Very Low Voltage with Accessories on - what is normal?

[TheFover]

Already mentioned in this thread is lower output due to a slipping alternator drive belt. Not mentioned as a cause of belt slippage is an incorrect drive belt. Use a known-good belt that meets specs. A Gates 7495 is one such belt.

Citations below, slightly edited, are taken from the Knowlegebase article about alternators from DMC Classic - including how to prove belt tension is adequate (use a wrench):

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"4 - Set the correct belt tension. Nominally the belt should be tight enough that the belt will deflect 1/2 inch when pressing with your finger. A better indication is to try to turn the alternator main shaft clockwise with a wrench. You should see the belt turn the engine slightly rather than slip at the alternator pulley. Too tight is better than too loose. A loose belt will slip under load and greatly diminish the output of the alternator just when you need it the most.

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The # 1 Reason for Premature Alternator Failure Including "No Output" Is a Slipping V-Belt!

In the good old days when all cars needed was a 40-60 amp alternator, slipping belts were not a common problem. Now that we need 100 amps and up, it has become a real issue. The higher the amperage, the more resistance an alternator has, and it takes more power to turn the pulley. Therefore the belt has to be tighter to prevent slippage. Most of the time the alternator belt doesn?t make a squealing noise when slipping.

Signs of belt slipping:

• Little or no output, discharged battery
• Alternator pulley extremely hot and starting to discolor
• Belt sticking to pulley or belt glazed or cracked
• Rust on alternator shaft
• Belt dust collecting on fan and front alternator housing
• Belt riding too deep in pulley groove
• Bearing noise

Yeah those are enough potential issues that its worth looking at the belt slippage, especially since I'm taking the alternator off. It seems tight enough based on the deflection. I don't know how one gets a socket on that pulley without removing the asbestos muffler heat shield, but what I have done is push on the alternator fan blades with a wrench and its extremely hard to get it to slip/rotate. If belt dust can get it to slip maybe that bearing old grease residue is causing some slippage. The sides of the pulley's where the v-belt grips are black from heat from belt friction I presume but unless you all tell me your pulleys are super shiny I assume that's normal. I'll consider all these things after I re-assemble after the alternator bench test.

[TheFover]

Cleaning grounds and installing a ground bus, along with getting rid of that stupid bundle of brown wires from the alternator to the bulkhead and replacing it with a thick battery cable, all solved my voltage issues back when I was dealing with them.

I'm 90% sure mine goes red battery cable from alternator to starter, red battery cable from starter to jump post, then brown cable bundle from jump post to bulkhead post. And red battery cable from starter to battery. I'll verify tonight.

[Rich]

Pulley test
No socket wrench. Use a 22mm open end or combo wrench on the alt pulley nut. Even a 7/8in wrench will do.

My pulley surface is shiny where belt contacts it. Belt is smooth but not glossy or glazed.

I don't know how one gets a socket on that pulley without removing the asbestos muffler heat shield

The sides of the pulley's where the v-belt grips are black from heat from belt friction I presume but unless you all tell me your pulleys are super shiny

[TheFover]

I think Dave wins. Load test points to bad alternator see pics. I also see what looks like bearing grease on the back side of the alternator too. This means this is the second alternator that has crapped out on me in 2 years, and I know these are good units so I'm no really sure what causing this, but I'm just going to replace my cooling fan motors too since the car is apart since I was cleaning 30 ground points and checking every connection. I'll report back when I get it all back together to confirm my idle voltage is back to normal when everything is installed.

Thanks again everyone for the help. Dave you're an invaluable resource here and I'll be getting another solid state fan-fail unit as soon as I get everything fixed.

Also please guys remember to post you're resolution to any issue you have so poor saps like me don't read 10 pages worth of posts and never find if what fixed the issue. Although I do understand often you fix multiple things at a time when the car is apart and you're sure exact which one fixed the problem.

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[Bitsyncmaster]

It probably is less stress on the alternator if you don't let the battery get to low. When the battery is low your pumping a lot of current just to charge the battery. Maybe keep a trickle charger on it.

[David T]

It probably is less stress on the alternator if you don't let the battery get to low. When the battery is low your pumping a lot of current just to charge the battery. Maybe keep a trickle charger on it.

Overtightening the belt can cause the bearings to run hot and then the grease melts and comes out. The grease can also damage the internals of the alternator so it won't work right (it gets on the commutator and brushes). If the alternator is always working hard it can overheat and melt the internal voltage regulator and/or the diode bridge, especially if your battery is always chronically undercharged. If you have a bad battery it will kill the alternator and if you have a bad alternator it will kill the battery. That is why you must always test them both if there is a problem. When I don't use my car I put a Battery Tender on it for 1 day once a month and a day before I use it. That way the alternator only has to "put back" what it took to start the motor.

[TheFover]

Overtightening the belt can cause the bearings to run hot and then the grease melts and comes out. The grease can also damage the internals of the alternator so it won't work right (it gets on the commutator and brushes). If the alternator is always working hard it can overheat and melt the internal voltage regulator and/or the diode bridge, especially if your battery is always chronically undercharged. If you have a bad battery it will kill the alternator and if you have a bad alternator it will kill the battery. That is why you must always test them both if there is a problem. When I don't use my car I put a Battery Tender on it for 1 day once a month and a day before I use it. That way the alternator only has to "put back" what it took to start the motor.

Yeah I agree with all this. I would leave it on a trickle charger all the time if I could, but the car spends a lot of time in a storage unit where there is no power source, so I just disconnect the battery. But even after sitting for a few weeks or more with the battery disconnected I should probably charge it up.

I'm pretty sure the belts weren't too tight - they had had 1/4" of deflection, but then again the water pump bearing busted recently too, so who knows maybe it was.

[MacStainless]

This thread is a good reminder that I also need to figure out what's going on with my system. Lately my volts have been lower even while driving. Though yesterday while out I my volt meter was spiking way up and my blower fans were going at crazy high speed. Only when I unplugged an accessory from the cigar lighter and turned the radio off, did it return to normal. Weirdly, when I plugged the accessory in a moment later and turned the radio back on, the voltage was fine. This freaking car sometimes. LOL