Relay questions

[70 Challenger Lover]

Doing a bit of research on relays for upgrading a couple cars of mine. I'm extremely challenged when it comes to electrical matters but I'd like to understand it better. In reading about relay upgrades for headlight wiring, it makes perfect sense and I can see doing that. My question is if a similar upgrade could be done to other things that draw a lot of current such as a blower motor.

I seem to recall the heater box with fan motor having a three position resistor with each circuit having a different amount of resistance to provide different fan speeds. I know there is a noticeable decrease in voltage by running everything through all those skimpy dash wiring harnesses and I am curious if there would be a decent improvement on air flow if I used relays. Particularly high speed.

If so, would the relays be used after the switch but before the heater box relay? And would I need a separate relay for each speed setting coming out of the switch?

[Rich G.]

A relay takes the load off of the switch. It won't effect air flow from the motor. At high speed you should have 12 volts at the motor and that's as fast as it will turn no matter how the 12 volts gets there.

[70 Challenger Lover]

A relay takes the load off of the switch. It won't effect air flow from the motor. At high speed you should have 12 volts at the motor and that's as fast as it will turn no matter how the 12 volts gets there.

I think in theory that's true but there is likely a voltage drop by the time the current travels through miles of skinny old wire and passes through the switch before it reaches the motor. That seems to be the case with headlights which is why folks highly recommend using a relay to ensure the lights get the full 12 volts. I was wondering if the same benefit could be realized with other devices like the blower motor. It would be nice to get a little more air movement on high.

I remember checking

[dodj]

The relays are really taking the load off of the bulkhead connector and associated connections that create voltage drop. By adding relays to the headlights, you take a high current demand device(s) and re-route the high current demand off the B/H connector and instead, route through relays in the engine compartment. Using the B/H connector only for signal current (very low)
For the blower motor, you still need to get the high current demand through the firewall to the interior. So there is no place to put the relay to relieve the B/H connector of that load.

[70 Challenger Lover]

The relays are really taking the load off of the bulkhead connector and associated connections that create voltage drop. By adding relays to the headlights, you take a high current demand device(s) and re-route the high current demand off the B/H connector and instead, route through relays in the engine compartment. Using the B/H connector only for signal current (very low)
For the blower motor, you still need to get the high current demand through the firewall to the interior. So there is no place to put the relay to relieve the B/H connector of that load.

I was thinking about that already. The two cars I have in mind I'm not overly concerned about originality or appearance. I was thinking I could run the current into the dash through something other than the bulkhead connector and locate the relay under the dash somewhere. I guess it comes down to whether I could expect any significant improvement. I suppose I could turn the blower on high and then connect a temporary bypass directly to the fan just to see if there was any meaningful improvement. If not then I wouldn't bother.

[Rich G.]

Just check the voltage to the wire going to the motor. If it's 12v then it's good and that's all you're going to get. Maybe you're motor is just tired after almost 50 yrs. If you're not concerned about originality you can put a bigger motor in.

[crackedback]

Ask Randy (1wild R/T) about the relay in the Green blower motor wire.   The motor does the same thing as the headlight relays or my power window relays do, it improve the performance of the motor/end unit.  The green wire to the blower motor and out of the switch is notorious for heat and inefficiency.  Put a relay in that circuit and you'll be surprised how fast that old tired motor will turn.

I've been trying to get a set up together to put a relay there.

A relay, when wired correctly, will provide a cleaner power supply to the unit it is firing...   no doubt about it. 

These cars may be a 12 volt system, however, they shouldn't be running at 12 volts with the engine running.  If it is 12 volts, you have a serious issue with the charging system.

[70 Challenger Lover]

Ask Randy (1wild R/T) about the relay in the Green blower motor wire.   The motor does the same thing as the headlight relays or my power window relays do, it improve the performance of the motor/end unit.  The green wire to the blower motor and out of the switch is notorious for heat and inefficiency.  Put a relay in that circuit and you'll be surprised how fast that old tired motor will turn.

I've been trying to get a set up together to put a relay there.

A relay, when wired correctly, will provide a cleaner power supply to the unit it is firing...   no doubt about it. 

These cars may be a 12 volt system, however, they shouldn't be running at 12 volts with the engine running.  If it is 12 volts, you have a serious issue with the charging system.

Thanks. That was the explanation I was looking for. I did a test on my 64 Vette just for curiosity sake since both my Challengers are not together yet. I tested the fan at high speed by jumping the fan directly to the battery (which is more like what the relay would do). There is a noticeable improvement to fan speed on high. Not a huge amount but it wouldn't surprise me if my Mopars had skinnier wires or other resistance factors from the Vette.

I have a feeling I'd see a bigger improvement on my Mopars. Not a real issue with non AC cars but on an AC car pushing air through several vents, I'd think Any improvement on fan speed would make the AC feel that much better.

The real reason for doing it in my view would be to minimize the amount of heavy current running through the dash harness and switches. It might not be stock but I don't see anything wrong in trying to improve the original function, particularly if you can do it in a way that is well hidden.

I found this on the Internet. Do you think it would work in our application?

[BIGSHCLUNK]

I did do a video on ROADMAN's headlamp relay kit. I can post a link if your interested

[70 Challenger Lover]

I might have seen that one. As stated, I'm definitely no electronics expert but I was thinking that a fan motor would pose a different challenge since you are running power to the fan at different speeds and still needed to use the factory resistor to accomplish that task.

[crackedback]

IIRC, it's a little more complicated than plugging in a relay.   There are diodes involved to prevent back feeding issues.

You can pull the power for the blower motor relay right off the fuseblock if you didn't want to run a dedicated circuit in from the battery/alternator.  The issue is the wiring/switch etc in the mopar harness.  It's basically one wire along the firewall to the blower motor connector.

[70 Challenger Lover]

Maybe I'll wait until you develop one for sale. Haha. Certainly don't want to screw it all up.

[truckinman466]

I'd be interested in seeing the link to your video Bigshclunk

[BIGSHCLUNK]

@truckinman466  here you go !

https://www.youtube.com/watch?v=k1_x8pSeq1M