Bad alternator? 00 merc mounty 5.0L

[JimMadsen]

Yesterday noted that the battery needle was very low. Plugged in my cig lighter volt meter and was jumping between 11.3 volts and 13.4 volts. Quickly got home and hooked up the trickle charge for about 12 hours. Volts while off is 12.2.

Popped the hood and saw that the belt was shiny. No frays or cracks, just that glazed look. Replaced that this morning and volt meter at the dash while running is 11.4 volts. I should have 14+ if alternator is working fine.

So.... Time for a new alternator?

Also, I removed fan and shroud and saw that the 11 blade fan is pretty cracked. Still in one piece, but.....

I find new ones on eBay in plastic. Those are 12 blade for the 4.0, then there are used 9 blade metal ones that are 5.0 pulls. Does it matter?

 

[koda2000]

fully charged battery = 12.6v. good alternator should put out about 14.6 volts, accessories/lights off, measured at the battery. also check the voltage at the "B" terminal at the alternator.

serpentine belts don't slip like old-style v-belts unless they're wet or you have tensioner issues.

Note: a bad battery will ruin an alternator, so also get you battery load tested.

 

[JimMadsen]

My nearest autozone where they can test battery and alt is 15 min down the road. How worried should I be hat it will not make it for the test?

 

[JimMadsen]

Sorry Koda, lack of caffeine here. Will do a few more tests on the wiring and post. Also going to charge up to 12.6v.



Battery charging now, was at 11.76 after I retested.

Alt to battery ohm test is .000

Car running with blower at max, volts AT battery 11.25.

Not sure how to test volts out from alt....

 

[J_C]

Alternators can get leaky diodes, or there could be other parasitic drains. Hook up your trickle charger with battery disconnected from everything else, top off the charge (If it will) and leave the battery disconnected for a few hours then measure voltage while still disconnected from everything. Cig lighter volt meter may or may not work sufficiently if the lighter outlet has significant corrosion and/or tobacco tar in it causing poor contact. Inspect and clean the contact if warranted.

It is likely the alternator is bad. Voltage should be higher running than turned off, but a bad alternator, over time, wears out batteries too. Turn off accessories incl. fan not needed to drive the 15 minutes and leave it sitting running. Does voltage go up, down, or stay the same while running? If it maintains 11.25 you should be able to make the trip. If voltage keeps dropping then maybe, or maybe not... depends on rate of drop and state of charge when you start the trip.

 

[JimMadsen]

What are thoughts on the fan blade.... Interchangeable?

 

[JimMadsen]

Went out for a bit and do not like leaving the charger running. 12.35 volts out of car when I left. 12.2 when I got back, will check it again in the am.

 

[SoHK_alumni]

What are thoughts on the fan blade.... Interchangeable?

The plastic blades are lighter, quieter, more effecient and perhaps better looking.

The metal ones simply work. No history of cracking and flying apart, tearing stuff up under the hood and dinging the hood.

I'd not go with a used plastic but I would probably opt for the less expensive option between new plastic and used metal.

But, my 5.0 has a metal blade on it!

 

[JimMadsen]

Thanks, eBay for a metal blade here I come