# AC Heater Master Relay how does it ground



## alanmay0 (Sep 3, 2015)

Hello I have been trying to figure out why my ac and heat controls do not work on my 67 GTO. I must add with the help of Scott. Most of my problems came down to the master relay that is attached to the firewall with two small screws. Attached is a wiring diagram supplied by Scott that shows the master relay being grounded after it wraps around the coil. Well my question is how does it ground. Also attached is a picture of the master relay with the casing removed and a picture of the case itself. No way can the case be used as the ground because as you can see in the pictures the case does not touch any metal or wires inside the casing. 

Now with the cover removed and the relay connected to the wiring harness and I ground the opposite side of the one gray wire going into the relay the points inside the relay close and the ac clutch is energized and the fan motor starts to work. Of course this is with the ignition in the on position. If I turn the ignition off then grounding the master relay does nothing. 

So it is obvious that the master relay has to be grounded but how.
That is my question.
thanks for the help in advance


----------



## PontiacJim (Dec 29, 2012)

Alan, not a clue on working with AC, BUT, noticed your plug has 5 spades and the diagram has 3 spades where the wiring harness plugs in.

Did a Google search and found a master relay for 1968 Pontiac and up and it had only 3 spades like the diagram. Is that the correct wiring diagram for year 1967? My thinking is that perhaps one of the spades is actually used for grounding? Could one of your 5 wires coming off the wire harness be a ground?


----------



## alanmay0 (Sep 3, 2015)

That is a good question. This diagram was provided to me by Scott. It does appear to be accurate. My master relay does have 4 spades coming off of it. The two browns use one spade. The black and pink use one spade and the gray wire uses one spade. That leaves one spade left unused per the new wiring harness I have and as per the harness I took off of the car. Both harness are the same and do not have a wire running to the empty gray spade. 

Like I said when I did add another gray wire to the open/unused spade the blower worked.

Now the update as of 11/11/17. When using the not used spade as a grounding spade ONLY the 2nd and 3rd speed work using the fan speed selector inside the car. When the unit is turned to outside or inside (which is the AC) the ac clutch works. I get no LOW or HIGH speed in the fan. Now there is the large black and pink wire running to the 4 speed fan switch which is suppose to be HIGH but when the switch is moved to high nothing happens. If I take 12v and touch it to the black and pink wire at the switch the HIGH speed works. 

I have checked the resistors in the ac unit and they work. The main relay seems to work when I ground it and the switch, which is new, only works speeds 2 and 3. Now I pluged in the old switch and once again I only get speeds 2 and 3. Very confusing to me. I know very little about electrical systems and can't figure this out.

So I am missing the high and low speeds.

Any and all help will be greatly appreciated.

thank you in advance


----------



## Goat Roper (Oct 28, 2014)

I had problems with mine like you are describing and the fix ended up being one of the tabs wasn't going into the Molex connector properly.
I discovered it by taking the relay off the firewall and looking at it while I plugged in the Molex.
I adjusted the tab, plugged it in then attached it to the firewall.
The problem was plugging it in from the bottom and not being able to see.
I spent two days farting around with it before I found the problem.


----------



## JNYBOY (Sep 28, 2017)

alanmay0 said:


> Hello I have been trying to figure out why my ac and heat controls do not work on my 67 GTO. I must add with the help of Scott. Most of my problems came down to the master relay that is attached to the firewall with two small screws. Attached is a wiring diagram supplied by Scott that shows the master relay being grounded after it wraps around the coil. Well my question is how does it ground. Also attached is a picture of the master relay with the casing removed and a picture of the case itself. No way can the case be used as the ground because as you can see in the pictures the case does not touch any metal or wires inside the casing.
> 
> Now with the cover removed and the relay connected to the wiring harness and I ground the opposite side of the one gray wire going into the relay the points inside the relay close and the ac clutch is energized and the fan motor starts to work. Of course this is with the ignition in the on position. If I turn the ignition off then grounding the master relay does nothing.
> 
> ...



...Ever get it figured out ???


----------



## alanmay0 (Sep 3, 2015)

Not yet still working on it. Thanks


----------



## JNYBOY (Sep 28, 2017)

Not sure of where you are at with this, but on my 66', the plug-in with the two browns measured just 3 ohms to ground, Possibly the trail to grnd that is needed ? do you have that reading ?


----------



## alanmay0 (Sep 3, 2015)

no i have not measured that because it looks to me that the double brown is hot when the relay is closed allowing current to flow to the blower motor resistor. I am thinking of buying a new resistor and seeing what happens. I get 3 and high but no low or 1 when I ground the master relay. I have been told that the only purpose of the master relay is to give you the high speed on the blower. The other speeds use the resistor. I wish someone was an expert with the master relay and how it effects the resistor. You know what color wire does what etc. I know that the small black wire is low, the red and white wire is med and the black and orange wire is high and the brown wire supplies the voltage to the resistor. But I do get hot to the brown and the red/white on med and then to the black/orange and brown for high but then for low I get voltage to all the colored wires. Just don't understand how it all should work. Of course all this only happens when I ground the master relay. Otherwise nothing.

Really need someones expertise in this matter. Hoping someone will come forward.


----------



## Goat Roper (Oct 28, 2014)

Hey Alan

How are your grounds from the engine to the firewall?
Have you buzzed engine to firewall with your DMM?
Cleaned the paint under the banjos and behind the relay?
Have you buzzed from the housing to the tabs to see if any of them ground to the housing?
I had grounding issues I had to trace out and I also added engine to dash ground that fixed the flickering in the instrument panel and GI.

This diagram is for a '66, maybe it will help.


----------



## alanmay0 (Sep 3, 2015)

Thanks for all the info. I have the laminated copy of the wiring diagram that you posted. You are missing the point. The master relay box is not meant to be the ground for the relay. If you look at the pictures I posted you would see that nothing metal touches the relay case thus it does not ground. So my question is still the same the wiring diagram shows the master relay grounding so I am asking how.

thank you for your information


----------



## Goat Roper (Oct 28, 2014)

I don't think it grounds, the motors and compressor ground on the other side of the units.
Look at your compressor, two tabs one it the switch leg and the other is ground.
Fan motor if memory serves is one wire and it grounds itself on the housing.
Make sure you have good contact on the resistor on the blower housing.
Sorry I can't be of more help, I had so many bad wires I replaced the under hood harness and the A/C harness.
So brittle the insulation was crumbling.


----------

