Current going to Blower Motor can be higher than Headlights - why no relay mod?

[CountryBumkin]

Just posting this for discussion purposes only.

I was looking at the circuit for the AC/Heater Blower motor and I see that it is a 30 Amp fused circuit (and that the "full power" to the Blower goes through the "AC/Heater Function Selector Switch".

So I'm wondering if sending 30A through the Selector Switch for the Blower is, or should be, a concern.

We are doing/recommending the "Headlight Relay" mod to keep the headlight switch from melting, and the headlight circuit is on a 16GA fusible link (which from my research shows that a 16GA fusible link is designed for a circuit with 20 (maybe 25) amps max). Therefore the Blower circuit has (potentially) higher current flow than the headlight circuit.

If this is true why are we not seeing melting of the AC/Heater Function Selector Switch plug?

Why is a "relay mod" similar to the headlight mod not needed here too? Is it because the AC/Heater Function Selector Switch electrical connector is just better (heavier duty) than the headlight switch?

[Gary Lewis]

Actually, there has been some discussion about that on FTE. The thread heater recirculated air has a few posts about that. Further, I am planning on a significant upgrade on Dad's truck's wiring, as discussed in Dad's Wiring. And on post #12 of that discussion you'll find the diagram below showing how I'm going to do it. (Yuk! That didn't come across very cleanly. )

But, as Jim said on FTE, the issue is with the small size of wire going to the fuse box. So, while the current draw of the blower motor may be ok for the size of wire running from the fuse box to the motor and back to the switch, by eliminating the load of the motor from the original fuse box the load on the wire feeding it goes down and everybody is happier.

In my plans the power distribution box, liberated from a later F150, will go right behind the battery. So there will be a serious power feed to it, and then the big items will branch off from there, leaving what's on the original fuse box to be fairly minimal. Plus, most of the lighting will use LED's, so that reduces the draw even further.

[85lebaront2]

First, look at the entire wiring diagram for the blower motor. On our trucks the blower motor is fed 12V through the function selector switch, the 30 amp fuse or breaker feeds both the blower and the A/C compressor clutch, yet on the heater only there is also a 30 amp fuse. The blower speed control is on the ground side of the motor.

If you want to add a "relay mod" you need it on the power side of the motor, but controlled by the function selector switch on A/C models. The brown/orange wire provides 12V to the blower motor so that would have to be connected to one side of the relay coil (85 or 86 on a Bosch relay) with the other side grounded. If you really want to unload the blower switch, the a second relay to ground the motor in high would be needed.

If someone really wants to go through all this, then I can give you diagrams and suggested locations for the relays. Only issue I have ever had is the plug inside the cab where the HVAC harness goes through from inside to outside, that plug gets hot enogh to become stuck together.

[CountryBumkin]

First, look at the entire wiring diagram for the blower motor. On our trucks the blower motor is fed 12V through the function selector switch, the 30 amp fuse or breaker feeds both the blower and the A/C compressor clutch, yet on the heater only there is also a 30 amp fuse. The blower speed control is on the ground side of the motor.

If you want to add a "relay mod" you need it on the power side of the motor, but controlled by the function selector switch on A/C models. The brown/orange wire provides 12V to the blower motor so that would have to be connected to one side of the relay coil (85 or 86 on a Bosch relay) with the other side grounded. If you really want to unload the blower switch, the a second relay to ground the motor in high would be needed.

If someone really wants to go through all this, then I can give you diagrams and suggested locations for the relays. Only issue I have ever had is the plug inside the cab where the HVAC harness goes through from inside to outside, that plug gets hot enogh to become stuck together.

I just typed out a complete explanation of Dark Energy and Dark Matter and how they interact with the Higgs boson- but the forum would not let me post. It wouldn't let me sign it. So I'm starting over - and skipping the physics this time.

