1978 ford lights flicker

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Hey guys. Dealing with my 78 ranchero here. Maybe "flicker" isnt the right word, but when the headlights/dome light/dash lights are on, they are constantly dimming and brightening back up. They never shut off, and they never get so dim they're unusable, they just look like the voltage is constantly fluctuating. engine RPM affects this absolutely none. It does it at idle, and does it identically at highway speeds. Parts store tested the battery and alternator and both are performing amazingly. (both were tested while removed from the vehicle) It always cranks nice and fast.. What are your thoughts on this guys? could the voltage regulator cause this? bad ground maybe?
 
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Regulator and grounds. Typical of Fords of this vintage. Easy fixes, just takes alittle time to clean everything up and replace regulator. Check battery ground really good too.
 
the grounds looked good, so i did what ive done a few times in the past; swapped the part (regulator) with the one in my 81 fairmont, and proved that to be the issue. Now, is it important for the regualtor to have a good (ground) connection to the fender, or does that not matter?
 
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also, i forgot to ask, any idea what this is for? In my ranchero (which has a rebuilt, not-stock, engine) this was in between the regulator, and the wiring harness. My fairmont does not have this. The vehicle wiring harness has 5 wires, and this reduces it to 3. I removed it completely, and it doesnt seem to be causing any problems..

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didnt learn much from the google search, besides its a "ford alternator adapter".

I also figured out this adapter is what was causing my problem. After removing this adapter from my ranchero, and swapping out the regulator with the one in my fairmont, neither vehicle experienced this problem anymore. (just to clarify, my fairmont never experienced this problem) For the heck of it i put this adapter back in the ranchero, and the problem immediately returned.

i hope having this adapter removed from my ranchero doesnt leave me stranded sometime soon..
 
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There is nothing out there on it. I can only guess the older alternators used additional terminals and the newer one doesn't. Probably a earlier wiring harness and later alternator factory installed or Vice versa.

If you had the old wiring diagrams you could figure it out.
 
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