Rapid Voltage Drop while Idling

[winslows]

Last night while showing the new truck/stereo to some friends, I noticed the voltage meter dropping rapidly and suddenly from about halfway to almost nothing. The stereo cut off and the lights became very dim.

Today I hooked up a portable jumper to the front battery and it jumped off, would not start before. The voltage meter dropped all the way down on the way home, no turn signals or brake lights or anything. The front battery was completely dead.

I am recharging it, and have taken both alternators in and had them tested, both tested good. The Gen 2 light had been blinking sometimes, and the drivers side alternator had a slightly loose belt.

Any ideas as to the problem?

 

[Cucvnut]

change your belts. if the alts are good your batteries are only for starting the truck if they are bad the truck will use the power from the batteries. might have a loose wire. look up alternator issues on here.

 

[Warthog]

Does the GEN1 light come on with the key on and engine off?

Have you charged and load tested the batteries? Front one runs the entire truck. Sounds like the front battery has croaked.

 

[winslows]

The drivers side alternator had the loose belt and I believe thats the one that charges the front battery. Anyways I just realized a PO bypassed the pulley on the bottom of the engine and so when I went to autozone with belt part numbers we saw a huge difference in size and tried to find something close. I'm about to go back and get the correct ones, hopefully its just the belts.

 

[winslows]

Does the GEN1 light come on with the key on and engine off?

Have you charged and load tested the batteries? Front one runs the entire truck. Sounds like the front battery has croaked.

Yeah I am charging the front battery as I type it has an hour or so to go before I can load test it, its looking borderline weak on the last load test when not fully charged but not dead like this. I really think it might have been the belts weren't giving the alternator enough and so the battery slowly died.

 

[winslows]

I replaced the belts, everything was working fine last night, now my front battery is dead again this morning.

 

[Red_506]

Probably need a new front batt, check the rear to be sure it is good as well. Diesels have a tendency to kill both batts if one goes bad.

 

[winslows]

Edit: My thought is that the front battery has gone bad in a way that it charges slower than the car is drawing, so it slowly runs down, but then once it finally recharges fully it tests well. I don't understand electronics well enough to know if the alternators can bypass the battery to power the car? My other theory is some sort of wiring issue.

The batteries and alternators all tested as good, and I used a multimeter to check continuity between the first alternator and front battery, which there was. The battery will take a charge from a trickle charger, once charged it tests as great, but then it runs down in the car and dies. It does take a significantly longer time to charge from dead then my dads 5 year old Optima Red Top but they also have different CCA ratings and such. Would that be a sign of a bad battery?

The Gen 2 light comes on from time to time, but it tested good at an alternator rebuild shop, and the belt is good and tight. Only the front battery runs down. This all started after I hooked an amplifier up to the front battery, but the battery has died on a long trip when the alternators should have been charging the batteries.

I tested the first alternator at auto zone, and they had to have a vehicle, so I told them to use the 75 Cadillac 8.2liter alternator since I saw on here it was a workable replacement. They weren't to confident in their testing machine, but if I understand the schematics correctly the 2nd alternator charges both batteries.

I don't want to just start throwing parts at it because I'm between jobs at the moment. This is driving me crazy though as this is my only vehicle and I'm trying to get it reliable.

 

[Warthog]

I don't want to just start throwing parts at it because I'm between jobs at the moment. This is driving me crazy though as this is my only vehicle and I'm trying to get it reliable.

Going back to my earlier question. Has the GEN1 and GEN2 lights ever come on when the key is in the RUN position and engine off? Just because the alternator tests good, without working GEN idiot lights the alternators will NEVER work. They are part of the excitor circuit

No need to start throwing money at it. Just spend a few dollars on a digital voltmeter and start testing the charging circuit.

The GEN1 (driver) charges the front battery. The GEN2 (passenger) charges the rear battery. Only under perect conditions do the alternators charge both batteries. There has been a tech article written about how this works.

Why not disconnect the new radio until the issue is fixed?

 

[winslows]

Going back to my earlier question. Has the GEN1 and GEN2 lights ever come on when the key is in the RUN position and engine off? Just because the alternator tests good, without working GEN idiot lights the alternators will NEVER work. They are part of the excitor circuit

No need to start throwing money at it. Just spend a few dollars on a digital voltmeter and start testing the charging circuit.

The GEN1 (driver) charges the front battery. The GEN2 (passenger) charges the rear battery. Only under perect conditions do the alternators charge both batteries. There has been a tech article written about how this works.

Why not disconnect the new radio until the issue is fixed?

I was about to do that until I accidentally stumbled upon an temporary fix that also makes me believe the whole problem has been belts. I replaced one of the belts that was loose, my dad who has much more experience than me thought the others were tight and not worn enough to bother replacing at the time. The batteries kept going dead. I also do not have a transmission gear indicator, and find myself accidentally in 2 instead of D a lot. I was in D and thought I was in 2, so I shifted it into neutral accidentally and stepped on the gas.

When the motor revved up high, I noticed the voltmeter jumped from being centered in the yellow to dead between the green and yellow. When I got it back in drive there was some squealing from the belts for a bit. It seems like it needs to be revved up high for a second or two so the alternator belts will "catch". Since learning this trick I haven't had to put the truck on the trickle charger, and have been able to run the stereo as loud as I want. I plan on replacing all the belts soon, but I need to find employment first.