Winnipeg Free Press

Charging system likely culprit to vintage truck's starting woes

by Jim Kerr . Jul 21 2017

QUESTION: I have an old 1977 Dodge one-ton truck that I use on my acreage to haul wood, dirt and other things around. The truck isn’t pretty, but it does the job. The battery is good. If I charge the battery, the truck will start and run for most of the day, but then the battery goes dead. A neighbour advised me that it was probably the alternator not charging the battery, so I bought a new one and bolted it on. The problem is still the same. I don’t know what to do next and don’t have the vehicle registered right now, so I can’t drive it to the local garage. Can you help me out?— Jeff

Answer: While modern vehicles use computer control to operate the starting and charging systems, your vintage truck has a very simple electrical system. Because the truck starts and runs fine when the battery is charged, it is safe to assume that the battery, starter and electrical cable connections to them are OK. That leaves just the charging system, and you have already changed the alternator.

There are three wires connected to the alternator.

One is a large-diameter cable that runs from the large battery post on the back of the alternator to the positive post of the battery. You can test this wire by using a test light at the alternator connection.

It should have 12 volts and light up the test light all the time.

The other two wires are smaller and clip on to two terminals on the back side of the alternator. One of the wires will be either blue, red or orange in colour.

This is a power feed to the alternator and power comes to it from the ignition switch when the ignition is turned on. This is the same electrical circuit that feeds the ballast resistors on the firewall that are used for power to the ignition and to the power terminal on the charging system voltage regulator, which is bolted to either the firewall or the inner fender of your truck. It is a small electronic module about the size of a package of cigarettes and has two wires running to it.

Because the truck runs, there must be power to the ballast resistor block and there should be 12 volts to this wire on the alternator and the power terminal of the voltage regulator as well. Test the wire connections with a test light.

If it lights when the ignition is on, all is good.

If it doesn’t turn the test light on, then you could look for a broken wire in the wiring harness to the alternator, but the easiest repair is to run another wire from the power wire at the ballast resistor to the back of the alternator and tape it to the existing harness.

The third wire on the alternator is typically green in colour. This wire goes from the alternator terminal back to the voltage regulator. The regulator controls the charging output of the alternator by varying the ground connection on this green wire, which varies the current through the rotor of the alternator. The rotor is simply a big electromagnet that spins inside the alternator and is turned by the engine drive belt. By varying the strength of the electromagnet, the charging output is controlled.

You can momentarily ground this green wire for a second or two and the alternator will put out full voltage of 14 volts or higher. You can measure this with a voltmeter or simply turn on the headlights and watch them brighten when the wire is grounded and the voltage goes up.

If the alternator charges when the green wire is grounded at the regulator connector, then the regulator is faulty, but before replacing it, clean any rust from the regulator mounting. The regulator must ground to the body to work properly and if the mounting is corroded, then that could be the cause of your problem.


Question: When changing from winter to summer tires, I looked at my brake pads on my car. How do I know when it is time to change the pads? — Sheldon 

Answer: When the brake pads are worn to the thickness of a quarter, you need to change them right away. Otherwise, they may wear all the way through and damage the rotors. You can change them sooner and I would recommend that so you don’t have to inspect them often. City driving can wear the brakes very quickly.