Electrical Problem

Greetings,
My son's 78 Chev 4X4 has a frustrating electrical problem.(aren't they all!)
Under acceleration the amp gauge needle drops way down, and the
headlights dim, and the heater fan slows way down.
At idle the needle goes way up, and the headlights become bright, and the heater fan speeds up to what I figure is a normal speed.
Another twist to the problem is; if you are going along and brake down firmly the headlights brighten and the fan speeds up to normal. It doesn't make any difference if you do this in gear or in neutral.
Things done so far: Had alt tested: It tested good. Replaced both battery cables Replaced battery (even though old one tested good)
Any suggestions?
Thanks!
Ray
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sounds like you may have a loose alt belt. If you can turn the fan blade on the alternator with your hand it is too loose. another thing to check is ground from battery to frame. Common problem is the ground usually goes to engine the engine is mounted on rubber motor mounts and the frame looses ground. just run a 10ga. wire from battery neg. to a clean part of frame by the battery

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JimAuto,
Thanks for the reply. I put a new belt on, the old one was not the correct one, but it didn't help. I checked the ground straps from the back of the engine and they are in place. Next I'll try your suggestion by adding the ground wire from the battery to the frame.
Things done to date:
Had alt tested: It tested good. Replaced both battery cables Replaced battery (even though old one tested good) Replaced Alt belt.
Thanks!
Ray
jimauto wrote:

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Tester wrote:

What finally fixed the problem was adding an additional ground wire from the neg. post of the battery to the body. All factory grounds were in place, but adding an additional ground fixed the problem.
Thanks!
Ray
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Tester wrote:

S-10 years ago wouldn't start sometimes until I would wiggle the ground wire. I pulled the bolt out and put a toothed lock washer on each side of the terminal lug so the teeth woudl bite into the metal good and it has been fine since.
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