alternator - 75 amp vs 100 amp

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Well-known member
Jan 30, 2006
Tempe, Arizona
hello all -
I have a 1991 class C Tioga with the ford 460, had it for almost 9 years - love it!
had a new alternator put in last year while on way to las vegas -? at the repair shop I needed
to wait while they went and got another 100 amp, because that's what I had originally (they said).
all seemed well, until this weekend, went alternator went bad - and I find out that the replacement
put in last year (carquest lifetime warranty) is only a 75 amp. how much am I missing out on added
performance, or is my one house battery not charging well enough?
I may have to buy ANOTHER new one if y'all recommend I go back to the 100 amp.
the repair shop (as yet unnamed) seems a bit reluctant to fess up to installing a 75 amp, and the receipt
does NOT mention 75 or 100, just a R+R alternator - but carquest says that number is a 75 amp.
did they just slap a 75 amp to get me to las vegas (with our Mom's), or is it really not needed?
help - rv is at mechanic 50 miles from home, and I'll be away for work tomorrow, I'll be back to check
here Wed.
thanks so much for any suggestions.
Figure the engine and car's OS needs around 15-20 amps, the rest is there for charging,

However if you normally drive this thing for 4 to 8 hours when you drive it.. Don't sweat the difference
thanks. what I have found is this - even though Ford has told me this came with a 100 amp alternator,
it's not correct. from what I understand, the 100 amp alternator is about 2" larger in diameter, and the mounting
hardware is different. So for the original owner to have replaced the alternator during the first 6 years of it's life was
doubtful, and especially doubtful he would have swapped the 100 for a 75 amp, seeing as he would have had to change mounting
brackets and all, it would make no sense.
so I have had a 75 amp alternator all along, and that's what I'm putting back in.
this post is done as far as I'm concerned.
happy traveling y'all.
Glad to have been of help.... I've replaced the OEM alternator with one of a larger capacity on my towed, howevermy towed is a GM not a Ford.  My reason for going heaver is Ham Radio.  I can burn up a lot of electricity when I press "Transmit", more so here in the motor home though as it's got the "Big Rig" in it
Our last 460 had a replacement 115 amp alternator that could barely keep up with demand, especially at night in the rain.
what accessories are you running while you drive that you need so much juice from the alternator?
I have no trouble with the 75 amp alternator being insufficient (that I know of).
100 amps sounds like alot, but remember it is at 12-14 volts.  That is about 1200 watts.  The alternator is sized for worst case loading.  Probably winter driving with: heater blower on high, windshield wipers going, head lights and tail ligths on, stop and go traffic (what does alternator put out at engine idle?), engine and vehicle electronics, and the radio going, and rear heater(?) blowerr.

In most conditions you don't push the limits....unless you were driving home from work in Buffalo this week.  :eek: :eek: :eek:
joester said:
what accessories are you running while you drive that you need so much juice from the alternator?
I have no trouble with the 75 amp alternator being insufficient (that I know of).

We usually had heaters, lights and an inverter running TV and kids games as well as a couple of lights in the rear bedroom. Just add a digital voltmeter to your chassis cigar lighter and see if it drops below 13.2 volts or so.
The size of the alt is important depending on the accessories you use, a/c, cd, blower motor, etc. this will draw from battery charge. with alt's bigger is better and worth a few extra dollars.
Another consideration is that the 100 amp alternator may be somewhat better built than the 75 amp version. IT may be that it can outlast the 75 amp version.

This has to do however with how long the 75 lasted as opposed to the original 100 amp.. If the 100 amp lasted say 8 years and the 85 lasted 7.. I would not think the savings would be that great.

But then.. I've had an alternator that did not make it 30 days

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