Very Low Voltage with Accessories on - what is normal?

Hey guys,

I'm super frustrated with this situation and trying to crowdsource some new ideas or diagnostic approaches from the impressive brain-trust on this forum.

I've scoured the posts here trying to improve my low voltage situation at idle. I may just need new cooling fans (mine are stock), but I'm not totally convinced there aren't other factors at play because I feel like my idle voltages are too low when I start adding accessories. The other day on the road the with the headlights on at max A/C on the voltage dropped below 12V too many times, and my understanding is it started impacting the spark and fuel pump, the voltage couldn't recover and the battery died a slow death. Luckily I noticed it dropping, and made it home just as the battery died as I was pulling in.

Here's the voltages I'm reading at idle with a multimeter across the pos/neg posts in the bulkhead area behind the passenger seat.

Headlights on + Max A/C - 12.0V (it holds low 12's for a minute or so but then drops to 12.0V)
AC on Speed 1 - 12.9V
AC on Speed 2 - 12.7V
AC on Speed 3 - 12.4V
AC on Speed 4 - 12.2V dropping to 12.0V in under a minute
AC on Speed 4 at 1750 RPM - 14.3V
Just cooling fans from otterstat, no cabin fans - 12.5-12.9V
Vent mode, no AC, just cabin fans Speed 4 - 13.5V
Just headlights 13.8V
Headlights + cooling fans 12.8V

Car off - 12.5V
Running with no accessories steady at 14.1V

Battery light comes on in accessories mode

I've got a 1-year old DCMH alternator, 2-year old battery, alternator belt is tight, tried a battery swap, updated fuses and fuse block, relays and relay terminals, I've swapped the changeover relays (the gold kind), checked the connectors for corrosion even at the washer bottle, I've cleaned as many grounds as I can find including the one on the chassis for the cooling fans, DMCH also put in an additional ground from the frame to the engine, and I've ran a jumper cable from the cabin fan shroud to the neg battery post. No joy on any of those.

My cooling fan relay runs very hot - always has. I was using Dave's solid state fan-fail relay and it worked great for awhile, but it also ran very hot and eventually cracked under the excessive heat I presume, and now I'm just running the 20 amp in-line fuses for each cooling fan. When I was using Dave's fan-fail relay I don't recall seeing the low voltages, but I'm not totally sure, and I don't really understand how they would have improved the voltage, but that and a water pump install was the last thing that changed before I started noticing the low voltages.

I'm out of ideas except for getting new cooling fans - mine are stock and the motors show some rust on them.

Sorry for the long post, but if you made it this far it'll help weed out suggestions and maybe you have an idea I hadn't though of?

Much respect to you all out there dealing with this machine, but hell if it ain't worth it when she's running good! :driving1:

Bad connections don't increase load current, it drops load current. So don't go chasing that. It sounds like your cooling fans should be changed but my guess is your alternator is bad. When it was new did you have low voltage problems? If a diode in the alternator goes bad it's current output drops in half.

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Originally Posted by Bitsyncmaster

Bad connections don't increase load current, it drops load current. So don't go chasing that. It sounds like your cooling fans should be changed but my guess is your alternator is bad. When it was new did you have low voltage problems? If a diode in the alternator goes bad it's current output drops in half.

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You may be onto something Dave. 2 years ago I got a new DMCH alternator and it started dying a year after install. Battery light would come on, output was very low driving. I never bench tested it, DMCH just replaced it and the problem was fixed. But now I'm having electrical issues a year later, albeit different symptoms, which started after the water pump was installed which I noticed because water pump bearing grease started spraying all over the belts. The water pump got replaced, but maybe the residual grease on the belts is causing some belt slippage at high alternator loads? I do still see some residual grease on the belts. I've wiped them down a bit but there is still some there and maybe it lubed up the pulley walls. I'm not even sure if belt slippage would explain low voltage only under high accessory load. The belt tension seems pretty tight. It deflects in the middle about 1/2" if you push hard.

Maybe both alternators where fine and there was just a little grease spraying on them causing them to slip? Do you think bench testing the alternator will reveal anything given it seems to charge the battery fine when you rev it and the battery light doesn't come on this time, but maybe something in voltage regulator is damaged. Not sure if a bench test will reveal that. But this is the best lead I have right now.

From your voltage readings is seems the alternator is not keeping up as the load increases. Yet the 14 volts with little load would normally be considered acceptable. You might want to clean and tighten the main alternator power connection. However, Dave is spot on about diode bridge failures impacting capacity. The alternator is fairly easy to remove and many auto parts stores have a test stand available to evaluate. Take them both in and have them tested.
Ron

It is normal for the voltage to decrease when you have a lot of load on the electrical system and the motor is idling. The alternator can't output at it's full rated capacity at idle. The battery is supposed to hold the voltage up for a short period of time, like when you stop for a traffic light or a stop sign. Otherwise you are moving and the motor isn't idling. If you think you have a problem clean ALL of your connections, make sure the belt is not slipping (even if it is tight it can slip if it is glazed or the wrong size), and have the battery load tested. Make sure you don't have your rear window defroster on, it is easy to hit the switch with your elbow and not realize it.

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.

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Originally Posted by dn010

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.

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I have the stupid bundle of brown wires from the jump post to the bulkhead post, but good battery cable from jump post to starter to alternator with very clean connections. I could replace that but it has to go stupid brown wires once its in the bulkhead. I see if I can find a battery wire from jump post to bulkhead. :thumbup:

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Originally Posted by David T

It is normal for the voltage to decrease when you have a lot of load on the electrical system and the motor is idling. The alternator can't output at it's full rated capacity at idle. The battery is supposed to hold the voltage up for a short period of time, like when you stop for a traffic light or a stop sign. Otherwise you are moving and the motor isn't idling. If you think you have a problem clean ALL of your connections, make sure the belt is not slipping (even if it is tight it can slip if it is glazed or the wrong size), and have the battery load tested. Make sure you don't have your rear window defroster on, it is easy to hit the switch with your elbow and not realize it.

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Yall have me concerned it could be alternator/belt related even though its only a year old and the belts seem tight. I'm elbow deep in this, there's no reason I shouldn't pull the alternator get it bench tested and put new belts on that don't have grease-residue on them. I have them sitting in my garage. At the very least it'll help me rule it out.

It's been a while since I've messed with the original engine/configuration, but I'm pretty sure that the bundle of brown wires is supposed to go from the alternator to the bulkhead post (not the jump post). The Jump post should only have one battery size wire going down to the starter.

Also, while you can try new belts, I don't think that is your problem. You'd probably get squealing if it was, but the alternator spins pretty freely, unlike the AC compressor for example that would really have trouble if there was grease on a belt.

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