Diagnosis with electrical basics

Basic knowledge streamlines electrical repairs

A STEP-BY-STEP APPROACH CAN TAKE THE HEADACHE OUT OF COMPLEX ELECTRICAL SYSTEM DIAGNOSIS

Electrical issues can be some of the most difficult problems to isolate and repair. Given all the electronics on today’s vehicles, electrical issues can be challenging and frustrating for even some of the best technicians.

In this feature, we’ll discuss some issues that can occur and how to employ a diagnostic strategy using a variety of tools to help find the root cause of the problem.

ISSUE 1 — INTERMITTENT NO CRANK, NO START VEHICLE: 2014 JEEP WAGONEER

This Jeep came into the workshop for an intermittent cranking issue. There was no real pattern to the issue but it had been occurring about fortnightly over the past few months. The vehicle had had a battery replacement about two weeks earlier; and the starting issue had occurred since then.

The first step was to test the battery and charging system, using our battery/charging system tester. We found no faults and both appeared to be functioning correctly.

Our next step was to scan the Jeep for codes. There were none, but we noticed that many of the monitors had not completed. Incomplete monitors can occur either by someone clearing codes or by a loss of power to the PCM. We decided to monitor the charging system to determine any signs of an intermittent issue. Using our scan tool, we monitored the generator duty cycle and voltage sense.

In the course of monitoring, we noticed that the voltage would drop to under 12 volts and the duty cycle would drop to zero. We also noted that the voltage would occasionally rise to around 15 volts, slightly higher than normal.

We suspected some high resistance somewhere in the charging system was causing the computer, which controls charging output, to intermittently undercharge and overcharge the system. One of the best ways to test for resistance in the circuit is to perform a voltage drop test using a DMM in its voltage min/max setting.

A voltage drop test is much more reliable than using an ohmmeter because the results will also show the ability of the circuit to deliver enough current. For example, if you were to use an ohmmeter to test a single strand of 16-gauge wire, it would show continuity and zero resistance, just as a length of 16-gauge wire will show continuity and no resistance. But if you tried to pass any kind of load through the single strand of wire, it would most likely burn.

Performing a voltage drop test will show you the amount of voltage that is available at the circuit when current is flowing. A good rule of thumb for passing a drop test is a drop of less than one-half volt for most circuits.

No resistance but a single strand cannot carry any appreciable load

We performed a voltage drop test on the starting and charging systems and found a drop of a little over two volts on the ground side. We noticed some minor corrosion on the ground connections for the battery and body grounds.

We cleaned the ground connections and retested the system, and found the voltage drop readings had returned to normal ranges. After re-scanning, we found the charging system functioning normally. That was it: the customer has reported no further starting issues.

One of the best ways to test for resistance in the circuit is to perform a voltage drop test using a DMM in its voltage min/ max setting

ISSUE 2 — INTERMITTENT HEADLAMP OPERATION VEHICLE: 1999 PETERBILT 379

Intermittent electrical issues can easily be some of the most troublesome problems to diagnose. It can take hours to determine the cause, and many more hours if the vehicle is not experiencing the issue when it’s in your workshop. In this case, the truck had lost power to both headlamps on high and low beam, but the tail and marker lights worked.

Compounding an intermittent issue was the fact that this was an older semi-truck with multiple owners; therefore, many hands had been behind the dash and had made modifications and repairs to the electrical system. A once-organised routing of wires and connections had turned into a rat’s nest of splices, added connections, and wire taps that made identification very difficult. One significant factor in

Using the min/max feature shows the voltage drop on the ground side of the circuit

Rus and corrosion on positive terminal More corrosion on the ground cable

Connect one end of the DMM to the headlamp wire, then touch the other probe to each pin at the engine side of the bulkhead connector. When you hear the tone, you have found the correct wire

locating the correct wires to the headlamps was that many of the wires behind the dash were white and we couldn’t see the numbers on many of them.

Additionally, correct wiring diagrams are difficult to locate because many trucks of that era were built from the factory and sent to motor-body specialists who finalised wiring connections.

