2000 Ford Explorer



February, 12, 2013 AT 2:16 AM

My dash battery gauge fluxes when im only idling and in gear. Also I upgraded my alt to a 200a, do I need to upgrade my 175a inline fuse between the alt n battery? If so, how high do I go

4 Answers



February, 12, 2013 AT 2:50 AM

Where did you find a 200 amp generator and what are you using to run it? It will not produce any more output current than what the electrical system needs unless you do a full-field load-test. At that point it would blow a 175 amp fuse. If you install a larger fuse, the wiring will be the weak link in the chain and will be overloaded.

If the dash gauge is bouncing around, the common causes are loose or corroded battery cable connections, loose ground wires, and one defective diode inside the generator. The load test would identify a bad diode. The maximum output that could be developed would be exactly one third of its rated value, and "ripple" would be very high.



February, 12, 2013 AT 4:09 AM

Its a high output alt bought off a website for high proformance audio systems. I have done the BIG 3 all done with 1/0 and I kinda figured the fuse was good for the alt on that princable.
As for battery cables, and grounds. They are all tight and clean. But the defective diode in the generator? You refering to the alternator? Or another part on my truck.
The alt is brand new and only 3 hours on it so far. Til I get a new battery, lil driving as possible. Also, battery at idle checks out at 14.2v so all is good there. Just this gauge jumping is really annoying lol
also, if this helps. At idle, lil heat on, lights, and foot on break. The gauge goes down. Dash dims.



February, 12, 2013 AT 4:11 AM

******also its in gear.



February, 12, 2013 AT 5:28 AM

What you're describing is typical of a bad diode. Technically speaking, you have an AC generator but when you call it an alternator, everyone will know what you're talking about. Chrysler was the first to use an AC generator in 1960 and they copyrighted the term "alternator". As a former instructor it was important to always use correct terminology to avoid confusion, and I'm still used to doing that.

It's also important to understand that it will not produce more current than what is needed by the electrical system. It only has the CAPABILITY of producing more when it is needed. All generators need movement, (rotation), as one of the three things needed to cause current to flow, so they are always less efficient at lower engine speeds. It still should be able to keep up for most conditions when at idle. For the output voltage and / or current to drop means it can't keep up at that engine speed. That is typical when one diode is defective. AC generators put out three phase output which is very efficient, but with one bad diode, one phase is lost. The result is it will only be capable of developing exactly one third of its normal rating, which in your case is around 65 amps.

65 amps is plenty to run the electrical system under normal conditions but "ripple" also comes into play. Here's two sad drawings from my web site that shows ripple. The voltage regulator monitors that voltage and tries to adjust the generator's output CURRENT to maintain proper system and battery voltage. Think of a large water pump having to pump the proper volume of water, (current), to fill a municipal water tower at the same rate people are using water, to maintain the proper weight of water in the tower which produces pressure, (voltage).

When the voltage regulator sees the drop due to the missing phase, it bumps up the strength of the generator to try to get the voltage back up. Later it sees the output from the working phases is too high and tries to cut back. The result is flickering lights due to pulsing voltage levels.

I would start with a load-test on your system to verify the generator can produce near its rated output. If it can, for the dropping at idle, look at the size of the pulley. It takes a lot of horsepower to run a regular generator, and to run a really large one often they will put on larger pulleys so they drag the engine down less. That will show up at idle as the same drop in voltage as a defective diode would cause.

Because of the lower efficiency at low speeds, one part of a proper load-test is to raise engine speed to around 2000 rpm. No generator or alternator will produce its maximum rated output at idle.

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