the circuit as the sending unit heats or pressure changes….this translates to a higher reading on the face of the gauge. It’s not an on off switch, but more of a sliding scale dictated by the resistor inside the switch.

So if the wire itself doesn’t care, then why does this make a difference in the harness part number?

Two very important reasons-

Gauge and warning light sending units may have different connection methods. A part designed for gauges may have a ring terminal, while the warning light harness of the same year uses a push-on blade. Both work, but they are certainly not interchangeable, and a factory harness will only have provisions for the correct, matching sender.

Since the warning light and gauge are both looking for different signals, installing the correct sender at the end of the wire is very important. If you hook your gauge harness to a warning light sender, you’ll never receive meaningful information from your gauge. In fact, you may not see any reading at all.

As with many parts of a build, a little extra time to make sure you’re adding the right component can prevent troubleshooting down the line!