Engine run on with A/C on - The 1947 - Present Chevrolet & GMC Truck Message Board Network

walker · 12-17-2007, 10:41 AM

I ran across an interesting problem wit my '67 the other day. It has always run for a few seconds after the key was off and removed from the switch. This is not dieseling, or anything like that just normal running. I had played around with fuel mixture and timing to try to make it go away, but it never changed.
Yesterday I had turned it of after turning off the heater fan and it shut right down, so I restarted it and turned the fan on and shut the engine down. It would runs for a few seconds after the key was off when the fan was on, but if I turned the fan off first it would shut right off. Puzzling, the only thing I could think was happening was that the fan was somehow inducing electric current into the ignition system.
Has anybody else run cross this, and what has been done about it?
BTW it has an HEI dizzy in it and has been converted to an internal reg alternator by jumping the wires at the VR (I found the directions on the site somewhere).

Longhorn Man · 12-17-2007, 02:25 PM

Where do you have the HEI wired to?
Most likely, if you move the HEI wire somewhere else, it'll kill the run on that you are having.
And you are absolutly right, your blower motor is creating just enough voltage to cause the HEI to fire. This is a common issue when an electic fan is installed.

walker · 12-17-2007, 09:04 PM

Thanks Longhorn. I'll have to look and see where in the fuse box I wired it to but I seem to recall the A/C wire in or near the same spot. I'll look in the morning and plug it in somewhere else to try it out.

Longhorn Man · 12-18-2007, 05:17 AM

Failing that, yoiu can put a diode inline on the heater wire, that'll block the power.

mrein3 · 12-18-2007, 06:56 AM

Mine does the same thing but only when the fan is on high. When I converted to AC I bought a new AC fan switch and the plastic plug for the back of the switch. After putting it all together I discovered the factory relay I got with my parts buy was bad. So I plumbed in an aftermarket relay ($5 at Checkers and that relay matches all the other relays I've added for other stuff).

Well the power for that high-speed relay feeds back to the HEI I added.

My solution is simple. If I shut the truck off and it still runs, I reach down and flip the fan off high.

12-17-2007, 10:41 AM	#1
walker Registered User Join Date: Jul 2003 Location: north of Phx AZ about 30 miles Posts: 698	Engine run on with A/C on I ran across an interesting problem wit my '67 the other day. It has always run for a few seconds after the key was off and removed from the switch. This is not dieseling, or anything like that just normal running. I had played around with fuel mixture and timing to try to make it go away, but it never changed. Yesterday I had turned it of after turning off the heater fan and it shut right down, so I restarted it and turned the fan on and shut the engine down. It would runs for a few seconds after the key was off when the fan was on, but if I turned the fan off first it would shut right off. Puzzling, the only thing I could think was happening was that the fan was somehow inducing electric current into the ignition system. Has anybody else run cross this, and what has been done about it? BTW it has an HEI dizzy in it and has been converted to an internal reg alternator by jumping the wires at the VR (I found the directions on the site somewhere). __________________ Andy,Phx AZ '67 C-10 (Ahhh, done at last. Well there is that disk front end I want to put in and...) "23 C-Cab-sold '48 Ford 8N tractor(still working) '67 Scout(Now on the road) '70 MG B.-sold

12-17-2007, 02:25 PM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Engine run on with A/C on Where do you have the HEI wired to? Most likely, if you move the HEI wire somewhere else, it'll kill the run on that you are having. And you are absolutly right, your blower motor is creating just enough voltage to cause the HEI to fire. This is a common issue when an electic fan is installed.

12-17-2007, 09:04 PM	#3
walker Registered User Join Date: Jul 2003 Location: north of Phx AZ about 30 miles Posts: 698	Re: Engine run on with A/C on Thanks Longhorn. I'll have to look and see where in the fuse box I wired it to but I seem to recall the A/C wire in or near the same spot. I'll look in the morning and plug it in somewhere else to try it out. __________________ Andy,Phx AZ '67 C-10 (Ahhh, done at last. Well there is that disk front end I want to put in and...) "23 C-Cab-sold '48 Ford 8N tractor(still working) '67 Scout(Now on the road) '70 MG B.-sold

12-18-2007, 05:17 AM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Engine run on with A/C on Failing that, yoiu can put a diode inline on the heater wire, that'll block the power.

12-18-2007, 06:56 AM	#5
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,253	Re: Engine run on with A/C on Mine does the same thing but only when the fan is on high. When I converted to AC I bought a new AC fan switch and the plastic plug for the back of the switch. After putting it all together I discovered the factory relay I got with my parts buy was bad. So I plumbed in an aftermarket relay ($5 at Checkers and that relay matches all the other relays I've added for other stuff). Well the power for that high-speed relay feeds back to the HEI I added. My solution is simple. If I shut the truck off and it still runs, I reach down and flip the fan off high. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

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