AUX Relays

[Bitsyncmaster]

I had solid state AUX relays I built about 6 years ago. But when I installed my "BATTERY RELAY" I needed to have the ground wire for the AUX relay go open circuit to turn that AUX relay off even if the key was on (the control voltage is +12 volts). Well that did not turn off my solid state AUX relay. I did a lot of design work on my solid state relays again and have come up with a circuit change that will "turn off" the solid state relays with the ground removed.

So I was going to make up some new solid state AUX relays and measured the current capacity needed for the first AUX relay. With the engine running current flow is 7.8 amps, add the wipers onto high speed and the current jumps to about 15 amps (varys as the wipers move). What am I missing? Why did they use a 70 amp relay for that circuit?

[DMCMW Dave]

I had solid state AUX relays I built about 6 years ago. But when I installed my "BATTERY RELAY" I needed to have the ground wire for the AUX relay go open circuit to turn that AUX relay off even if the key was on (the control voltage is +12 volts). Well that did not turn off my solid state AUX relay. I did a lot of design work on my solid state relays again and have come up with a circuit change that will "turn off" the solid state relays with the ground removed.

So I was going to make up some new solid state AUX relays and measured the current capacity needed for the first AUX relay. With the engine running current flow is 7.8 amps, add the wipers onto high speed and the current jumps to about 15 amps (varys as the wipers move). What am I missing? Why did they use a 70 amp relay for that circuit?

Probably because it was on someone's shelf in huge quantity and cheap. It's the same part as the "main" relay, there is economy in parts re-use as well, although that's probably not anywhere near that much load either.

That also explains why I've NEVER seen one fail. Ever.

[Patrick C]

That also explains why I've NEVER seen one fail. Ever.

And with that comment, Dave just jinxed the entire fleet. No one expect their car to start tomorrow.

[DMCMW Dave]

And with that comment, Dave just jinxed the entire fleet. No one expect their car to start tomorrow.

It will be one of those parts where they start dropping like flies in "year 35" . . . . .

[Bitsyncmaster]

Yes I know they never fail contact wise. But there was someone with one that shorted to the case. My reason to go solid state is just to reduce the heat in the relay compartment. Each of those 70 amp relays are always on when the engine is running. With 86 ohms on the relay coil that is 2.3 watts of heat (at 14 volts) for each relay.

[Bitsyncmaster]

I checked the current on the second AUX relay and that is less than one amp until you run an accessory.

Rear defrost uses 8 amps.
Window motors about 6 amps but spike when they hit the full open of full closed. I did not want to keep it running there to measure currents. AC clutch and mirrors are other loads.

So I cut the AUX relay wires and installed a standard relay socket for the two relays. I don't think you would run more than 30 amps in each (maybe if you jam both windows at the same time).

The first AUX relay has a 10 AWG power wire and 10 AWG load wire. Second AUX relay had two 12 AWG power wires (spliced with the relay mating pin). I butt joined those two wires and ran one 12 AWG to the relay socket. The load wire was also 10 AWG. I do have relay socket pins for 12-10 AWG wire.

I'm going to glue my new solid state AUX relays onto aluminum angle to mount them like the OEM relays mounted.

My solid state AUX relays use 0.06 watts of power each.

[Bitsyncmaster]

Have been thinking of why some of the circuits on the "ACC" AUX relay were not wired to the "RUN" AUX relay.

Why would you need the rear defogger on without the engine running?
Same goes for the AC compressor. Although it's nice to test your cooling fans.
Same goes for blower motor.

Don't think I will change the wiring. That would be fuse block wire changes. Only gain would be not accidentally running your battery down with the rear defogger if your just listening to the radio.