96 f-150 with 5.0L, dual electric fans and underdrive pully's. I've been running this combo for > 1yr. and no problem until now.

Had 2 different parts store test alt, one said bad diode and other said "only 32amps output, both agreed bad alt.

Replaced alt with a new 95amp unit. During the daytime no problems even with a/c on, needle stays around "M" with only a slight deflection at the start-up of any equipment (headlights, radio, ect..). With the a/c on, 1 of the electric fans runs, its hooked to the compressor. At night with a/c, headlights and I'm sure at least one fan on, lights will go dim at idle. My indications besides dim lights at idle are: the gage indicates on the low side (around N in normal). Light turns green and RPM >1000, the gage never really seems to go back toward "M". At 2000RPM its still at "N" even with lots of stuff shut off, its slow to come back up if at all. During the daytime it jumps back to "M" when the RPM's come up.

2 times in the last week after driving at night i've had to jump start the truck in the morning and am concerned about being stranded somewhere.

Does it sound like to much of a draw on the system with all the equipment running and it can't charge the battery? Will a 130amp alt fix the problem if this is the case or is it something I'm missing? Why for the last year no problem until now. The battery checked out fine and I've removed and cleaned the terminal posts at the battery.

Any help would be appreciated
Thanks, Randy

 

Where do you have the electric fans hooked up at? Is it at point #3 in the diagram above, with a relay?

 

Yes, they are hooked up at point #3. Also noticed that after driving for some time during the daytime with lights and a/c on, the charge needle does indicate progressively lower during stop and go traffic and is slower to move up the scale after increased RPM, meaning that it is not a night time with all equipment running problem. Today after my daily 20 mile drive to work the indication was at "o" in "normal" which indicates about 10-11volts (guessing). The fans will only run with key on. Also they have the temp probe in the fins of the radiator, so they also have a temp activated sensor to turn them on. I will check the temp setting to ensure that they are not coming on to soon. The settings on them, low temp fan set at 195 degrees and the second fan at 205, also the high temp fan set to run with ac compressor. Maybe the low temp and a/c fans should be the same fan, with second fan set at high temp only. I set them up so that the low temp fan would run at 195 and the second fan would run when a/c energized to distribute the wear evenly between the two fans. When a/c is on the high temp fan runs constantly. Together they are drawing around 50-60 amps i'm guessing, the fuses for each are 30amp and have not tripped one yet.

Randy

 

Also, now that I'm thinking--would a bigger Alternator help, say a 130 or 200. I inquired at auto zone and advanced auto about a bigger one, but they could not help me. A 3g ford type with 130 or 200 amps should fit?

Randy

 

Well, I am concentrating on two things you said in your first post. It worked ok before, and it takes a while to recover.

I am thinking you have a lot going on there as far as electrical load. I am going to guess that either a connection, or a small part of the harness may have cooked over time, causing a poor connection. Most of your load is the fans, so I would look at point #3, and see if it looks melted. I think I would go ahead and take it apart, and make sure all the connections and the threaded stud look ok. I would also look at the alternator harness, to make sure nothing has gotten warm on it.

You could put a larger alternator on, but I am thinking it may not make much sense to slow the alternator down with pulleys, and then have to buy a larger alternator to try to make up for it. The larger alternator will not perform very well if it's turning slow.

If you think about it, energy in is going to be close to energy out. You took the mechanical fan off, and added the electric fans. The electric fans will be an advantage because you can use them only when you need them, unlike the mechanical fan which has some drag all the time. But this would only be an advantage going down the highway at speed. Around town, you will probably use close to the amount of horsepower to turn the alternator, which is powering the fans, as you would the mechanical fan. And then you may be shooting yourself in the foot with the under drive pulleys on top of that, and the alternator is being handicapped.

 

Thanks for your help Dave!!
I removed the under pullys and put the low temp and a/c on 1 fan, set 2nd fan to high temp. Running down the road uses no fan until a/c is on. I moved the low temp setting to 195-200 degrees so that it doesn't come on while driving down the road but only in stop/go traffic. The charging gage does not move lower that mid point (m in normal) now and I have 13.8-14.2 amp across battery terminals. I did notice that when running 2000RPM and I turn on a/c that RPM drop 800 and slowly rise. Do you think that this may be part of the problem? The a/c is drawing a lot of power and may be an indication of a pending problem that has not surfaced yet? When the a/c comes on the alt gage barely twitches, so I know the alt is working good.

Thanks again
Randy

 

You may need to take your throttle body apart and clean it up. Also clean the IAC valve. The IAC controls the idle speed, and these parts can get gummed up over time.

The only thing I can see that may be wrong with the AC is if it was overcharged with Freon. This would make it load the engine down more than normal. I thought the AC compressor circuit had a wire going to the computer to tell it to boost the idle whe the AC is on. This would be another reason to clean the throttle body and the IAC valve.