HVAC / Blower Relay box

J

j5ball

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1968 Lemans / 350 / 2bbl / AC.

Caveat. When I got the car none of the HVAC was working. I do not know if the previous owner had everything hooked up correctly or not. I am trying to get the HVAC working and I am having an issue with getting the HVAC (or Blower) relay box working. I will try to describe what I am finding.

I bought a new relay switch for the car - BWD R378. I tried to get a schematic on the relay but BWD is having problems finding the schematic.
This relay box sits on the firewall in the engine compartment about middle of car. There are 3 prongs on the relay switch.

The middle prong is connected to a gray wire coming from the HVAC control mechanism inside the car. This grey wire has 0 volts when the HVAC control lever is in the off position. This grey wire carries 12 volts to the relay box when the HVAC control switch is NOT in the off position.

One of the other 2 prongs has a larger brown wire connect to the HVAC control mechanism. This wire specifically connects to the blower switch which controls if the blower is off, low, med or high speed. I can connect a line from the battery to this brown wire and get the blower to function on low, med and high speeds. So know the brown wire can carry current, the blower switch works, the wire leading from the blower switch to the blower motor can carry volts and the blower motor works. There is also a smaller brown wire coming from the same prong that connects to the a plug on the AC box.

The third prong, there is a black with orange stripe wire coming from the main wires through the fire wall at the drivers side. I cannot find this wire on any schematic in the manual and I have no idea what this wire does. But this wire is connected to the 3rd prong on relay box. I don't know if this a voltage wire or a ground wire. What is its purpose on the relay box?

Do I really need this relay box? Whats is its purpose?

Thanks . . . JIMB
 
Do you have the factory manual or a Hanes? The factory manual goes into great detail on how everything functions and has detailed wiring diagrams. It is a must have on any restoration. The oldest manual I have is 1970 and can I check tonight and try to answer your questions. What does BWD stand for? You are describing the master A/C relay and it permits high amperage current for the compressor etc. without that amperage having to be present at the switch on the dash. You do realize there is a blower motor resistor on the A/C box in the engine compartment which controls the blower speed and should be in the blower speed switch circuit?
 
Yes. I have a 1968 Pontiac service manual that covers the GTO, Tempest and Firebird. The Lemans is a sub model of the Tempest. I usually can find everything in this manual and its been correct. This is the first time I cannot seem to connect the dots. I have no idea what BWD stands for - its just the manufacturer of the relay switch I bought.

Yes - I agree and do see the wire in the blower speed switch circuit.

When you refer to "master A/C relay and it permits high amperage current for the compressor" in your reply . . are you talking about the black wire with the orange stripe (from my post) ?
 
Got with the relay manufacturer and found out how it functions.
The middle prong is connected to the grey wire. The grey wire carries 12 volts when the HVAC control arm is moved from the off position. When the HVAC arm is moved to an on position the grey wire carries 12 volts to the relay switch. This creates a magnetic field in the relay box which then closes contact points between the left and right prongs. Thus creating a connection between the left and right prong.

So, one of the prongs is the brown wire that goes to the blower switch on the HVAC mechanism in the cabin.

I am assuming then that the back orange stripped wire is a power wire coming from somewhere.

When the HVAC control arm is moved from the off position the contact points in the relay box are connected. So if the black orange stripped wire carries power then this power is then connected to the brown wire which is connected to the blower switch on the HVAC control mechanism.
______________________________
 
ok. Went ahead and unwrapped all the wires . .. .I really did not want to because now I have to wrap them back up .. or take the easy way out and put a plastic tube on it.

Anyway. The black orange stripped wire comes from the alternator. There is a fuse in the wire near the alternator - a 30 amp glass fuse. Which by the way - IS BLOWN!!! So, it appears the power to the HVAC unit in the cabin is powered by the alternator. Off to get a 30 amp glass fuse.

More to come.
 
OK, so it sounds like your questions have been answered?
 
HVAC controls now work. The HVAC mechanism in the cabin receives its current from the alternator. I love these old cars!!!!
 
melsg5 said:
You are describing the master A/C relay and it permits high amperage current for the compressor etc. without that amperage having to be present at the switch on the dash. You do realize there is a blower motor resistor on the A/C box in the engine compartment which controls the blower speed and should be in the blower speed switch circuit?
Click to expand...


Where is this blower motor resistor located? I can not get my blower to work in the 'high' speed position. I have replaced the relay and am trying to figure out what to do next.
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In the engine compartment on top of the blower case.
 
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