1961 - 1966 F-100 & Larger F-Series Trucks Discuss the Slick Sixties Ford Truck

Old truck electrical basics.

Old 04-09-2013, 07:43 AM
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Old truck electrical basics.

I was at the paint shop, rewiring my truck and discovered some of the guys know absolutly nothing about electricity.

Here are a few things you need to know about the DC (direct current) electrical system on the old trucks and how to use a cheap simple meter. Please feel free to add to this post, but please keep it super simple.

Power is provided by the battery or by the alternator / regulator when running. The alternator and regulator keeps the battery charged.

On most if not all 12 volt systems, the negative of the battery goes to ground. On very old 6 volt systems, the ground was positive.

Voltage is measured between positive and negative. You set the meter on DC volts and place the red probe to positive side and black to negative. If you use a clip, you can clip the black probe to ground and test a lot of points with the red probe. For example, the coil should be hot only when the ignition key is on. The wire going to the starter solenoid from the ignition switch should be hot only when the key is in start position and off when in run. The voltage actross the posts on a battery should increase when the alternator / regulator is working unless the battery is fully charged.

Voltage pushes electrons through the wires to the components then to ground to return to the battery. Grounds are essential to get a complete circuit from one post of the battery to the system and return to the other post. A bad ground will reduce or stop electron flow.

You may have good voltage going to the component as measured by a meter but the component still will not work unless you have a way back to the battery via a wire or ground through the component. Note the starter has only one wire to it because it is grounded to the motor which is grounded to the engine which is grounded to the battery via the frame. Any bad ground in that path can cause problems even if the voltage to the starter is good.

Voltage at the hot wire to all components should be very close to the voltage at the battery, if not, there is something in line causing a lot of resistance to electron flow or the electrons are going back to the ground via a different path. Most low voltage problems relate to dirty connections or gaps (open) circuits. It is best to check voltage under normal operating conditions. For example, you can have 12 volts supplied to the starter solenoid but when the starter engages, the volts can drop drastically if the battery is bad.

An open circuit means you have a gap in the circuit somewhere, it can be a broken wire or a component not working, light burned out, etc.

When using a meter to check for continuity, set it on ohms or the "Continuity" position that will "beep" when the two probes are touching each other.

NEVER measure voltage when in ohms or continuity position, it may burn out the meter or a fuse in the meter. The meter must have a good battery when checking continuity as it is providing power to push the electrons through the wires. It is best to check continuity with one end of the wire disconnected from the component or power supply.

You can check current flow in the "DC Amp" position but you have to have the meter in line .... a part of the circuit. This means you have to disconnect the wire and place the meter probes between the wire and the point you disconnected from. Most cheap meters will not measure high currents but are good to see if you are getting a battery drain when everything is suppose to be shut off. Another way to see if you are getting a battery drain is to disconnect the postive battery cable at night or in a dark garage and see if there is a spark when you touch the cable terminal back to the post.
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