I am pulling substantial currents to try to power a home a/c unit installed in the rear of my '87 E150. It was modified with a Motorcraft 3G alternator, good for something like 140 or 160 amps. It also has an isolator and deep cycle battery. It takes around 70 amps in normal running if I recall correctly.

The inverter and deep cycle battery are both very sensitive to voltage drops. If you want to charge the deep cycle by idling it, a 0.5v voltage drop can make the difference between a quick 60 amp charge and a nearly useless 10 amp charge.

I have good connections except for ground. Right now I have it bolted to the frame near the hood, but it's not a great connection. I'm not even sure where the ground strap is that ties the engine to the frame, nor am I sure if it has enough current rating.

Is there any way to bolt a quality ground straight on the alternator? Otherwise, where is the ground strap located on this vehicle with a 302 in it?

 

On my '86 there was no ground strap from engine to frame. The factory battery negative cable had a piece missing from the plastic sheath where a piece of metal was attatched to use as a clamp to bolt it to the frame. When I changed the cable, the new one had an extra wire from it that I bolted through the body to the frame.
Post your results about the A/C install. I've been thinking about the same set-up, if I can find something acceptable.
Munrow

 

I would use a wire brush in a drill to clean a spot on the frame, then bolt a ground there, and then do the same on the engine block. Where are you grounding the A/C unit? I hope it's not through the body... Keep that ground as short as possible, like to the frame back by the rear axle, or close to the unit.

 

I traced the ground wire from the starting battery and found where it meets the engine. But I don't see that ground strap to the frame either, and that's disturbing since the deep cycle battery, which deals with all that current, is tied only to the frame. I wonder just what component the 70 amps was going through? As far as I know the engine, tranny, and exhaust are all rubber-mounted! I'm lucky there wasn't a 14 gauge wire somewhere making a low resistance path which carried enough current to melt it. That would suck!

The a/c system worked while parked. While driving, sometime in the first 10 min or so the a/c shuts down and I don't know why. I never saw it happen. It can be restarted. Maybe the higher heat, I was in the shade while parked and it wasn't 120F inside or anything like it was when I took off out of a parking lot.

I never heard the inverter's warning tone. It makes a beep when the voltage goes down and that didn't happen. Someone suggested maybe the shaking of driving got liquid coolant into the compressor, but I would expect that to stall the motor and thus overload the inverter, and thus I'd hear a tone.

The idea requires a huge alternator, and not one that's just high capacity on paper (Powerline was only high cap on paper). Motorcraft 3G is a great one and it's commonly available. I'm still worried about it overheating, baking its rectifier, when the temps are high enough to make me want to turn it on. You need a power inverter with a high surge current to start it and like 1kw continuous rating. Needs 2ga cables or thicker (lower numbers are thicker). The interconnects are critical! A few milliohms of resistance will kill it. They should be gold plated copper or brass, and soldered onto the cable rather than one of those screw-down jobs. Install with electrical grease. If this is off a two-battery system with an isolator, the isolator needs to have a large rating, vertically mounted, and receiving airflow straight from the front, not through the radiator. Actually it should be as big as the alternator even if you don't have the a/c system like that since a dead battery will basically draw as much power as the alternator can put out.

Do not- NOT- use the stainless steel posts with wing nuts on a marine battery. They have a high resistance and it gets higher, apparently the boundary between the lead and steel corroded inside.

Frankly, seeing how the ground was neglected, I'm not sure why this ever worked. I will get enough cable to tie the engine ground to the deep cycle battery/inverter ground, but since I never heard the tone indicating the inverter was seeing low voltage I doubt it was the problem.

I have a custom digital graphics monitor reading the current in and out of the deep cycle to manage it. When I cleaned up the power connections, I went from 15 amps charging current to a half discharged battery to like 30, even 65 amps. Got the job done MUCH faster, good since that 302 guzzles gas at idle and I hate spending $5-$10 in gas just to charge the battery!

 

Hey, I found the ground strap! The battery ground only appeared to go straight to the engine. Upon closer inspection, when it ducks under a harness it actually terminates on the frame and another cable goes to the engine.

I found the Motorcraft 3G has a threaded hole in back for a ground, it has a fine pitch thread but I found one bolt in all my stock that would fit it. This is not only a shorter cable than running to the other ground, it means the current isn't going through the alternator bracket, steel engine body, etc.

Well the van's fridge has been draining the battery about 30 amp-hrs or so since this afternoon. I started it up and the deep cycle was charging at 88 amps! Before, I think the peak I saw was 67 amps.

I kicked on the a/c and it went down to 50 amps into the battery. So now it's not a matter of voltage drops, the alternator is actually getting maxed out. I turned off the engine and saw only 47 amps being drained, and that includes a couple of amps for the interior lights and if the fridge was running- and I think it was- another 4 amps for that. So I was mistaken about the current draw for the a/c- only 40-45 amps here.

So that 3G is outputting basically 100 amps at high idle. I throttled up and saw, with the a/c running, the charging current went from 50 to 88. So I think the 88 amp charging current is the max the battery will consume at the voltage the voltage reg set it at. And the max current is close to 140 amps at high rpm.

Well, now I know the problem won't be in getting power. This rocks. The charging current is awesome!

 

Glad you got it solved.. Now put in a hand trottle for stationary charging... That will help some, as you can hold the engine at 1500rpms to charge the batteries.