When using the ac/heater on my '89 comanche I6, the in dash voltage meter drops below 12 volts when running it eventually drains the battery until it's too weak to start the truck. It doesn't blow the fuse and still works. It only draws when the unit is turned on.
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Re: Ac/Heater pulling too much voltage when used
This actually could be a few issues. you'll need to check a few things.
First the simplest is that the Blower Fan is indeed faulty and drawing too much current, if that's the case that blower motor should be very hot when run. If so its likely the culprit.
But you could also have a faulty voltage regulator. this is built into the alternator and can be easily tested at most major parts stores for free(some while in the vehicle). I suspect this may be the case because you are not blowing fuses or breakers and the alternator inst keeping up with the load.
The other could be a short such as an abrasion on wiring to the blower or compressor unit.
And finally, double check that there is a good ground wire or braid between the engine and the Body. this could cause all kinds of charging and electrical anomalies if there is a bad block to body ground.
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I have seen many times a 12 volt car battery pass the diagnostic test performed at a repair shop but still fail while being used in the car. A simple test to do at home is the cranking amps test. Place your 12 volt meter on the battery and check for 12 volts. Then looking at the meter crank the car over. If the voltage drops to 10 volts with all accessories off then the batteries cranking storage amps is weak. A bad scenerio would be the battery drops to below 9 volts. Below 9.6 volts there is not enough electricty to provide good spark to fire the engine.
This could be the fuse check the fuse panel. This could be the resistor in the heater plenum behind the glovebox, it could also be the heater fan motor. The easiest thing to check is the fuse, then use a volt meter on the heater fan motor and if you have voltage there, it is the motor. If not it is most likely the resistor.
When they are on, it causes the alternator run harder. Check the voltage when they are off and on. If it drops below 12 volts, I would replace the alternator. If it doesn't, I would look at amperage draw from the blower motor. Mostly likely is the alternator.
Not sure I follow your problem. The alternator puts out 13-14 volts at idle but very few amps. If you turn on the headlights at idle the voltage may drop to 12 until you raise engine speed. The amperage goes up with engine speed. Is the battery voltage at least 14 when the engine speed is 1500RPM or more ?
Hello. Install amp meter on the battery main feed. One the amp meter is coupled
by direct or induced coupling. Start remove the fuses and watch the amp meter
and when the amp meter indicator drops that one of the circuit that have some
shorted or running. Now they can be more than one circuit that is operating when
First thing with the amp meter connect disconnect the alternator. Do get
wrong here. It a percaution if
you happen to concur a large short. Also, when disconnecting the alternator look
at the amp meter to see if there is a drop in current being used.
An alternator is a three phase AC generator. There are 6 to 8 high current
diodes hook up where it turn the three phase AC voltage to DC voltage half wave
+ side of the AC. Then this DC voltage being combined voltages goes to a
regulator which smooth out the voltage to 13.8 volts approx. The 13.8 DC voltage
is need to recharge the battery. Because the vehicle operates on 12 voltage
system it uses the battery for the constant voltages of 12 volts and a story
house of current. stewbison
Sounds like a defunt blower motor & or blower motor switch, lots of testing to be done to find which one is the culprit, useage of an ohmns and dc volt meter is necessary as well as a 12 v test light, also look at the condition of the fuse for the heater/a/c located in the under dash fuse panel should be labeled as such.
Fan will be under dash on passengers side , Removing glove box might be needed, Gm also uses a blower resister ro control fan speeds. First check your fuse and then check and make sure your getting 12 volts to the terminal at the blower, You could have a blown fuse, a bad switch, or a bad resister , if your getting voltage to blower motor then most likely it will be the motor. These are not to difficult to change. I hope this helps, please be aware I can only guess at what the problem is from my past experience.
There are two fuses for the fan. One is probably a 20 Amp in the fuse panel in the cab and marked "Heater". or "Heater/AC" The second is under the hood may be marked Heater, Blower, or Fan and should be 30 Amp. The one in the cab supplies switched 12 volts to the fan speed switch. The one under the hood supplies the 12 volts to the relay. The relay is normally energized by the 12 volts through the 20 Amp fuse in the cab and through the switch. I think you find that the fan has the normal speeds except for missing HI with the underhood fuse removed, and will turn off with the ignition that way. There is no hazard in operating it that way but you may want to replace the resistor/relay assembly before it gets too cold. The relay is a pretty trouble free device in most cases, maybe you can find a deal on a good one at a junkyard.
The whole idea with the relay under the hood started back in the Sixties believe it or not. There is considerable voltage drop in the wiring between the fan switch and the blower fan motor. By adding the relay under the hood right next to the motor, the voltage drop was reduced, both by the shorter wire, and by using heavier wire to boot. The motor gets really close to the full system voltage when the relay is closed. In your case where the relay keeps the fan running, it can drain a battery rather quickly too!