How Bidirectional Computer Controls Affect Your Diagnostic Strategy
for the rest, you need to make sure You connect your scan tool and check is wrong.
the scan tool you’re using provides as the Vehicle Speed Sensor. There are no The trick here is know that there’s
much access as possible. codes, and the VSS matches the speed- more than one computer, and determin-
Those cheapie OBD-II scan tools ometer reading almost precisely. No ing which computer is actually provid-
they sell at consumer outlets for just problem there, right? ing the system control for the problem
a few hundred dollars? Forget about Maybe. Maybe not. you’re working on. If it’s a shift prob-
them. They’ll be all but useless to you. The problem with this type of strat- lem, check the TCM or GEM against
So which one should you use? egy is you’re looking at the primary the PCM or ECM. A difference in the
First, see if the scan tool manufacturer computer signal; usually the PCM. But signals indicates you’ve found a prob-
also builds a scan tool for the OEM. that signal may be a constructed signal, lem.
If they do, they’ll probably provide not the actual sensor signal. And it may And don’t forget, just because
greater access to that manufacturer’s not even be the signal the computer is there’s a vehicle problem doesn’t mean
computer system in their general pur- using to control the system… in this the system will set a code. For example,
pose scan tools. case, shift the transmission. in this car, either of the two speed sig-
It’s always a good idea to “try This car also had a transmission nals were possible, based on the other
before you buy” when it comes to scan computer. And the transmission com- signals to the computers. Neither was
tools. Ask the tool company to let you puter doesn’t use the VSS to shift the far enough out of range to set a code.
give the scan tool a shakedown cruise transmission; it monitors the wheel But the difference indicated something
before writing a check. And then give speed sensors to measure vehicle speed. was wrong, and should be an easy clue
it one. Make sure you can get the data A check on that signal indicated the car to where to look for the problem.
you want from each of the computers in was moving half its actual speed. No
today’s cars. wonder the shifts were late.
Circuit Diagnosis: You’ll
Don’t sign that check until you’re Actually, this use of multiple com- Never Break Out Again
sure you’re getting the best access for puters and differing signal sources can One of the most basic diagnos-
your buck. be a valuable diagnostic tool, if you tic tools of just a few years ago was
know to look for it. Suddenly you the breakout box. You connected the
Diagnostic Data:
have access to comparison data. One breakout box to the computer and sud-
Don’t Trust the Obvious computer shows one speed, another denly you had access to every computer
The car comes in with a late shift. shows a different speed. Obviously one circuit, all from one location. You could
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Ask for Shayne Tobaben or Bruce Lonneman
34 GEARS January/February 2010
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