putting vintage air in my 41 pu everything finally working as it should finally except when blower is on the volt meter is barely reading above 12v has new blower motor. it is a sbc with stock alt for 76 1/2 ton pu.
That could be several problems. Is the alternator large enough for the total connected load? Where is the AC system and/or blower fed from electrically? Is the wire feeding that point from the battery large enough? Does the voltage drop even lower if you turn on other electrical items like the headlights? This sounds like a voltage drop issue (too small of a wire for the load), but without knowing just how the truck is wired will be difficult to diagnose long distance.
Is you battery new or getting some age on it. Your Alternator should put out between 13 and 14 volts,you might go to a auto parts house and let them check it to make sure it's running at peak conformance. Worse case scenario you might think about a 1oo amp alternator. HRP
It matters where the volt meter is drawing it's reading from. Check the battery voltage at the battery using a hand held volt meter with everything on to get a clearer picture of your usable voltage. I had a bone-stock winter beater 77 Camaro once that showed 11-12 volts with the heater motor on but the battery never gave me problems. I believe the volt meter was plumbed into the heater motor circuit..
Not a safe assumption.... Wire has to be sized for the connected load. A wire that's big enough without the AC connected could easily be too small with it connected; AC is a fairly large load. This is one issue I see with 'universal' aftermarket wiring harnesses and why I don't like them. Nearly all feed the fuse panel or panels with #10 wire, which for most 'normal' cars is big enough. But add a large load like AC (which can add anywhere from 10 to 20 amps additional load to the panel feeder) and you can easily drop a volt or two between points. For a full explanation of wire sizing and voltage drop, look here: http://www.jalopyjournal.com/forum/threads/wiring-101.843579/ The big reason for a #10 'limit' in aftermarket harnesses is once you go over this size, you need larger wire, different connectors (more $ and harder to find), and a different crimper (even more $). There are 'workarounds' for this limit, but that will make your wiring harness considerably more complicated.
Are you using a one wire Alt? They do not work well in a vehicle with accessories that draw a lot of power. You need one with a sensing wire wired to a junction block close to the heavy power loads. That way the alternator knows when a heavy load is on and ups the voltage as necessary.
Yep, that can be an issue. But beware, inadequate wiring/excessive drop to the junction point can cause the alternator to raise voltage to the point that you can overstress the alternator and/or cook your battery.
If your not having any battery problems I'd leave it alone. I don't have a voltmeter on my 63 BelAir and have no battery problems using A/C, air ride compressor and lots of other stuff. Gary
I don't know that I'd ignore this; it's definitely a symptom of something wrong, but how serious it is can't be determined until it's checked out.
Is your a.c. load picked up thru the panel?? Better is to run a wire with a relay protected via a circuit breaker from the starter or batt post to the a.c. and a "trigger" wire from the acc side of the panel thru the controls to close the a.c. repaly
agree with Rocky, I think it is just where your volt meter is picking up the voltage. If that volt meter is on the same circuit as the fan, it will pull the voltage down; even if the actual alt output and battery voltage is good for the rest of the system.
the truck is wired with rebel 21 wire harness and the air cond is wired according to vintage air. ol scrounger could you post or pm me a diagram of what you are talking about if I have a schematic I can usually figure it out thanx to the hamb all post are informative I will be checking all of them thanx again tom
If wired as per vintage, the main feed is not off the panel--the load wire with the relay (large red) is hooked to batt or starter lug
If was me, I would check the alt output against its design. If it is a 60 amp, for instance, and total draw is 55 or more, problem solved. Just get a higher amp alt. Dollar to a doughnut this is it. Ben
I ran into this problem this year, a much hotter summer. Running the blower fan on high, stopped with the electric fan on, I'm down to 12 volts. Turning the blower to low or medium, the voltage comes back up. On the highway, the electric engine fan shuts off, voltage comes back up. My wiring is sufficient, I'm just right on the edge of the alt. output. Winter project as summers almost over.
Did we hear where the voltmeter is plumbed to? At the very least I'd measure voltage at the bat posts at the high load condition. If the bat voltage was good, it might be the MOST I'd do.
Could be either a too-small alternator or too-small wire from the alternator to the battery. Find out what size your alternator is, then check for actual load. You don't want to run the alternator at more than 80% of it's full rating on a continuous basis. If/when you have the right sized alternator, check voltage (with all 'normal' stuff running; lights, AC, etc) first across the battery terminals, then move the positive test lead to the alternator output terminal. You should get a higher reading at the alternator, but if it's more than .5 volt more, the wire from the alternator is too small.
alt for 76 1/2 ton is 63 amps , IIRC they wired the blower motor high circuit thru a relay on firewall that pulled directly from the back of the alt ( bypassing the battery up till 73 amp alternators came along ) and the lower fan speeds went thru the ignition switch . and on high the alt was taxing out and caused ignition problems ( HEI misfiring at higher rpms ) , and this was with a minimal electric load on the system of the time ( lights and early HEi ignition , am/fm radio ) wasn't soon after GM went to the 73 amp alt . and things improved . check the gauge of the wire like steve said ( IIRC it should be 10 gauge to the bat and ac relay ) then if you have access to a amp probe , see what the fan is drawing amp wise on high . as the a/c clutch and fan combo might be the limits of the 63 amp alt and the running from the alt to the battery then to the fan might be a big resistance to the circuit . IMO check wiring ( feed and grounds ) and connectors feeding alternator ( have to have juice to make juice and a complete circuit ) and output sides ) and if they are all good then go with a higher amp alternator like a 85 amp .
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