Sometimes when wiring new items into an old car, you can run into unexpected problems. Such is the case when we performed the Auto Meter InVision Dash display installation into a 1972 Ford Truck with original wiring.
What we ended up with, was a case where the turn signal indicators simply did not work on the new instrument cluster. They did however work on the original cluster. Even though they worked, they were a bit dim. After re-tracing our steps and verifying that we were indeed using the correct wires, as verified with both, a diagram, and a test light, I was left “wondering what is going on”. It was then that I decided to use a DVOM (volt meter) to measure what was coming from the vehicles turn signal wires.
What I found was right about 5v. This is not normal, and the original truck turn signal wires should have been outputting much closer to 12v. This was enough to illuminate the incandescent bulbs dimly, however was not quite enough to trigger the LED’s used for the new dash, as they operate (based on the operating circuit that drives them) at 6v and higher. If I start the engine, and the charging system starts outputting, the signals would work on the new dash, due to the increase in both voltage and current. But not with simply key-on/engine off after the battery voltage dropped a little.
Realistically the old truck is a good candidate for a new wiring harness (and a new turn signal switch), but for now we are going to use a couple of relays to make them work properly.
Here is what you need. A standard 4, or 5 pin Bosch style relay and either a relay connector harness, or simply some ¼” female Quick Disconnect (also known as female spade) terminals. Since we are not passing through a high amount of current, your wire size can be anywhere from 22g to 18g wire for all terminals. Each terminal has a number next to it for identification.
- Connect 12v key on power, or battery power to terminal 30 of each relay.
- Connect ground to terminal 86 of each relay.
- Connect the vehicles right turn wire to terminal 85 of one relay.
- Connect the blue wire with red stripe from the InVision dash to terminal 87 of the relay with the right turn signal wire.
- Connect the vehicles left turn wire to terminal 85 of one relay.
- Connect the blue wire with white stripe from the InVision dash to terminal 87 of the relay with the right turn signal wire.
- Nothing connects to the center terminal (if using a 5 terminal relay) which is numbered 87a.
The turn signal indicators now function with the engine not running! Now, you may ask, if they worked with the engine running, why not just leave them alone and not wire in the relay(s)? The simple answer is that we wanted to make sure that they still functioned if the hazard flashers were engaged in the case of a breakdown. Otherwise, we could have left it as it was.
You can use the same style relay for the hi beam indicator, and even for powering the dash if you have low key on input voltage.
Many of us (myself included) have come to rely on a simple test light for checking for the presence of power (and also ground), but when something does not look or function correctly, don’t be afraid to ditch the test light in favor of a DVOM. This will give you the real story that in this case the test light could not!
