weak alternator?
Thread Starter
Pro
Joined: Apr 2005
Posts: 591
Likes: 7
From: Batavia IL
weak alternator?
My alternator is a 100 amp single wire type that I bought at a swap meet. It ran well and kept a 13 volt charge untill recently when I added msd ignition, an electric fan and installed my hot rod air conditioning system. Now the voltmeter says only 10 volts when every thing is running and just under 12 volts with the a/c, stereo, lights, and cooling fan off. I thought a 100 amp could handle this kind of load but maybe my altenator is bad or weak. I checked to see if the belt is loose or that the car will run with out the battery connected which it did just that the lights all got real dim. Is this a fixable problem I can do to the alternator itself or should I buy a 140 amp alternator?
Racer
Joined: Dec 2001
Posts: 328
Likes: 0
From: Dallas County Texas
Well, take it off and have it tested for free at a parts house
If it's OK, you can try running a wire straight from it to the battery. That is a fairly large battery - that needs to be 8 gauge or better to avoid heating up.
Mine just tested bad after a similar deal so I ordered a $40 rebuild kit with bearings from Jegs. If that doesn't work out on my nifty chrome case, I'll spring for the most cost effective of the 140A or 170A offerings out there now.
This is like the fourth I ate. This one lasted a lot longer until I took off the enlarged (reduced RPM) alternator pulley off it - which I will replace when I rebuild it.
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BTW, your ignition doesn't eat very much power. Neither does your A/C (blower motor is probably the same, I assume.) Your fans don't eat 35A in all probability. Real power eaters are big lights on four-wheelers and huge amps on stereos. 13V is a little low. 13.5+ is more likely and over 13V with everything engaged at over 1500 rpm.
Mine just tested bad after a similar deal so I ordered a $40 rebuild kit with bearings from Jegs. If that doesn't work out on my nifty chrome case, I'll spring for the most cost effective of the 140A or 170A offerings out there now.
This is like the fourth I ate. This one lasted a lot longer until I took off the enlarged (reduced RPM) alternator pulley off it - which I will replace when I rebuild it.
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BTW, your ignition doesn't eat very much power. Neither does your A/C (blower motor is probably the same, I assume.) Your fans don't eat 35A in all probability. Real power eaters are big lights on four-wheelers and huge amps on stereos. 13V is a little low. 13.5+ is more likely and over 13V with everything engaged at over 1500 rpm.
Last edited by WayneLBurnham; Jul 16, 2005 at 06:58 PM.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
There are tests that you probably can't do other than voltage if you don't have the tools. Volts are just one measurement. You also have amps. And you need to see if you're diodes are giving you full wave output. You could have an alternator that basically checks out on volts but isn't putting out enough amps. You can also have adequate volt/amp output but be missing wave rectification on all the diode peaks, which means you're missing at least 1/3 of your current. Not much of an issue on one bad diode on a non-computer car but the system works a lot harder to make up the current.
Racer
Joined: Jan 2005
Posts: 492
Likes: 0
From: Birmingham AL
It's a combination of the one-wire version, load, and wire resistance. There is a sense path in the multi-wire version of the alternator that uses the voltage nearer to the battery that compensates for voltage drop as a result of load loss. I'd start by switching over to the mult-wire version. You should also look at the wiring and see if there are any splices in the primary +12V line from the alternator. Splices will eat your lunch.
