Jim Grant's Tech Tips
'97 GMC Jimmy, Misguiding Compass
have a '97 GMC Jimmy with a Sunroof & Compass/Mileage unit. My compass
is 90 degrees off. When it says East, I'm really going South, etc. I've
tried the adjustment procedures in the shop manual, but they're good for
a few degrees off, not 90. When I pulled the compass/mileage unit down
& rotated it with the Jimmy parked, the compass didn't change. When
I drive, the compass changes, but is 90-off. If the compass is bad, why
does it move when I drive, but doesn't move when I rotate the unit? Is
there a compass sensor somewhere else in the vehicle?
first question would be, has the vehicle been in a collision of some sort?
Why would an accident effect the compass on your vehicle? A vehicle has
an inherent magnetic field, which just so happens to be accounted for
by the compass' electronics. If the vehicle is altered due to a collision
the magnetic field could be altered as well resulting in an offset of
the compass readings on the display. Or your compass is out of calibration.
But not to worry, your compass can be recalibrated. First you need to
find a parking lot large enough for you to drive in complete circles.
With the ignition switch on, press both the US/MET and the ON/OFF or MODE
buttons for about 10 seconds or so. The display will change and read "CAL"
or the letter "C". All depends on which system your vehicle
has. Once "CAL" or "C" is showing in the display all
you have to do is drive your vehicle in a complete circle three consecutive
times. If the systems working the display will change back to normal and
the compass should work as designed.