Question About A/C Relays On Early C3's

[toobroketoretire]

Does anyone know why the early C3's used two A/C clutch relays? They're mounted side-by-side and it appears the 1st one simply energizes the 2nd one which doesn't make any sense to me. I'm sure the 2nd relay is supposed to be energized under certain conditions but WHAT conditions? I bypassed the 2nd relay years ago and my A/C has always worked great without it. I guess I could study a wiring diagram and maybe figure it out but I was hoping one of you would know why the 2nd relay was used. I'm not going to lose any sleep over it but it has puzzled me.

 

[Antz81]

I can't find a diagram that shows any relays for the A/c clutch. Just the one for the blower.
Your car is a '71 right?

 

[toobroketoretire]

I can't find a diagram that shows any relays for the A/c clutch. Just the one for the blower.
Your car is a '71 right?

Yes, it's a '71 and the two relays are mounted side-by-side on the rear face of the heater core box right above the passenger's feet.

 

[Antz81]

I've found some more info on this now.

It's an anti-dieseling system, and was only used on the '71.

There is an anti-dieseling relay which applies power to the compressor clutch for a few seconds after the ignition is switched off. The extra load of the compressor on the engine helps to prevent any dieseling (if it is going to occur). If this relay fails in the open position the it will likely go unnoticed. If it fails in the closed position your A/C will be permanently on, causing a battery drain.

The second relay is a blocking relay which prevents the blower motor from running while the anti-dieseling relay is running. If this relay fails in the open position then your A/C will not work. If it fails in the closed position the your blower will run during the anti-dieseling cycle.
It seems both relays tend to fail in the open position.

 

[toobroketoretire]

I've found some more info on this now.

It's an anti-dieseling system, and was only used on the '71.

There is an anti-dieseling relay which applies power to the compressor clutch for a few seconds after the ignition is switched off. The extra load of the compressor on the engine helps to prevent any dieseling (if it is going to occur). If this relay fails in the open position the it will likely go unnoticed. If it fails in the closed position your A/C will be permanently on, causing a battery drain.

The second relay is a blocking relay which prevents the blower motor from running while the anti-dieseling relay is running. If this relay fails in the open position then your A/C will not work. If it fails in the closed position the your blower will run during the anti-dieseling cycle.
It seems both relays tend to fail in the open position.

Thank you for researching it. I knew there had to be a reason for it being there but couldn't understand WHY it was there after I bypassed it and found it worked great without it.