voltmeter reading
Thread Starter
|
Mountain Pass
Joined: May 2013
Posts: 110
Likes: 5
From: Memphis, TN
voltmeter reading
When I take my multimeter and attach it directly to the battery - engine not running - it reads 12.5 volts. When I take a reading with the engine running it reads 14.5 volts.
But when I look at my newly installed volt meter under the dash - 18 gauge wire, attached to the solenoid fuse - it reads 10.5 volts. And if I cut on the dash lights it drops considerably.
What gives?
CB3
But when I look at my newly installed volt meter under the dash - 18 gauge wire, attached to the solenoid fuse - it reads 10.5 volts. And if I cut on the dash lights it drops considerably.
What gives?
CB3
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
What solenoid fuse are you talking about?
The readings your getting with your multimeter are correct.
First, I would use something bigger than 18 ga. wire. My preference would be 14 ga., but that's just me.
I would find a switched 12v source to hook the voltmeter to, something like the radio or accessory feed. One wire goes to the power source the other goes to ground.
The readings your getting with your multimeter are correct.
First, I would use something bigger than 18 ga. wire. My preference would be 14 ga., but that's just me.
I would find a switched 12v source to hook the voltmeter to, something like the radio or accessory feed. One wire goes to the power source the other goes to ground.
Cross-Country
Joined: Mar 2008
Posts: 97
Likes: 9
From: KS
You are reading the voltage at a place that has a bad connection or corrosion upstream, i.e. undesired resistance in that circuit. That is what causes things to heat up and eventually fail.
It would be like reading the voltage between 2 resistors wired in series.
It would be like reading the voltage between 2 resistors wired in series.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
One important thing to consider is that you're using two different instruments to take two different measurements. You can't necessarily compare the two. Your dash voltmeter could have a different sensitivity level than your handheld meter, and vice versa. To eliminate the variables, disconnect your dash voltmeter and perform the same experiments that you did with your handheld meter. If you see the difference there, then you know the dash voltmeter itself is at least partially responsible. Otherwise, your dash voltmeter is picking up the drop across some unwanted resistance as described above.
18 AWG is plenty for a voltmeter; they are ultra high impedance and draw very little current. The advantage to going bigger (lower AWG) would be tensile strength, but it doesn't have anything to do with your problem. Agreed that I don't understand what you mean by "solenoid fuse," unless you meant the fusible link.
18 AWG is plenty for a voltmeter; they are ultra high impedance and draw very little current. The advantage to going bigger (lower AWG) would be tensile strength, but it doesn't have anything to do with your problem. Agreed that I don't understand what you mean by "solenoid fuse," unless you meant the fusible link.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Got it, you're tapping off the idle stop solenoid fuse in the panel, not under the hood. Typically you want to tie a voltmeter off the accessory bus (radio, etc) so that you can turn the key to ACC and still get a reading, without having to turn the key to RUN (which powers some more things up). What you have is not preferred but there's nothing wrong with it functionally.
You need to do the experiment I described, as that information will be the most valuable. Based on your description I suspect what you're seeing is a result of calibration and sensitivity differences between your two meters rather than unwanted resistance. The reason I say this is because current through the circuit you've tapped off of (and therefore the drop across the resistance between this point and the battery) should not change at all based on the dash lights being on or off. In other words I would suspect your meter would behave the same even if you tried another fuse. The experiment I described would prove this easily. This of course assumes no major wiring modifications by a previous owner. Otherwise, all bets are off as to what's what.
You need to do the experiment I described, as that information will be the most valuable. Based on your description I suspect what you're seeing is a result of calibration and sensitivity differences between your two meters rather than unwanted resistance. The reason I say this is because current through the circuit you've tapped off of (and therefore the drop across the resistance between this point and the battery) should not change at all based on the dash lights being on or off. In other words I would suspect your meter would behave the same even if you tried another fuse. The experiment I described would prove this easily. This of course assumes no major wiring modifications by a previous owner. Otherwise, all bets are off as to what's what.
Thread Starter
|
Mountain Pass
Joined: May 2013
Posts: 110
Likes: 5
From: Memphis, TN
voltmeter reading
I have now switched the connection to the radio fuse - no change in the reading.
CB3
CB3
Trending Topics
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
That's exactly what I said would happen, because the fuse you're using is not the problem. Refer to my previous post:
You need to perform the experiment I outlined in my very first post.
You need to perform the experiment I outlined in my very first post.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Thread
Thread Starter
Forum
Replies
Last Post
1977_f10028
1973 - 1979 F-100 & Larger F-Series Trucks
3
Jun 3, 2019 08:55 AM
Montync
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
10
Jan 26, 2017 05:55 PM
jschluter
1973 - 1979 F-100 & Larger F-Series Trucks
2
Jan 4, 2012 09:26 PM