I'm having a problem with my charging system on my 88 F150 XLT Lariat 4X4 with a 351 V8. I think (but am not sure) that I have a 75 Amp alternator. I have a sneeking suspicion that it might be wiring, but I want to eliminate the alternator first. The old battery froze, so I've replaced it with a new 650CCA Motorcraft Max battery.

Is it a fair test to remove the large wire from the alternator and place a voltmeter from the large terminal on the alternator and test from here to ground and expect to get 14.7V on the DVM? I'm only getting 12.1V at the battery right now.

 

You don't need or want to remove the charging cable.
With the engine off, but the key to run, check voltage on the green/red stripe wire coming to the alternator. You must have 12 volts there or the alternator will not produce any power.
With the engine running, check voltage at the large terminal, by back probing the plug. Should be 14ish volts there. If you have 14 volts there, check the other end of the power cable at the solinoid. If you have 14 on one end and not the other, the fuseable link is bad in the wire.
If you have the stock alternator, it is a 2G and the plastic plug is kind of famous for for getting slightly melted, and causing the plug to be loose. They don't call them flaming Ford alternators for nothing. You can buy a new plug and pigtail, or upgrade to a 3G.
Good luck Frank

 

Got home last night, and found the battery stone cold dead. I'm told there's not much pont to charging sytem troubleshooting without a fully charged battery. I threw a charger on the battery, and didn't do any more testing. I let it charge for 4 hours or so on 10A and then left it overnight on 2A and will let it charge throughout the day until I get home. I'll give these tests a try tonight. On thing I noticed that was kind of odd... the interior lights would go from bright to dim to bright to dim and the needle was jumping all over the place on the charger. So I'm worried that I might have a bigger issue that just the charging system. I noticed that after the charger was on for awhile that this stopped happening though.

I'll try a parisitic load test tonight too and post back (assuming the battery is charged)

 

I had something similar happen many years ago. The alternator had a bad diode and would discharge the battery when parked and turned off. It was still charging fine when it was running, but slowly sucked it flat when off. I think you could do a quick test for this by checking the green/red stripe wire with the ignition off. If it is hot, alternator is bad. Should be hot only when key is on. If it is not hot the alternator could still be bad, but if it is hot, you will know at least part or maybe all of the solution. You need to do this test by back probing the wire, with everything still connected.
Before buying a new one, most places can test your old one once it is out. I have a small place locally that rebuilds alternators, pretty cheaply. This shop has been around since the 1940s, so I'm very comfortable with their work. You might have a similar one also.
Good Luck Frank

 

Wow....what a gong show work has been this week. So it looks like tomorrow is truck day and I'll finally be able to do some "real" troubleshooting on this truck of mine. I've had the charger on the battery at the 2A setting now for 48 hours. And the battery is not charged. Lights were dim on the truck when I opened the hood. I disconnected the battery and set the charger on the 10A setting. I'll leave it like this for about 4 hours or so then I'll set it back down to 2A where it will sit all night. I can't see my new battery being toast, but maybe it's possible. I'll check the voltage tomorrow.

I'm also going to try that parisitic load test tomorrow morining; I suspect it will fail, and the first thing I'll disconnect is the alternator. I bet that fixes it and that will confirm what you said about having an alternator with a dead short in it. I'll try those other tests too, but the wiring colors you mentioned don't match up with my truck. The alternator has two connections. One is at the 11 o'clock position an the the other is at the 4 o'clock position. I was a little surprised not to see a large red lead from the back of the alternator, and it looks like the middle of the three leads at the connector from the 11 o'clock position is the charge lead. It looks like it goes straight to the solenoid. So there's two other leads there that I'm not sure which one is the field lead. I think the connector at the 4'oclock position is the external regulator mounted on the back of the alternator.

 

check all the ground straps like especually the one from the engine to the fram make sure that one is good my truck was acting the exact same way and that had something to do with it i actually replaced the WHOLE charging system during the process though hah

and replace the alternator plugs (both) they are known to go bad, parts stores won't have them in the back they are both on the shelf in the electrical isle

 

http://fordfuelinjection.com/public/...ALT_wiring.gif

This link is to a wiring diagram for the 2G alternator. I'm still assuming you have the 2G. Ford used two small black wires to carry the load to the starting solenoid. They come together and run thru a fusible link. If the alternator is bad and has been drawing power for several days, the quickest test you can make is lay your hand on it. It should be warm to the touch. If it is, the alternator is probably bad and the next step is pull it for bench testing and /or replacement.



Ford Fuel Injection Alternator Files
This link is to all the info for the different alternators. It has pics, and wiring diagrams. Very useful info that you may want to bookmark.

Remember the real basic tests you can do on the green/ red stripe wire. It has to be hot with the ignition on and not hot with the ignition off. If it is not hot with ignition on, your problem is before the alternator, and this must be resolved before you do any other trouble shooting.
If it is hot with the ignition off, the alternator is probably bad and draining the battery. An easy way to backprobe this is to just find and accessible section of the wire, and slip a sewing needle into the wire. I slide it thru the insulation and insert half the length of the needle parallel with the wire, not in and out the other side. This gives and nice clean firm place to attach a small jumper wire with alligator clips, or just to touch the tip of the meter probe. It is not pulling a lot of amps, so this small connection is perfect for seeing if power is present or not. When done, one wrap of electrical tape seals it back up with no damage to the wire.
Like someone said good grounds are essential for everything to work properly.
Good luck Frank

 

Well, it's all fixed up. I did a parisitic load test and it was drawing 4A!! So based on what you told me with that old alternator of yours, I pulled the the top plug from the alternator, and no more load. I pulled the alternator and off to Waterloo Ford and and $140.00 later, no more load and it's charging. Voltage at 1500 RPM is 14.97 volts and is 14.7 at idle. I can see from your diagram that I was looking at the wrong spot for the leads too.

Job done and thanks for the info. I had no idea an alternator could short inside and kill a battery!

Oh yea, it is the 2G alternator that I have. If anyone is curious, the Motorcraft replacement number is# GL-520-RM

 

Glad you got it fixed. Did it come with a new pigtail?
If not you really should see about getting one and changeing it. The old plug tends to loose it's grip on the metal parts, and increase resistance. The high resistance causes heat and more resistance and fairly common for it to cause a fire. Occassionaly it gets the garage the truck is parked in. Had an Explorer go up in flames locally last year. Someone in the forum had a small fire in their F150 couple years ago. The new plug is made with different technology and different plastic. For a few bucks and a few minutes it is pretty cheap insurance.
Good Luck Frank

 

It did not. When you say pigtail, I'm assuming a new connector for the non-regulator plug on the alternator?

 

I have not done this myself because I went to a 3G alternator. Other members of this forum have reported their new/ rebuilt alternator came with a new plug that had a few inches of wire attached. The wires are easily spliced into the exisiting wiring after the old plug is cut off.
One member said using the pigtail was part of his warranty. The new materials and manufacturing proseess used to make the pigtail, eliminate the problems that resulted in fires. There probably should have been a saftey recall on this several years ago, but Ford did not do it.
The pigtail may beavailable at any auto parts store, or this site may have them. This Motorcraft site seems to have most connectors used on our old trucks.
MOTORCRAFT® WIRING PIGTAIL KITS FOR FORD, LINCOLN OR MERCURY VEHICLES

Good luck Frank