1971 Covette AC Blocking Relay
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From: Ohio
1971 Covette AC Blocking Relay
Does anyone know the part number for a 1971 AC blocking relay the is mounted on blower box on inside of the car. Is there a replacement for it?
Ted
Ted
Last edited by thelal1; Aug 8, 2020 at 10:30 AM. Reason: Mispell
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From: Ohio
Relay
Some idea that GM eliminated after 1971.
The Anti Dieseling relay when turning the key off shot power to compressor to put drag on engine to help shut it down. Not to have run on.
The Blocking relay would kill power to the blower motor not to run after turning the key off. THAT is the proper name that GM has on diagram. Blocking relay.
Then when turning on AC , carb selenoid would kick in, increasing rpm so engine would handle extra load. If not would probably cause engine to stall at lower rpm, causing ac compressor probably to lock up.
After not being able to find an actual 3 prong "blocking" relay, I gave up and bypassed both.
Also when I jumpered the wire on anti dieseling relay, I installed a 20 amp inline fuse in that line just in case with easy access to it.
The Anti Dieseling relay when turning the key off shot power to compressor to put drag on engine to help shut it down. Not to have run on.
The Blocking relay would kill power to the blower motor not to run after turning the key off. THAT is the proper name that GM has on diagram. Blocking relay.
Then when turning on AC , carb selenoid would kick in, increasing rpm so engine would handle extra load. If not would probably cause engine to stall at lower rpm, causing ac compressor probably to lock up.
After not being able to find an actual 3 prong "blocking" relay, I gave up and bypassed both.
Also when I jumpered the wire on anti dieseling relay, I installed a 20 amp inline fuse in that line just in case with easy access to it.
Last edited by thelal1; Aug 9, 2020 at 11:50 AM. Reason: Part name
Le Mans Master
Joined: Jun 2006
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From: Tampa Florida
From 7-11-2005:
https://www.corvetteforum.com/forums...lem-relay.html
https://www.corvetteforum.com/forums...lem-relay.html
Yes, the blocking relay is different than the blower relay. As I'm guessing you've learned, the blocking relay is next to the anti-dieseling relay, and both are located behind the passenger's side dash pad in the interior of the car.
Both the blocking and anti-dieseling relays are used by the a/c system to control when the compressor kicks in.
The anti-dieseling relay engages the a/c compressor when the ignition key is turned off. It does so to add drag to the engine in case it experiences dieseling after turning off the car.
The blocking relay ensures that power is applied to the compressor if the a/c controls are powered.
Both of these controls are simple SPDT relays and can be replaced with aftermarket relays...unless you're a purist.
Both of mine went bad long ago, and now I don't even use a blocking relay, and my anti-dieseling relay is bypassed.
Good luck...
Both the blocking and anti-dieseling relays are used by the a/c system to control when the compressor kicks in.
The anti-dieseling relay engages the a/c compressor when the ignition key is turned off. It does so to add drag to the engine in case it experiences dieseling after turning off the car.
The blocking relay ensures that power is applied to the compressor if the a/c controls are powered.
Both of these controls are simple SPDT relays and can be replaced with aftermarket relays...unless you're a purist.
Both of mine went bad long ago, and now I don't even use a blocking relay, and my anti-dieseling relay is bypassed.
Good luck...
Race Director
Joined: Sep 1999
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From: Plano TX
Hate to disagree with a 15 year old post but...
This is not correct. Previous post is correct. The blocking relay stops power provided by the anti-dieseling relay from back feeding into the blower motor. This is because there is a connection between AC clutch and blower motor so that it runs on at least low speed whenever AC is on.
The anti-dieseling relay is not a normal relay. It also has the 2-5 second timer built in.
The blocking relay ensures that power is applied to the compressor if the a/c controls are powered.
Both of these controls are simple SPDT relays and can be replaced with aftermarket relays...unless you're a purist.
Last edited by zwede; Aug 9, 2020 at 01:09 PM.
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From: Ohio
Relay
Mine was corroded on inside. I checked with Zip, Wilcox, Ecklers, Paragon, Parts Geek, Bosch, Electric Limited, etc.
The ones say that they have a unit to do the same thing but have 4 prongs.
I Wish I could find an original 3 prong that was a made for blocking relay .
Ted
Last edited by thelal1; Aug 9, 2020 at 04:59 PM. Reason: Description
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Race Director
Joined: Sep 1999
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From: Plano TX
Thread Starter
Pro
Joined: Jan 2016
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From: Ohio
Relay
Check with https://www.facebook.com/thecorvetteben/ if he has a good used one.
Ted
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From: Ohio
Relay
That isn't the Blocking Relay,, 2072624 is the Blocking relay number as far as I can find out for now. This is the one I need.
That relay number 3996204 is the anti dieseling relay. There are lots of those out there. I have a new one of those. Bought it for a spare.
Thank
Ted
That relay number 3996204 is the anti dieseling relay. There are lots of those out there. I have a new one of those. Bought it for a spare.
Thank
Ted
Melting Slicks
Joined: May 2002
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From: Laporte, IN
Thread Starter
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Joined: Jan 2016
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From: Ohio
Relay
That's the anti dieseling relay. I have 2 of those. It's the blocking relay mounted next to it on blower box that I need.
Had a source give me a number today that I am checking on. # 2072624. One of it's functions is to kill the power to the ac blower when ignition is turned off while the anti dieseling relay sends power to ac Clutch to help stop the engine from run on after key is turned off.
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From: Ohio
Le Mans Master
Joined: Jun 2006
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From: Tampa Florida
Relay
That's the anti dieseling relay. I have 2 of those. It's the blocking relay mounted next to it on blower box that I need.
Had a source give me a number today that I am checking on. # 2072624. One of it's functions is to kill the power to the ac blower when ignition is turned off while the anti dieseling relay sends power to ac Clutch to help stop the engine from run on after key is turned off.
That's the anti dieseling relay. I have 2 of those. It's the blocking relay mounted next to it on blower box that I need.
Had a source give me a number today that I am checking on. # 2072624. One of it's functions is to kill the power to the ac blower when ignition is turned off while the anti dieseling relay sends power to ac Clutch to help stop the engine from run on after key is turned off.
Have you seen this one online?
https://www.hiltopautoparts.com/prod...olet-corvette/
https://www.mavdrixh.com/index.php?m...ducts_id=46576
Last edited by 71 Green 454; Aug 10, 2020 at 09:28 AM.
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From: Ohio
That's the number but my old one shows 3 Prongs only. That one has 4 prongs. If you have one out, send me the number. Don't pull your dash.
Thanks
Ted
Do you use both relays or have you pulled them and jumpered the Anti Dieseling relay plug.
Thanks
Ted
Do you use both relays or have you pulled them and jumpered the Anti Dieseling relay plug.
Le Mans Master
Joined: Jun 2006
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From: Tampa Florida
Melting Slicks
Joined: May 2002
Posts: 2,473
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From: Laporte, IN
Here are your A/C relays in the parts book go to 9.277
http://www.gmpartswiki.com/getpage?pageid=40748
http://www.gmpartswiki.com/getpage?pageid=40748
Last edited by PJO; Aug 10, 2020 at 10:36 AM.