My thoughts on this are to limit the high current through the AC/Heat Function Selector Switch by installing a relay in the engine compartment near the Blower. The original circuit through the Function Switch would only be used a the control for the relay. This will protect the Function Switch - but doesn't do anything to protect the Blower Switch, but current through Blower switch is split between switch and resistor ( I don't think switch ever sees full current load).

 

[ArdWrknTrk]

First, look at the entire wiring diagram for the blower motor. On our trucks the blower motor is fed 12V through the function selector switch, the 30 amp fuse or breaker feeds both the blower and the A/C compressor clutch, yet on the heater only there is also a 30 amp fuse. The blower speed control is on the ground side of the motor.

If you want to add a "relay mod" you need it on the power side of the motor, but controlled by the function selector switch on A/C models. The brown/orange wire provides 12V to the blower motor so that would have to be connected to one side of the relay coil (85 or 86 on a Bosch relay) with the other side grounded. If you really want to unload the blower switch, the a second relay to ground the motor in high would be needed.

If someone really wants to go through all this, then I can give you diagrams and suggested locations for the relays. Only issue I have ever had is the plug inside the cab where the HVAC harness goes through from inside to outside, that plug gets hot enogh to become stuck together.

I've replaced the socket for the resistor *twice* in my truck....

No relay on the hot side is going to help that.

[Gary Lewis]

I've replaced the socket for the resistor *twice* in my truck....

No relay on the hot side is going to help that.

Mike - Where you suggested is where I have the relay planned for Dad's truck. But, as you and Jim point out, that won't help the blower speed switch nor the relay. However, I've not had problems with either of those, so I'll take my chances.

[85lebaront2]

Mike - Where you suggested is where I have the relay planned for Dad's truck. But, as you and Jim point out, that won't help the blower speed switch nor the relay. However, I've not had problems with either of those, so I'll take my chances.

If you use a relay on the ground (switch) side, it only needs to be on the high leg as that is the highest current. If you use a Bosch relay, and use 30 as the blower motor, 85 or 86 to the high position on the switch, other to a 12V when the system is on source, then 87a would be to the switch and 87 to ground. You will probably need a diode in the switch leg to prevent the system from sticking on high.

[ctubutis]

Not to be the Devil's advocate, but...

I disconnected, cleaned & serviced each wiring harness and made it OEM+ quality again, and I don't have these issues, it works as designed.

Which is a crappy design, the power-distribution stuff they came out with in the later years seriously kicks our gen's ass.

And it's a fair amount of work to take all that stuff apart, it's not something one "just does" unless they have serious time on their hands. ;)

[Gary Lewis]

Not to be the Devil's advocate, but...

I disconnected, cleaned & serviced each wiring harness and made it OEM+ quality again, and I don't have these issues, it works as designed.

Which is a crappy design, the power-distribution stuff they came out with in the later years seriously kicks our gen's ass.

And it's a fair amount of work to take all that stuff apart, it's not something one "just does" unless they have serious time on their hands. ;)

Yes, the power distribution systems on the later trucks were far better than ours. Placing the fuse/relay box by the battery and using small-gauge wire to/from the switches solved a lot of problems. And distributing relays around the engine compartment, as they did later, helped as well.

I don't believe it is going to be terribly difficult to upgrade Dad's truck significantly by using a later power distribution box. As shown, I have a plan but haven't executed it yet. We shall see.

[85lebaront2]

Not to be the Devil's advocate, but...

I disconnected, cleaned & serviced each wiring harness and made it OEM+ quality again, and I don't have these issues, it works as designed.

Which is a crappy design, the power-distribution stuff they came out with in the later years seriously kicks our gen's ass.

And it's a fair amount of work to take all that stuff apart, it's not something one "just does" unless they have serious time on their hands. ;)

The more I dug into Darth's factory wiring the more I became convinced that Ford's electrical design team was on something pretty powerful (I want some!). It's pretty bad when Chrysler's cheap salvation (K-car) had a better setup, even though it was full of fusible links.