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73-87 Chevy _ GMC Trucks => 73-87 Chevy & GMC Trucks => Topic started by: 76_Scottsdale on September 04, 2010, 05:38:30 pm
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Hey all. I've been having intermittent charing issues in my '76 K20. The voltmeter stock gauge almost always rides below 13 volts even at cruising rpms, and when it is idling i can notice dimming lights and what not. I have a brand new battery, new 100 amp alternator, and I tested the charging system when running and it said over 14 volts and 70 amps, but even during the test my gauge was below 12 volts. I would think it's the gauge but every now and then I'll start the pickup and it will jump right to 14 volts, no dimming, and stay there. Any ideas?
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Gauge
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that's what i thought but i actually have dimming lights at idle when it's low on the gauge, then when it right at 14 there's no dimming at all.
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If you have the stock alternator, it should be like 63 amps or so, or maybe less.... My 78 came with a 63A Alt. I had the same thing on my 1978 and I went to the parts store and got the 78 amp alternator for an '83 (c10 with a 350 and A/C) and it bolted right in, no mods and stopped it from dimming at idle and while driving. I just got the bone stock replacement, not a hp one It fixed me right up.
USUALLY, If your lights and stuff are dimming only at idle, its too low a rated alternator, assuming the battery and all connections are good. Trust the voltmeter 1st, but check the gauges connections to try to sync them up. sorry i don't know where the gauge hooks up to everything else. You might need to take connections apart and clean them, like down at the starter where the battery cables ad alternator chargins wire goto. If a connection looks bad it's probably bad, if a connections looks good, it may still be bad ;D
The amps the alternator is rated for is usually fine when cruising, but at idle they can put out as low as 65% of their rating depending on the make and model vehicle.
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Sounds like you need to do some voltage drop testing.
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That's what I was thinking Vile, but I'm not quite sure how to do that and where to start. I know I'd like to have a heavier gauge wire off the alternator anyways so I may try stepping up to 6 gauge and see if that helps. All connections look good. I'll take some pics and post them up.
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I know on my 94 when I had similar problems between changing the alternator and fixing a bad ground it fixed my problem and that was with stock although only like 70 amps or So its enough to have a computer controlled truck run so I doubt you although isn't enough unless you have a amp or two runnin
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bet you have a bad ground some were.
if you doubt the guage do a voltahe test on the bat terminals with a volt metter..
i have had to re-do ground on all the old vehilce i have come acros either by wireing new ones or cleaning up the old grounds
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does the voltage drop at idle or is it the same voltage no matter what you do? if your trucks alt is stock you're using either a 42A or a 63A alt depending on if you have AC or not, and the one on my 78 never charged above 13.0 since it was new with that alt, it has AC but thats about it. if it drops at idle only, a bigger alt will fix it, if its constant you got connection problems, assuming battery and alt are good.
Did you actually take the connections in question apart and clean them? Unless the charging wire is broken, its big enough for a stock alternator.
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Take your + meter lead to the B+ terminal on the alternator and you - meter lead to the junction block. Record your readings. Check it again with the engine running when it's acting up record your readings. Report your results.
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Ok will do Vile thanks for the help guys. Also, could I get an idea of exactly where all the grounds are on this pickup? I'd like to re-do those also. Thanks.
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With something like that problem all you should have to worry about is the main bat wire
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I kind of lose the main bat after it goes off the alt. Should it be going to the main block then down to the starter?
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Ok, so my Bat + wire was going to my battery isolator as the PO had dual batteries. It was single battery only when when I got it so I removed the isolator. My bat + is now going to a circuit breaker on the firewall, which has the wire going to the starter off it. I removed the wire from there and put it directly to the + of the battery to rule out the breaker/ length of wire. The Voltmeter still read very low ~ 8-9 volts, even with revs it went to only ~12, and the motor was hesitant like it wasn't charging like usual. Do I now test to see if my ignition wire is starting the alternator every crank? Maybe it starts once in a while that's why it charges at 14 volts sometimes but it usually is below 12? Any thoughts?
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Take the truck or just the alternator to autozne or advanced and the will test it for you for free
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I am betting the Isolator with only one battery is part of your issue....
They are not real good when there isn't 2 batteries attached....
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isolator is removed, alt is attached to the main block on the firewall. I actually work at Autozone and I did a charging test on it, it read at over 14 volts and 70 amps but the gauge was still reading at about 10-11 volts and when it did the load test it went down to almost 8. This is what leads me to believe the gauge is the culprit.
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sounds like. test how much power you got at the fuse box. haha another zoner
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Yeah yeah laugh it up gotta pay for gas and beer during college somehow...
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Not laughing to hard I used to be one about 5 years ago. Ill walk in to a store and use the computer my self better than most then kids can. Like today I did a parts search online for for hinge pins made sure the store had them then once I got off work I forgot the number and this kid didn't know me didn't know I knew what I was doing and he was a Pam. So as soon as I walk up to the computer he jumps on it and I tell him what I need he looks it up. Then tells me they can't get it but if I want I can look in the help section and he walks away so I find it. And take it back to him take the computer back and show him how to look it up. He thensays well now I know how to do it next time. Bad part is when I was a red shirt I was getting paid more than most the managers and they get pissed when they would find out