Roadrunner777
Explorer Addict
- Joined
- February 5, 2011
- Messages
- 2,044
- Reaction score
- 346
- City, State
- Bemidji, MN
- Year, Model & Trim Level
- 94 4x4 Sport 88k
I just wrapped up a thread troubleshooting the electrical side of a fuel pump problem, and it reminded me of a problem I had last fall. In both cases, troubleshooting was temporarily side-tracked because of some unique traits to Digital Voltmeters. So, while it's fresh in my mind, I'm going to throw this out for the record.
Digital voltmeters are designed to have as little influence as possible on the item being measured. This leads to a problem if you remove a lamp or motor and measure the voltage going to it because a poor connection (a corroded splice for example) can pass 12 volts at no load, but will fail in actual use, with the lamp or motor connected. If at all possible, measure voltages with the device connected. Alternatively, you can use a cheap screwdriver type tester with a 12V bulb in the handle to simulate the load.
The other place where you can get tricked by this is using a digital voltmeter to read a pin from a computer, like the various control modules in our Explorers. A disconnected pin can show a floating voltage that will lead you down the wrong path.
If you are having trouble imagining this, maybe this will bring it around: We have all had a battery that showed the correct voltage but when you put a load on it, it fails. The idea is similar.
Digital voltmeters are funny. They will try to read and display whatever they can get at... a lot of voltmeters will show slight varying voltage with nothing connected at all.
I suppose what I am trying to say is... a digital voltmeter is great, but you have to be aware of false readings, and it's not a bad idea to have one of those cheap screwdriver type testers around to double check things that don't seem right. They have the added benefit that they don't need a battery themselves... so there's that.
Digital voltmeters are designed to have as little influence as possible on the item being measured. This leads to a problem if you remove a lamp or motor and measure the voltage going to it because a poor connection (a corroded splice for example) can pass 12 volts at no load, but will fail in actual use, with the lamp or motor connected. If at all possible, measure voltages with the device connected. Alternatively, you can use a cheap screwdriver type tester with a 12V bulb in the handle to simulate the load.
The other place where you can get tricked by this is using a digital voltmeter to read a pin from a computer, like the various control modules in our Explorers. A disconnected pin can show a floating voltage that will lead you down the wrong path.
If you are having trouble imagining this, maybe this will bring it around: We have all had a battery that showed the correct voltage but when you put a load on it, it fails. The idea is similar.
Digital voltmeters are funny. They will try to read and display whatever they can get at... a lot of voltmeters will show slight varying voltage with nothing connected at all.
I suppose what I am trying to say is... a digital voltmeter is great, but you have to be aware of false readings, and it's not a bad idea to have one of those cheap screwdriver type testers around to double check things that don't seem right. They have the added benefit that they don't need a battery themselves... so there's that.
