Author Topic: Some fuses always have power?  (Read 599 times)

Offline runnergeorge

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Some fuses always have power?
« on: May 12, 2020, 11:00:03 PM »
Hey everyone! First time posting! I have a 75 Cheyenne 350 fleetside Lwb. This is my first classic truck and I am learning everything trial by fire so please don't rip my head off too badly lol.

So, I recently noticed that after not driving the truck for about a week I had a dead battery. It has been engrained into me to always check fuses and grounds to start with. I checked them after I charged the battery and it I found my A/C heater fuse was reading just about 12 v with the ignition off. To my knowledge I have followed the wires in that circuit according to the wiring diagrams on here and I didn't find anything of great concern. So my question is are some fuses supposed to be energized even with the ignition off?

Thanks, and sorry in advance I'm a super noob.

Online bd

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Re: Some fuses always have power?
« Reply #1 on: May 13, 2020, 03:24:14 AM »
Some fuses will measure battery voltage at all times (B+), some will measure battery voltage with the ignition switch in the run and crank positions (I+), and others will measure battery voltage with the ignition switch in the run and accessory positions (Acc).

Where is the fuse that concerns you located?  Have you used an ammeter to measure current draw from the battery with everything switched off?  Does the truck have a clock or aftermarket radio?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline runnergeorge

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Re: Some fuses always have power?
« Reply #2 on: May 13, 2020, 10:57:54 AM »
The fuse is for the A/C heater circuit. I have a multimeter that I am learning to use through this, that should read amps right? And yes it does have an aftermarket radio, but the radio only turns on when when the engine is running or with the ignition in the accessory position. I cannot turn it on by just pressing the power button. No clock, the emergency brake light is where the clock would be.

Online bd

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Re: Some fuses always have power?
« Reply #3 on: May 13, 2020, 11:27:49 AM »
More specifically, is the fuse located in the fuse box or in the engine compartment at the firewall junction block?

Most multimeters have a DC Amps scale, typically no greater than 10 amps.  Often the current scales peak at only a few hundred milliamps.  Don't use an ammeter to measure current greater than the range capacity or the meter can be damaged.

If the radio has a memory circuit it will draw a few milliamps continuously, though not enough to discharge a battery over a couple of weeks.  Charge the battery.  With the engine off, turn the headlamps on for ~1 minute.  Turn the headlamps off and measure the voltage across the battery terminals and write it down.  Disconnect the battery ground cable and let the battery sit for a week.  Measure the voltage across its terminals again and compare the measurement to the voltage you recorded initially.  They should be the same.  If the voltage decreased, the battery may have an internal discharge.  The alternative is to have the battery load checked professionally.  Once you verify good battery condition you can go about checking for external draws.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline runnergeorge

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Re: Some fuses always have power?
« Reply #4 on: May 13, 2020, 11:45:27 AM »
O! My bad. The fuse is in the fuse block under the dash. Iíll do that headlight test starting today. The battery should be good because I bought it new from autozone just last week. What fuses always have B+? N reference to the A/C heater fuse I used the wiring diagram from here and it looks like itís I+?

Online bd

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Re: Some fuses always have power?
« Reply #5 on: May 13, 2020, 12:20:40 PM »
The A/C-Heater fuse is supplied by a modified I+ circuit that is battery voltage only in the Run position of the ignition switch.  It de-energizes in all other ignition key positions.  Try jiggling the key cylinder to see if the fuse goes dead.

STOP LPS - TRAFFIC HAZ and TAIL LPS - CTSY should be the only two fuses that are constant battery voltage.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline runnergeorge

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Re: Some fuses always have power?
« Reply #6 on: May 13, 2020, 04:32:31 PM »
Update! I charged the battery, turned the headlights on for about a minute, off and measured. As I was measuring and writing the voltage down the voltage was dropping. I could visibly watch the number on the multimeter go from 12.73 down to about 12.65 in a matter of minutes. I havenít had time to check the amps as I got busy with other things. But I will test that today as well as any other fuses that may have power with the ignition off.

What could cause voltage to drop that significantly?

Online bd

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Re: Some fuses always have power?
« Reply #7 on: May 13, 2020, 05:16:41 PM »
A fully charged 12-volt lead-acid battery produces 12.6 volts (or 2.1 volts per cell).  Charging the battery entails applying a voltage across its terminals that is greater than the voltage produced by the fully charged battery.  The greater charging voltage leaves a residual surface charge on the battery that is unstable.  Gradually, the surface charge bleeds down and normalizes at 12.6 volts.  Removing the surface charge is the reason for turning the headlamps on for 1 - 2 minutes.  After about one week with the battery disconnected, recheck the voltage across its terminals.  It should still be 12.5 - 12.6 volts.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline runnergeorge

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Re: Some fuses always have power?
« Reply #8 on: May 13, 2020, 10:37:20 PM »
Got it! I will update in a week. Thank you!

Offline runnergeorge

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Re: Some fuses always have power?
« Reply #9 on: May 13, 2020, 10:39:44 PM »
I also jiggled the key cylinder (in different positions to cover my bases) and the my test light stayed lit.

Online bd

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Re: Some fuses always have power?
« Reply #10 on: May 27, 2020, 07:57:27 PM »
Any update on this?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline runnergeorge

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Re: Some fuses always have power?
« Reply #11 on: June 29, 2020, 04:24:54 PM »
Yes! Sorry about that, other projects got in the way of updating the forum. I traced the wires in the AC/heater circuit and found the fan speed selector switch was in real shape, the spade connections on the unit were able to move and looks like they shorted out on the metal backing of the unit. So I popped a new one in and the truck now holds a charge!