TRACING A CIRCUIT WITHOUT A WIRING DIAGRAM

One of the best strategies for locating a lighting issue is to start at the power side of the circuit, then proceed to both ends of the circuit as directed by the results of each test. If you don’t have a wiring diagram, start by using a circuit tester at the fuse panel — or, in this case, the circuit breaker — to determine if there is power at the source.

We connected a power probe and found power at the main fuse that supplies power to the switch. That made sense, because the tail lights were operating normally. We then tested the switch to determine if there was power on all terminals of the switch. This switch has four terminals, which can in some cases lose power on one side of the switch, and that could cause the tail and marker lights to work and the headlamps not to. There was power on both sides of the switch when the switch was in the ‘on’ position.

Our next step was to check at the headlamp for power at the connector. There was none. Using the power probe, we applied power to the headlamp to make sure we did not have an issue with either a ground or a burned-out lamp. The lamps on both sides of the truck illuminated.

So far, we had determined the switch, fuses, and lamps were all good, so we needed to determine where the flow of current was interrupted between the switch and the lamps. There was a bulkhead connector at the firewall, where we have seen corrosion issues on similar vehicles. Again, one of

the problems was that we could not easily identify which wire, or which of the three large multi-wire connectors, was for the headlamps. Since it is somewhat difficult to observe the lamps and test the bulkhead connector at the same time, we either needed two technicians to test this circuit or to use a little trick.

Since it is somewhat difficult to observe the lamps and test the bulkhead connector at the same time, we either needed two technicians to test this circuit or to use a little trick

HOW TO FIND THE RIGHT CIRCUIT

Set your DMM to the audible tone ohms setting. Disconnect the bulkhead connector and the headlamp power wire at the headlamp. Connect one end of the DMM to the headlamp wire, then touch the other probe to each pin at the engine side of the bulkhead connector. When you hear the tone, you have found the correct wire. Keep in mind that many heavy-duty trucks use a separate wire to each headlamp, so you should look to see if there is a pin for each side.

Once we found the correct pin, we could reconnect the bulkhead connector to determine whether there was power to the pins going to the headlamps. Our test showed no power. Using the power probe we were able to determine that when we supplied power to the pins, the headlamps illuminated.

Now that we were able to identify which pins at the bulkhead connector were used, we could trace them back to see where the loss of connection between the switch and the bulkhead was. There were numerous wires behind the dash, and, quite honestly, not much room to get to each connector without completely dismantling the dash.

The wires are mostly white and with numbers missing, tracing can be difficult

Charging system data from AutoEnginuity Scan Tool

Since headlamps use quite a bit of current, we assumed there would be at least one relay, and quite possibly two relays, for the headlamps. We had previously looked for relays at the fuse / circuit breaker panel near the driver kick panel. Even though there were relays located there, none was for the headlamps.

Using the DMM in the ohms mode, we checked for resistance from the bulkhead connector to the switch. The circuit tested as ‘open’, no continuity. Pulling as much of the harness behind the dash as we could, we found a six-way connector from the headlamp switch to another section of a harness. We tested the wires from the switch and had power on both sides of the connector.

After pulling another section of the dash, we were able to track part of the second harness to an added bank of relays. Using the power probe, we determined that there was no power to the battery + side of the relays.

Tracing the wires from the relay to their power source, we were able to find that someone had used a wire nut to connect wires from a positive source to the battery + relay wire. In my opinion — and per best practice in electrical repair or modification — wire nuts or wire taps have no place in a professional repair shop. We located three other connections behind the dash that had wire nuts and two that used a wire tap connector. Our recommendation to the owner of the truck was to repair the system correctly by adding a fused power strip and repairing all the incorrect wiring with proper terminal ends and soldered connections.

After the repair, the lamps all worked correctly and the customer stated that, as a bonus, the static in the radio was gone — an issue he had not complained about.

With the intermittent nature of both these issues, the problem could have been much harder to find had we not started with basic best practice tests and observations that found poor connections and corrosion.

We were able to track part of the second harness to an added bank of relays. Using the power probe, we determined that there was no power to the battery + side of the relays

TODAY’S LESSON

One of the best ways to improve technician performance and avoid wasted effort in any workshop is to provide basic electrical training to technicians. It is a small investment, and for many technicians, it’s their greatest opportunity for improvement.