Alternator current
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Cross-Country
Joined: Nov 2023
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Alternator current
1995 F150 4.9 engine. At idle with AC on max my meter reads 12.32v. Bring the idle up moderately and hold it at those rpm’s my meter reads 14.54v. Any idea’s? Bad alternator? Thank you for your help.
Cargo Master
Joined: Mar 2007
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The alternator appears to be working fine. Without knowing the current, no one really knows.
ALL alternators are not capable of charging max current at idle. I know, many Bosch alternators have two specs [idle and high rpm], not certain if Motorcraft does. If they were designed where they could deliver max current at idle, they would over spin at engine redline. All alternator tests are done at something more than idle. Usually, 2k.
Do a voltage drop on the cables to/from the battery. As far as I am concerned, Ford cables are too small. Since your cables are probably due for an update, now is a good time to resolve that.
ALL alternators are not capable of charging max current at idle. I know, many Bosch alternators have two specs [idle and high rpm], not certain if Motorcraft does. If they were designed where they could deliver max current at idle, they would over spin at engine redline. All alternator tests are done at something more than idle. Usually, 2k.
Do a voltage drop on the cables to/from the battery. As far as I am concerned, Ford cables are too small. Since your cables are probably due for an update, now is a good time to resolve that.
Cargo Master
Joined: Mar 2007
Posts: 2,856
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Note this spec. The RPM mentioned is alternator RPM, not engine. Notice how it's incapable of getting to the rated alternator amperage at lower alternator RPM's
https://www.bosch-motorsport.com/med...ator_150_a.pdf
Another site that shows RPM is vital when testing an alternator
https://balmar.net/balmar-technology...output-curves/
In the end, a proper alternator test is not done at idle and needs to measure the current as well as the voltage. When the alternator is tested we will both know.
One often thing overlooked is how much is the electrical loads on the vehicle. If you are at idle, with everything on [ie, rear window defrost, heated seats, defroster at max blower speed, engine aux fan on, wipers full speed, headlamps on, etc] the alternator would probably not keep up at idle. I know some manufacturers increase the idle speeds when there are those additional loads.
Years ago I worked on a car, battery voltage low, alternator current in the negative, high RPM test was producing +5A. I threw in an alternator. WTF? Same result. Must be a bad alternator. Shotgunned a second one. Same results. That was the day I learned that if the glow plug relay is stuck on, it will consume +40A and the 50A alternator could not handle that load. I put the original alternator in, installed a new glow plug relay, and returned the 'shop tested' alternators to the parts dept. They were not happy at all, but then again, nobody is happy at the auto dealership.
https://www.bosch-motorsport.com/med...ator_150_a.pdf
Another site that shows RPM is vital when testing an alternator
https://balmar.net/balmar-technology...output-curves/
In the end, a proper alternator test is not done at idle and needs to measure the current as well as the voltage. When the alternator is tested we will both know.
One often thing overlooked is how much is the electrical loads on the vehicle. If you are at idle, with everything on [ie, rear window defrost, heated seats, defroster at max blower speed, engine aux fan on, wipers full speed, headlamps on, etc] the alternator would probably not keep up at idle. I know some manufacturers increase the idle speeds when there are those additional loads.
Years ago I worked on a car, battery voltage low, alternator current in the negative, high RPM test was producing +5A. I threw in an alternator. WTF? Same result. Must be a bad alternator. Shotgunned a second one. Same results. That was the day I learned that if the glow plug relay is stuck on, it will consume +40A and the 50A alternator could not handle that load. I put the original alternator in, installed a new glow plug relay, and returned the 'shop tested' alternators to the parts dept. They were not happy at all, but then again, nobody is happy at the auto dealership.
Thread Starter
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Cross-Country
Joined: Nov 2023
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Had it tested. Bad voltage regulator and diodes. Picked up a new one. I’ll install in the morning. Thanks for the help.
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Cargo Master
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Laughing Gas
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From: Carnation, Wah
I probably ought to replace mine, too- it's been dropping out at idle when there's a load on it, too.
Charges fine at speed, but if one field coil diode's out, the next one might be sooner than later...
t
livin' on the edge (of charging)
Charges fine at speed, but if one field coil diode's out, the next one might be sooner than later...
t
livin' on the edge (of charging)
Lead Driver
Joined: Jun 2021
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Use the opportunity to take an place a ground wire at the back of the alternator case. Alternators have to ground, low voltage increases amps , temperature increases resistance. Resistance to ground means less ground. IF your engine ground is the stock (CRAP) one, then its probably not fully grounding either. A new cable on the back of the case is EASY to do. The threaded hole on the case rear is the same size as the top ear on the 4.9 cases, likely the same for other engines. I found a metric bolt just long enough, and flanged, to stick a lugged cable into the rear of the case, and ran it directly to battery negative.
Someone may find I was inaccurate in the statement above bolded, which if so please tell me how so. I am trying to learn all the good things electrical law and such, and as I understand it, low system voltage, means higher amps. Those higher amps will strain weak connections (high resistance) and potentially even pop fuses .
Someone may find I was inaccurate in the statement above bolded, which if so please tell me how so. I am trying to learn all the good things electrical law and such, and as I understand it, low system voltage, means higher amps. Those higher amps will strain weak connections (high resistance) and potentially even pop fuses .
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