The advantages of having the right tools for the job can make the difference between getting it done quickly and beating your head against the wall. One tool that can make your job a lot easier is a scope or graphing meter (figure 1).
Almost every job that comes through your door today will lead to some electrical diagnosis. These diagnoses can include checking the fluid level and condition, checking for codes, and looking for problems in the data stream. If you don’t find anything substantial with these checks, it usually leads to probing wires and checking for signals.
The most important things to know when checking for these signals are where to probe and what the signals are supposed to do. In most cases, that information will be available in your shop wiring diagrams. If you happen to be using an automotive-specific graphing meter or scope, such as Snap-on’s Vantage or Verus, you may have that information built into the meter (figure 2).
Here’s an example where a graphing meter made a difference in diagnostic time: The vehicle was a 2008 Ford F250 with a freshly rebuilt 5R110W. The transmission would intermittently lock the tires while driving down the highway, with no codes in memory. Of course, no one told the technician that the problem was there before the rebuild!
So the technician connected a scan tool and monitored the data during a road test. Turns out the transmission would bind-up in 6th gear and showed no abnormal solenoid commands when it occurred. To cause a bind-up in 6th, the coast clutch, intermediate clutch, or low reverse clutch would have to come on. If the low reverse were the problem, the bind-up would have been more random, and occur in any gear, so the technician ruled that out and monitored the coast and intermediate clutches.
He used the Vantage database to identify the connector pins (figure 3) and backprobed the solenoid command wires. A set of 6’ extension leads (figure 4) made it easy to read the meter from inside the vehicle during the test drive. It wasn’t long before the technician saw the intermediate clutch solenoid commanded on in 6th, causing the bind-up.
Since the problem was completely intermittent — cold, hot, might happen once a week or maybe several times a day — his first thought was a wiring harness or an internal computer issue. A visual inspection of the harness revealed no serious damage between transmission and the computer.
At this point, the technician disconnected the battery and wiring harness from the transmission and the computer. This should have isolated the individual circuits: While probing from one solenoid command wire to the others at the transmission end, there should have been no continuity.
A wiggle test revealed otherwise: the intermediate and overdrive solenoid command wires would intermittently develop continuity when wiggling them where the loom passed through a bracket and rested against an air conditioning line.
There was only a small amount of scuffing on the loom, and no holes. But the wires inside had their insulation rubbed off, allowing contact between the overdrive solenoid and the intermediate clutch solenoid command circuits.
When the computer energized the overdrive solenoid, it would feed power to the intermediate solenoid wire, and the intermediate clutches would come on. This would cause the transmission to bind, locking the wheels.
One of the main benefits of a graphing meter over a regular meter is that it allows you to look for changes in the electrical signal. This is particularly useful if you’re trying to find an intermittent problem. Sure, the MinMax or Record features that many DMMs share can provide a similar capability, but the graphing meter or scope provide a visual history, both for your diagnosis and to show the customer.
Intermittent electrical problems can be some of the hardest to diagnose, but the right tools can make your life a lot easier.