The alternator on my 85 4+3 seems to charge my battery just fine until I turn the AC on, then there is an immediate discharge of my battery. The dash is showing the water temp at first then switches to voltage mode and shows red discharge bar. Any help out there?

Todd

 

There is something wrong with your AC system. It might not be the compressor, but something is shorting out. Someone here has wiring diagrams for the AC system. You are likely to have a relay that is shorting out and draining the battery.

 

Thanks, I have a service manual but not much of an electrician. System is blowing cold. I'll start with relay.

Todd

 

Suggest having a charging system & a battery capacity test done before component draw (load) tests.

At 2000 rpm the alternator should be capable of 90% of its rated output with all accessories off. The remaining 10% of the alternator output is used to drive the ignition & fuel system while the engine is running.

A 200 amp load on the battery for 15 seconds (engine off) should result in a battery capable of maintaining 10.6 volts. If unable to maintain 10.6v during the load test recharge & re test. If still unable to maintain 10.6v after 15 seconds replace the battery.

How old is the battery? It may be sulfating.

To test for a sulfating condition: Hook a volt meter to the battery, engine off if the battery is still in the car.

Charge with a 30 amp charger for 10 minutes.

With the charger working & after 10 minutes, if the volt reading is now over 16 volts the battery is sulfating & on its way out.

If the charging system & battery pass the tests check the amount of amps required to run the AC compressor. Also test the amps required for the blower motor while operating on high speed.

Normally the compressor & blower motor system is fused & the fuse will blow if the current draw is excessive.

I suspect either the charging system or the battery is at fault.

 

Sorry I should have included more info. When I first got the car the dash was black with no visual at all. So when the I was having to charge the battery or jump it when I didn't have enough power to crank I took the battery and alternator out to have load checked. All was good. Only after I got the dash working did I notice that the voltage was droping when I turned on the AC. Although loads appear when lights are turned on and during start the voltage immediately recovers. Again the battery has not let me down when out and about unless the AC is turned on.

What would you next step be?

 

Suggest 1500 rpm, no accessories on. Connect a volt meter to the battery & read the voltage about 3-4 minutes after starting the car, it should be 14.4 to 14.7v.

Compare the accessory volt meter to the cars volt meter to see if the volt meter in the car is accurate.

Turn on the AC & compare both gauges again. Actual voltage with the AC on should be about 13.5v.

Return the engine to idle & compare both instruments again. Voltage should be in the 12.5 area at idle with AC on.

If the voltage readings on the meter are ok the charging system & battery are ok. If the gauge shows a lower reading the gauge is faulty.

If both meter & gauge correspond all the time & you have 12v or less @ idle there is a charging or battery problem.

Also check battery connections for clean & tight including starter wire & the ground wire to the engine/frame.

Inspect the alternator wire @ the bat stud on the alternator. Look for corrosion between the crimped connector & wire. Clean/replace as necessary.

A note on flat dash Vettes (attari dash): Instrument clusters are prone to ground failures & require additional ground wires.

GL

 

My Fbody had a short in the cooling fan motor. When the AC came on, the fan came on and did wierd stuff. Till it went up in flames. For real. Unplug the cooling fan and see what happens. May or may not be it.

 

Good chance the alternator is not putting out enough of a charge at lower rpms. Maybe someone put a light duty alternator in (80 amps). Make sure your replacement has a rating of at least 130 amps.

 

Idle and cruise or just idle?

Low speed, idle output is usually no more than 25 to 30 amps (use a Clamp-on meter if you want to see what it is). The fans can suck up 10 of that, a weak Blower Motor on high 25, and resistance in 25 year old wiring takes the oomph out of it, so it isn't unusual to be on battery at idle (which is where GM puts it if you read some of their Bulletins chastising Dealers for replacing good alternators). Unplug each component to see where the heaviest draw is and consider replacing that item. Getting it back to Factory would involve replacing the harness, but you should be able to make it better. Also, there's a voltage drop across the ignition switch, so the Display is going to be up to .5 volts lower than a reading at the Battery. Factor that in to your troubleshooting.

 

First it occurs at high and low rpm.
Thanks everyone, next time under the hood I will dive into all these possible culprits.

 

Well, at cruise, you're fans aren't on (or shouldn't be), so that should rule them out. I'd suspect the Blower Motor is sucking the life out of it, so disconnect it and see what happens. Take a good look at the Connector and if the wires are discolored or melted, it's dropping volts so take apart the weatherpak and make sure the crimps are good. If there's no change, the clutch coil is on it's last legs, though bad ones usually draw enough to knock out the a/c fuse, so you're going to have check the harness too. Check resistance across the coil connector - should be no more than a couple of ohms. Or, while the Blower Motor is disconnected, observe your meter (hooked up to the battery). The lack of air flow will cause the compressor to cycle and if you get your juice back with the compressor off - blower motor out of the picture - I'd say you need a new clutch coil.