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Hey, everyone!

We are halfway through our road-trip up to Dearborn from Atlanta. The car ran great on the backroads up to Chattanooga. Before we got on I-75 we put the top up and turned on the factory AC. As we merged into traffic, the radio shut off and the blower shut down. I also noticed the gauges dropping, however the car continued to run fine. I pulled over and everything started up again, then after 10 seconds shut down again. It continued to do this with regularity until I shit off the AC, at which point everything was normal. 
Later, as we drove I turned it on again and it was fine. After our next stop, it did the same thing, and I decided to just leave it on, and after about 2 minutes of driving it stayed on and was fine the rest of the way. 
What’s going on?  The only thing I have changed was a new solenoid. All connections are tight on it. I don’t have a voltage regulator with the one wire alternator installed. 
@Midlife, is there a fusible link somewhere?

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Remind us, is yours a 69 or 70? Fusible links started on the 70s, but the only one I'm aware of is at the solenoid, and would cut the main power. Once it is gone, power wouldn't be restored without replacing the link. This sounds like a loose connection somewhere.

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Sounds like something going on at the fuse box, as radio, backup lights, and blower motor/AC are powered by ACC and fused by 2 fuses.  Might be a bad ignition switch or bad/nearly broken wire for ACC at fuse box.  Usually, it is the BAT line that breaks, not the ACC line.  Something is going on that when more power is pulled through those lines, continuity is lost due to higher resistance.  I'd check your fuse clips first.

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Thanks for the replies! The car is a 69. 
While I will check what you all have mentioned, I have another theory that I haven’t tested: When I installed the new starter solenoid, it was a black color versus the gray color of the old one. Since that’s when my trouble started, I looked again at the online descriptions of the starters. Apparently the gray one is heavy duty. Could this be the issue? 
By the way, we made it to Dearborn from Atlanta! The power issue cycles through about three times and then stays on, so we’ve survived the travel. I’m posting on Instagram if you want to see our pics: @stevestang1969

D5D30095-D329-4476-A33B-AC0868160529.jpeg

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While the engine is running, the only thing the starter solenoid does is provide a connection point between the main harness, battery, and alternator via the large post on the side.  I don't think the starter solenoid is your problem, unless the connection is loose. 

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On 7/13/2022 at 10:29 PM, nymustang1969 said:

Check the plug on the back of the ignition switch. It is common to have a burnt up connector on the plug on cars with a/c due to the higher current load. Back in the day when you bought a new Motorcraft / Ford ignition switch it came with a new pigtail to make a proper repair.

Bingo! This was the issue! At one point on the trip, I went to start the car and nothing happened. I reached behind the switch and pressed on the wires in the connector, and tried again. It started fine after that. I just installed a new pigtail, and all seems happy! Quite a bit of corrosion on the old wires. 

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Glad my response was able to help you out. Also while I was behind the dash I put a relay on the a/c blower motor circuit. Between the blower motor and the clutch on the compressor that circuit can draw 25-30 amps. Putting that circuit on a relay will reduce the higher current load on the ignition switch.

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5 hours ago, nymustang1969 said:

Glad my response was able to help you out. Also while I was behind the dash I put a relay on the a/c blower motor circuit. Between the blower motor and the clutch on the compressor that circuit can draw 25-30 amps. Putting that circuit on a relay will reduce the higher current load on the ignition switch.

FYI, when I did the schematic for the 69s, I measured all the loads. The blower motor on high was 13A, and the AC clutch solenoid was 3.2A. I'm not sure why yours would be so high.

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