My 94 F150 XLT SuperCab 4.9L, 5 speed manual has 138,390 miles on it. I bought it 4 yrs ago with 89K miles already on it. Since that time I have put 4 alternators on it, three aftermarket and one OEM. At over $200 a piece, I have had enough. I constantly have a hot elecritcal smell in the cab. The ammeter shows normal but far to the right until about 15 minutes after driving. Seems to go through an alternator per year. Dealer service department says they have no idea what the problem is. They suggested disconnecting all the chassis ground wires and checking for corrosion. I have done some troubleshooting. Good grounds and continuity from battey to chassis to starter. Oh, I have replaced the starter & solenoid, battery, and positive cable from battery to starter. When engine is just started, I have 14.7 volts from battery to chassis and Ammeter needle sits on the L of normal. It takes about 45 minutes to get the hot electrical smell. Ammeter needle is at M. I turn on the headlights, radio, and A/C and the ammeter dips to less than half when engine is at idle. Voltage reading at battery reads 12.8 volts. Ammeter and voltage reading at battery increase with RPM to normal (needle to M and voltage to 13.5). I took a good look at the alternator and noticed the sticker said it was a 75W model. I thought XLT's with all the electrical bells and whistles needed a 90W or greater alternator. I called the dealer and they crossed my part number to the the OEM specs and said the only listed alternator for my model was the 75W alternator (internal fan and voltage regulator)I have installed. They said the 90W models were used on the V8 model engines and not the I-6 that I have.
Anyway, I still smell hot wires and cannot track it down. Is my alternator to weak for my truck? What happens when I have an undersized alternator? My battery never goes dead. I even let it set two weeks sometimes before I drive it. Never any problem starting it.


I have an after market Sony CD palyer that was already installed before I bought it.
I don't have any stereo amps that would draw excessive current.

 

Great question! I can't think of any clear answers but I have a few comments.

Try getting rebuilt alternators from your local autoparts store. They are a lot cheaper and the quality is just fine. Sometimes they will come with a warranty - which could be handy in your case. I assume you replaced the alternator because it quit charging the battery. If you replaced it because it was screaming or whining - that would be caused by too much tension on the belt killing the bearings or the pulleys not lining up.

I would expect an undersized alternator to do one of three things. 1) The excessive over current will heat the windings of the alternator until the winding insulation melts and shorts out giving you no voltage or very low voltage. 2)The excessive over current will burn out the rectifing diodes giving you no voltage (2 diodes) or half voltage (1 diode). 3)I am a bit uncertain on this one: at a given rpm, after the alternator has exceeded max excitation of the internal field, as the accessories draw more and more current, the voltage may begin to drop - caused by the resistance of the windings as described in #1 above.

14.7 to 13.5 Volts is just about right for a healthy charging system. Dropping to 12.8 seems low unless you are at idle with a large current load. Bringing your rpm up above 2000 should bring that 12.8 up to at least 13 - 13.5 Volts

A loose belt can cause alternator problems also.I don't think that this is the problem but make sure that you can not easily turn the alternator pulley by hand.

I think the comment about looking for ground problems is good advice. Remove, repeat, REMOVE, the negative battery cable end that attaches to the block - no matter how good it looks or tight it feels. Clean the block and the terminal and reattach tightly.

I would spend some time under the dash looking for that hot wire. If you can smell it, the insulation will usually be discolored or brittle. Feeling for the hot connection might be easier than looking. Keep in mind that the hot wire could be located in the engine compartment and the fumes could be blowing in thru a firewall penetration.

Could your alternator be 75 Amp unit (not watts). Or is the W just a part number? In any case, I would look for the hot wire smell first, before worring about the alternator size.

Good Luck