A scan tool complements rather than replaces a traditional voltmeter in a successful automotive service department. The voltmeter always has been and will continue to be the most-essential electrical measurement tool for professional technicians.
Undoubtedly, a scan tool is an extremely powerful and necessary diagnostic device.
But when all is said and done, a scanner is only one piece of a successful service center's entire diagnostic arsenal. Time and time again, I have watched an over-reliance on scan tool tests backfire on service personnel (no pun intended).
This over-reliance may prevent the service department from meeting their customer's expectations. Clues of failed expectations include an inconclusive diagnosis, a total misdiagnosis, wasted time and needless parts replacement.
OK, scan tools provide a wealth of data and provide it quickly. But years of shop experience have taught me — among others — that these data may be just the beginning of a potentially lengthy, painstaking troubleshooting process.
This seemingly slow process does fix the vehicle correctly the first time. And as I have emphasized repeatedly in previous columns, fixed right the first time is key to long-term success for any auto service provider.
In previous columns, I also have stressed the need to take control and maintain control of the service transaction.
An essential element of this control is recognizing that an accurate diagnosis on a modern vehicle may require a set of methodical tests — above and beyond a quick glance at a scan tool display.
Before I proceed, please remember that many technical topics are only as difficult as you choose to make them.
With that in mind, let's review an extremely simplistic but effective example of a scan-tool check complemented by a voltmeter test.
The throttle sensor, which first appeared on computer-controlled engines years ago, reports throttle position to the computer.
For instance, a common format is that the engine computer feeds a 5.00-volt "reference" into the throttle sensor (among other sensors.)
Then the throttle sensor varies this reference voltage according to engine conditions and reports it back to the engine computer. The voltage it reports back to the computer is commonly called a sensor output signal.
For your information, the throttle sensor usually reports an output signal of approximately 0.50 volts at idle and 3.60 to 4.30 volts at wide-open throttle.
Next, imagine that the check-engine light on a customer's car is illuminated. A technician connects a scan tool to this car; the scanner reveals a trouble code for an abnormally low throttle-sensor signal. The signal could be abnormally low for one of several reasons.
For example, a damaged wire or loose connection could be limiting the reference voltage going into the throttle sensor. What's more, the throttle sensor itself could be failing.
Or, a damaged wire or loose connection could be restricting the voltage coming out of the throttle sensor and heading back to the engine computer.
Suppose that there's a bad connection or damaged wire on either the "reference voltage" side or the "reported signal" side of this throttle-sensor wiring.
If so, a scanner cannot confirm and pinpoint the actual problem. Instead, a competent tech must trace the wires going from the throttle sensor to the computer, measuring voltage along the way.
An abrupt, abnormal change in voltage indicates a damaged wire or bad connection. There are some shortcuts a tech can take.
But this essential circuit-tracing exercise still consumes a certain amount of time — especially when a vehicle's components are difficult to reach.
The bottom line is that owners, managers and service sales associates cannot and must not assume that a scanner can do unlimited testing through a vehicle.
Instead, a competent tech may have to finish a diagnosis with a voltmeter, oscilloscope or other troubleshooting instrument. Sometimes, there's simply no substitute for these highly focused measurements.