Does anybody know where the blocking relay is on a 71 BB car? I've looked to the right of the heater box under the right side dash, it doesn't look like anything was ever mounted to the box. The car has been kinda restored though.

 

Do you mean the one that switches off the compressor at low temp? There's a switch, not a relay, inline with the green(?) wire going to the compressor clutch, it's mounted through the AC box on the blower side (4 o'clock from the blower)

 

That's were it should be. My 71 has them there. The AIM shows it on C60 Sheet E9. Just to confirm, the blocking relay was only used with AC equipped cars.

 

I found that one, but there's no juice going to it when the compressor is supposed to be on. There is juice at the wheel switch though. Only thing left in the circuit is the blocking relay, which I can't find. I sure hate to bubba it, but running out of options. I'm not going to pull the heater box to look for the blocking relay harness which may or may not be there. I'll try to fish out the harness from behind the heater box, if I can't, I'll cut the wire at the wheel switch and run a new one to a relay, then out to the thermo switch you speak of.

Thanks for the confirmation Duffman

 

The blocking relay should be mounted next to the anit-dieseling relay on the heater box passenger side under the dash pad. They are still mounted on my 71 but I have yet to find out if it works.

 

Yeah, I found them. Some joker had hidden them in the console, I didn't even see them till I traced the harness that was supposed to go to the heater box to under the front fiberoptics in the console. We have a new blocking relay coming. It's basically just the compressor relay, no clue why they call it a blocking relay.

 

Did this ever fix your problem?

I also have a 71, and I found a black relay mounted under the console and fiber optics, which I think is one of those relays, either the blocking or anti-diesel. There is only one relay, then another connector with two heavy guage purple wires, and a single connector with a gray wire just hanging loose under where the ashtray is. Its not the ashtray light or cigarette lighter, they both work fine.

Anyone have a clue what those purple wires go to, and what the part# is? I think it's the other relay, but without knowing the first one, I can't know the second one.

I also traced down the wires leading to the "thermal switch" on the passenger side of the evaporator housing that was mentioned earlier in this thread. No switch, just the wires, and a hole where the switch goes.

Anyone know the part number, and where to get this thermal switch?

As you can see, I am slowly trying to put the AC back in operation if I decide to use it. Car was a bit hot last August by the feet. "Bit" is an understatement. It was so hot, I had to switch feet when driving any distance. Blower didn't work (shorted and melted wire), and I found that the heater bypass control valve had a bad vacuum line. That explains the constant heat on the feet. Also the fresh air vent plenums didn't open either. Seems like I got them all working now. Reminds me of the game "mouse trap" with all the different gates to get working.

Was bored over the winter, so I replaced the Engine Harness and AC Harness, redid the AC vacuum control lines, the console controls, and the blower switch, and fixed the clock while I was there. Found the oil line with a slow leak to the oil pressure gauge. Kind of snowballed for a while, but coming together nicely.

Okay, last question. I did a pre-flight check on the harness to see if the lights and blower works without any smoke. (USAF Avionics training) I guess I did pretty good, everything worked...almost. The blower only works in the 3rd switch position. When I flip it to HIGH, the motor stops, and I can hear the relay under the hood flip. That makes sense(I think) since I do not have the compressor hooked up at this point. But why doesn't position 1 and 2 work on the switch? (New switch)

Appreciate any and all feedback.

 

Look straight up on the passenger side, below the lower dash pad adjacent to the interior light. Both the anti-diesel and blocking relays are there, screwed to the RH side of the interior Harrison air box plenum.
The anti-diesel relay triggers the AC compressor clutch to engage for a fixed time period on engine shut down. It's job is to add drag to the motor to prevent run-on.

When you switch off the engine, the anti-diesel relay over-rides the settings on the heater/AC control panel to FORCE the compressor to engage. Unfortunately, the side-effect of simply turning the A/C on would be the inadvertent start-up of the heater blower motor. The blocking relay kicks in at the same time to kill the blower motor.

The wiring harness that runs to the two special A/C relays has a single, ring loop terminal, ground wire on it. To make the system work as it was designed, that ground wire HAS to be connected to the mounting screws holding the relays.

 

Thanks for the hint on the other relays. Anyone have an idea on the other questions?