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Odd Voltmeter readings on dash ?

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fisher1648

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I've got an 87 K5 and am starting to notice some odd readings on the in-dash voltmeter. I have the stock instruments. Normally, with the engine running it shows approx 13 volts and stays pretty consistent - has been that way for years and seems normal. Recently, when I turn on my fan the reading goes to 10V or so and is proportional to the fan speed - faster = lower Voltage reading. Similar drops (but not as drastic) occur when I turn on lights, etc...

I replaced the fan motor thinking it was about to die and pulling a bunch of power. No change

My next thought is it's either the battery or alternator. The battery is 4 yrs old and not showing any signs of troubles. I measured the output voltage from the alternator and it was 14.40 Volts.

What do ya think ?
 
try measuring the output of the alternator with the fan at different speeds and see if it changes. it might be ur alt. regulator if it changes a decent amount.
 
Were you measuring voltage with all the accessories on? Sounds like an unloaded measurement to me.

Try again with everything on high.
 
Now that you guys mention it I think I measured the voltage with the fan on low...I'll re-measure with the blower fan at different speeds to see if there is a correlation.

What do I conclude if it stays around 14V and doesnt drop when the fan is on high? I'd assume the Alternator is good in that case - right ?
 
Correct. The fan draws a large amount, but it's not really worth watching it through each setting. Put it on high, put the high beams on, and anything else that uses a fair amount of juice, and check the alternator then.

Check voltage (under a heavy load) at the alternator, at the battery, then at the fuse panel. Up to a 1V loss at the fuse panel is to be expected, but shouldn't be much loss to the battery.

It's normal to see *some* drop in voltage off the alternator as load increases, since the alternators aren't putting out much at idle RPM's. If it's not maintaining the battery under full load at idle however (I'd say 13.5V+) something is definitely wrong.
 
Here are my numbers:

Battery voltage before starting = 12.58

While running (no fan, no lights, etc):
Alternator Voltage = 14.71
Batt Voltage = 14.56
Fuse panel voltage = 14.05
Guage reading = over 13, guessing close to 14

While running (high fan, headlights highbeam, fog lights):
Alternator Voltage = 14.45
Batt Voltage = 13.83
Fuse panel voltage = 12.76
Guage reading = 11, maybe a little less

Battery voltage right after test = 13.20

Would I be correct concluding the problem most likely lies with the battery ?
 
Doesn't look like you've got anything much out of the ordinary. The drop between the alternator and fuse panel seems a bit much, I might try and clean all the connections I could, just to try and make an attempt to get a bit more into the cab and to the heater blower.
 
OK - but in the 16yrs I've had the truck I dont remember ever noticing the guage dropping too much below the 13 mark in the past. Maybe I just wasnt looking very hard at it. Now I cant keep from constatly checking it....:doah:

I'll try cleaning up the connections to see if i can improve the voltage at the fuse panel. Off-hand, I've never noticed a power distro strip or anything similar in the engine compartment - where does the main power coming into the cab attach ? I must have missed it....

Thanks in advance -
 
Essentially it comes off the starter solenoid, to the junction block on the firewall (a few inches to the passenger side from the brake booster) to the fuse block. You can measure voltage at the junction block, as there is not likely much you can do at the fuse panel connector itself...GM used some sort of grease on it that's a real pain to deal with. If voltage is very close at the junction block and cab side of the fuse panel, I wouldn't mess with that connector.
 
Doubtful, since the voltage off the fuse panel changes by 2 volts, which is where the voltmeter is getting it's reading. Gauge reading changes around 3 volts, I'd consider that well within acceptable since the change mirrors the actual voltage at the panel.

Sounds like the gauge may be off a bit in the display, but it's not necessary that its dead on accurate, only consistent, which from the measurements given, it looks like it is.
 
I started thinking about the age of the battery and how the heat (AZ) and bouncing around for 4 years might have begun to take it's toll....I swapped in a known good (basically brand new and charged properly) equivalent battery and my measurements were the same. This is good info since it definately rules out the battery. Yeager already figured that...

I'm thinking since my alternator values are consistant (load VS. no-load) I can confidently rule it out. Right ?

I removed and cleaned the connections at the starter solenoid with the same numbers. I spotted the firewall/fuse-box connector and that looks clean but I didnt mess with it.

At this point I think the primary issue is I've simply never really noticed the difference at the guage when my blower fan is on high...:crazy:

What does your guys' guage show when under high load ???
 
a little late on the reply but i see the same large voltage drop with the blower fan on high. I have ruled out the battery as well as the alternator and am convinced that its the blower control switch drawing down the power. I'll change mine soon or atleast diassemble to inspect.
 
Mine has always done this on my '90 'burb, it even flickers with the turn signals on. Never worried about it.
 
Try removing and cleaning all grounds under the dash. Common problem with dirty and or loose ground connections. Also make sure that the negative battery cable is good and clean and tight and that the smaller 10 gauge wire is properly grounded on the fenderwell or radiator support like it should be.
 
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