Mechanics

WHAT KIND OF ISSUES WOULD BE CAUSED BY RUNNING A SMALLER ALTERNATOR

1990 Jeep Cherokee

I baught an alternator for a 4 cylinder 1990 Jeep Cherokee Pioneer and it turns out it will be going on a 6 cylinder. There seems to be up to a 30 amp diffence between some recommended alternators and the one I have. Will using the one I have be ok, or will it cause problems. Thanks for any assistance provided.

TWITCH
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TWITCH
May 31, 2012.




Need correct AMP output for vehicle less will cause a many problems

Docfixit
May 31, 2012.
What kind of problems will it cause? And is it something I can do as a temporary quik fix untill I get another alternator?

Tiny
TWITCH
Jun 1, 2012.
Actually will not cause a problem under most conditions. You rarely need the alternator's full output to run everything that is turned on with some current left over to recharge the battery. If you ever do exceed that current, it is going to be for very short periods of time and the extra current will just come from the battery. You likely won't even notice except the head lights might dim a little.

Alternators are self regulating as far as current output goes and you just won't get more than it is designed for. You CAN potentially run into a problem by installing an alternator with a higher output capacity than what came on the vehicle. They will only develop the amount of current needed by the electrical system and no more. The vehicle's needs haven't changed so just because a new alternator might have a higher capacity, that doesn't mean it will produce more current. What CAN happen is it will produce its maximum capacity during a professional load test. Almost all alternator output circuits are fused in some way. Most newer vehicles use a bolted-in fuse under the hood but in the '70s through the early '90s most cars used a fuse link wire. That was a smaller gauge wire with special insulation that won't burn. If you look at any Chrysler wiring diagram from the early '80s, you'll see they show three different "hypalon" wires with different colors to differentiate different gauges and current ratings. One of those three was used on each car depending on which alternator it was built with. If you install a new alternator with a larger capacity, as I mentioned, it will produce exactly the same amount of current as the smaller one, ... Until someone performs that load test, then it could produce more current than the original fuse link can handle. The rest of the wiring can already handle the higher current.

Since fuse link wires take some time to burn open, they usually survive that load test. The test is only performed long enough to get the reading, perhaps two to four seconds.

To put things in perspective, when one of the six internal diodes fails in an alternator, you will lose exactly two thirds of its capacity. That means a common 90 amp unit will only be able to deliver around 30 amps. That is not enough to meet all driving conditions but very often people don't have a problem or even notice. System voltage will drop at idle and if the alternator can't keep up with demand, the battery makes it up. On newer vehicles with lots of computers, people usually notice weird things happening from the unusually high "ripple" that is being developed. That can confuse some computers. The point is, if a car can still function when the alternator loses two thirds of its capacity, it will definitely work fine with an alternator with 30 amps less maximum capacity.

I hope this doesn't confuse the issue but if it helps to clarify my explanation, think of the hydraulic oil pump on a piece of construction equipment like an excavator. That pump has to deliver enough gallons per minute to drive all of the cylinders and hydraulic motors with a bunch of capacity left over and unused. If you install a larger pump with more volume, that doesn't mean it will run anything faster; it just means there is more leftover capacity that isn't being used. You, in effect, want to install a pump with less capacity. As long as the functions you're using require less capacity than the pump can deliver, you won't notice a problem. It's those few times you turn on more items at once than the pump can supply that things will slow down, but they won't stop completely.

What happened to your old alternator? If it was a Chrysler or Nippendenso unit, they typically fail from worn brushes. Those can be easily replaced instead of replacing the entire alternator.
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Caradiodoc
Jun 1, 2012.
I should have clarified why I was disagreeing with Docfixit in this case. Your Jeep only has an Engine Computer. Newer vehicles have all kinds of additional computers to do things computers where never needed for before and many of them are very sensitive to low voltage and voltage fluctuations. THAT'S where you can run into problems but they will only occur when that smaller alternator can't keep up with demand. That's when the voltage will fall off and cause computers to do unusual things or even stop working. You're going to find that new smaller alternator works just fine in your vehicle.

Caradiodoc
Jun 1, 2012.

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