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Hello,

I have a 1984 Cadillac Sedan Deville, beautiful car, 48k original miles that I purchased fairly recently. It has the 4100. Despite a new battery, alternator, and belts, I am still having issues with the battery draining down while driving, eventually leading to the loss of all power. At this point I am not sure what to address next and wonder if anyone else has run into this issue.

Thanks!
 

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70 Deville 78 Seville 92 Deville 03 Deville
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You need to verify battery voltage when the car is not running and then running to see if it is charging. The auto parts stores like to do this for free if you don't want to buy your own multimeter.
 

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'84 Eldorado and Seville
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If it was the alternator I would think it would set the dash light to come on that the alternator isn't working.

A solenoid in the starter not fully disengaged can drain a battery.
 

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1980 FBC
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Always make sure the water pump belt is tensioned and new enough to feed the stepup pulley that drives the alternator pulley.

With the engine off try to spin the alternator by hand by grabbing the cooling fan round cover in front. If you can spin it manually, it is not working properly.

Be careful not to ever overtighten the alternator belt on an HT-4100 because water pump replacement is no picnic.

Again, fresh, properly tightened belts are important. Some brands break in and need an additional adjustment. If you hear a belt squeel for more than a very short time it will be glazed and damaged. The glaze might wear off but this is a critical application.

The alternator gets current limited power through the dashboard idiot light. That part of the dash is fed by long metallic traces on a flexible plastic strip and can be one source of trouble.

Those alternators generally sucked. I replaced them with newer, better GM alternators.

A good load test with a carbon pile and graphic fast acting voltage display in the right hands can find most problems.

Some batteries will turn to crap is left dead, even if they are newer.

Best of luck.
 

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While the car is running and shortly after starting it, the voltage, as metered at the battery terminals, should be over 12.5 volts. Will probably be north of 13 volts.

Turn on the headlights, stereo and heater/AC. The voltage should still be above 12vdc.

If the voltage is at 12.0 or lower, the alternator is not working properly.
 

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Discussion Starter #6
I have tested the voltage, and it is fine when the car starts up on a fully charged battery, and the alternator appears to be charging the battery. At some point, I lose my voltage, the car loses power and the battery becomes completely drained, and I'm not sure why. The battery is brand new and I thought changing the alternator would be the answer, but it's doing exactly what it had been with the old alternator.

Thanks for your replies!
 

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If the alternator idiot light circuit has an intermitant open the alternater will quit charging and give you no warning.

A digital voltmeter connected even inside the car to monitor when voltages gets too low.
Then when it acts up is the time to start testing,

Really need to check voltages at different locations & voltage drop.
Best to also have an amp clamp.
Check the alternator output after startup & under different loads.
Look to see if the battery is charging or discharging.
Look for any excessive loads.

From the internet & the OP it's hard to determan if the alternators are charging or the car is running off a fully or overcharged battery.

If you don't have access to a DC amp clamp, good volt meter, load tester it might be better to pay a shop to check it out. It's basic auto tech. But too hard to write what takes some classroom time.
 

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I have tested the voltage, and it is fine when the car starts up on a fully charged battery, and the alternator appears to be charging the battery. At some point, I lose my voltage, the car loses power and the battery becomes completely drained, and I'm not sure why. The battery is brand new and I thought changing the alternator would be the answer, but it's doing exactly what it had been with the old alternator.

Thanks for your replies!
I've seen this on more than one occasion where the alternator would be find on cold start. But, after heating up for a while, the alternator would not charge. One in particular was RPM dependent. If the motor revved above 2500rpm, the alternator would not charge.

The internal regulator is probably going bad on your alternator. When in doubt, rip it out. Good luck!
 

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The alternator is an older style that still used the "D" test hole.
By grounding the "D" hole terminal it bypasses the regulator and full fields the rotor.
 

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The alternator is an older style that still used the "D" test hole.
By grounding the "D" hole terminal it bypasses the regulator and full fields the rotor.
Interesting troubleshooting test-point. Did not know that existed. Thank you
 

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Not sure if the D hole is accessible on an HT-4100 car.

Used that trick once in auto shop 1978.

After that always had a VAT-40 for testing.

Dinosaur_out.
 

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The idea with the "D" hole was to allow componant level repairs to the alternator.
Ford & Dodge had external regulators.
While it could be done with jumpers there was a simple orange plug in tool that covered the big 3 external regulators.
Just unplug the regulator & plug in the test plug & the alternator would be full fielded & unregulated.
At some point in the 80's most shops went away from rebuilding componants like starters, alternators, & distributors.
My guess is it was easier to quote an exchange fixed price, didn't need bench test equipment, & probibly more profitable.

The alternator used here was based on a 1970's internal regulator design.
To permit the dignoses of an internal regulator they designed the "D" hole.
Truth is if you're changing the entire alternator, the "D" is really not needed.
Because it don't really matter what part inside the thing is bad.
Unless you got a problem child or warrenty rejection issue.

By the late 80's to 90's GM went to a different design not made to be disassembled in the auto shops.

The good thing about those old alternators is you could get all the parts & rebuild yourself easily.
They could be "clocked" & rotated as needed easily.
The bad is 80a to 100a like used here was a high output charging system.
Few years before 63a was the stock big alternator.
But they are so cheap now, can't say if it's worth rebuilding.

The Sun Vat 40 was the shop test tool to have.
Along with a good auto DVM.
The newer Vat 60s went to junk with the digital displays.
 

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For the OP the alternator only needs a few connections to work.
12v+ at the insulated screw terminal in the back.
12v+ at the regulator terminal.
The both go to 12v+ but most of the time are connected with 2 seperate wires going 2 different directions.
Reason is the one landing on the regulator is trying to sence battery voltage.
Which is probibly slightly lower than the 12v+ terminal at the screw terminal.

12v+ run thru a 194 lamp or resister to the other regulator terminal.
12v- to the metal by the mechanical connection to the brackets & engine.

the 12v+ & 12v- needs to get back to the battery thru the battery cables & fuse link on + side.
 
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