The Stovebolt Site Forums The Shop Area The Electrical Bay Heater/Defrost Wiring

Hello all,

I'm at the end of a 12v conversion on my 3600 and I was wondering if anyone had a wiring diagram that included the heater/defrost motors in it? I have two variable switches and two motors in the unit itself. I haven't added in in-line resistors yet and that was next on my list, however the new 12v mechanical flasher socket seems to have a loose wire. When I wiggle it with the engine running, both heater motors turn on. This seems impossible since both switches for them are in the 'off' position, but that's what's happening. I just want to chase the heater wires back to make sure they are correct. I didn't have any issues like this with 6v, so this seems pretty odd to me.

Thanks!

The '48/'49 Chevy truck wiring diagram I just pulled from The Old Car Manual Project doesn't show the heater. Must have been an option.

Because of the "2 fan motor" heater/defroster you have I'm guessing your 12 volt conversion wiring system is aftermarket. Don't you have a diagram?

This might be a Canadian thing or dealer installed. I guess there won't be any diagrams. Would it be best to wire this to the aux connector on the light switch?

That's what I would do. Tie the feed for the two switches to the aux terminal of the ignition switch.

Two 12v fan motors will pull quite a bit of power, our school bus fleet at work runs 20 amp fuses for each two motor heater using 14 gauge wire. The wires are quite long so the big wire is needed. They use every bit of the 20 amp fuse, just a little corrosion at the connection will knock out a fuse or trip the circuit breakers. These are likely bigger motors than yours.

If you will be running both motors at the same time, I would use a relay with a heavy wire off the amp gauge as a relay feed, then use the ignition source for the switch to turn on the relay.
Powering off the light switch could leave you in the dark if running lights, defrost, and heat all at once. The light switch should have a built in circuit breaker designed for the lighting load, adding in two motors could put the amp load too high for the breaker.

I don't drive in winter and have never opened the valves to allow coolant to flow into the heater core, so these are more for show and the defrost is likely the only one I would ever use. That being said, I never had an issue with them before the conversion. The blue-wire from the three prong flasher socket seems to be tripping the fans to come on. Any idea how this could happen?

Sounds like you have a short between circuits (insulation gone from both wires and the copper touching.) A rewire should fix that.
Joe's idea of using a relay to isolate the heavy load from the ignition switch is a good one. Pretty easy to do. If needed you could run a separate heavy gauge wire from the Starter terminal (with a fuse) to the relay, but the Amp gauge as a source should work OK. If you're still running the 6V wiring, the gauge should be good, as using 12V on the same load originally run on 6V only draws half the amps.

Seeing as how a 48 Chevy truck didn't leave the factory with turn signals, it will be hard to diagnose this problem without an up to date wiring diagram.

Well, the heater fans have old cloth covered wiring and I'd be surprised if they weren't original or a dealer add-on. The fans are both wired to the ignition switch. The 7 wire signal switch seems to be wired correctly, with black/blue going to the flasher relay. Power to the relay from the aux on headlight switch. Problem is all terminals on the flasher have 12v (or about 9 when it lights the signal lights back and front. ). But no flashing. It was working previously for a bit when running but now the relay doesn't move. It did say it was for LEDs but maybe that wasn't correct. Also, the little indicator bulb in the signal housing doesn't seem to get power either, which is the blue wire. I'll try picking up another flasher, but I don't know where to go from there if it gives the same result.

Some of the LED specific 3 prong flashers have a separate ground wire for the electronics inside the can. Any chance yours is that way?

If you are reading 9 volts on the wire between the flasher and power with a regular (not electronic) flasher you'll never get enough current thru the bi-metallic strip to heat it up and make the flasher work. As a troubleshooting step, disconnect the flasher power wire from the headlight switch and hook it directly to the ammeter + side to see if the flashers work there

Well, I have seen the flashers with the ground wire and I assume those are for low-voltage LEDs. I just swapped out my rears for incandescent and it flashes as it should. The indicator light in the signal switch still doesn't work though but I've gone this long without it, so I guess I'll live. For the heater fans, it looks like it was a loose wire causing them to come on. I still have to grab 12v bulbs as they are illuminated switches and put the resistors inline. Since I have two fans, I'll probably look to mount the resistors to a piece of aluminum or something and hand under the dash.

Better yet, mount the resistors inside the air ducts of the heater to keep them cool. All muti-speed resistors packs are mounted in the air stream as way to cool them, might as well follow them.

If you want LED tail lights / brake lights, install a 1157 bulb and socket up in the frame rail out of the weather and out of site to create a load on the flasher, the LED's will then work as they should. #2357 is a incandescence bulb with the same tail tail light as a 1157, but a brighter brake light. Paint the inside of the tail light white and do away with the LED's. #2057 is a Halogen version of the 1157, they get way hot, so you need glass lens. They will melt plastic.