WARNING about emission testing of a Hummer on a dyno!!!

As someone mentioned, you must not try this with a TT4 truck. It will attempt to drive off.

Also, on the 1996 and later (OBDC II) vehicles, an error code will be set and the "check engine" light will come on. This (the light) will usually cause an inspection failure. The error code is "unknown drive ratio". It results from parts of the trucking spinning and other not.

Picture the truck transmission in gear, the motor spinning, and the speedo reading zero (which it will if you are spinning the fronts only). This confuses the computer control system. (The speedo sensor is on the rear output of the T-case.)

Also, if during the test the operator presses heavily on the brakes, he may exceed the bias of the center diff and cause some of the engine torque to be transmitted to the other axle, causing possible damage to the vehicle.

The only way to correctly do this test is to remove one of the drive shafts and operate the T-case in the Lock position. (Note: this only works for non-TT4 trucks.) In Colorado, they do not allow this type of testing anymore because they (the gov't) cannot certify that it was done correctly. They will allow on-road testing, but currently there are no testing stations certified for this type of testing. Go figure.

The bottom line is... don't allow the Hummer to be dyno-tested unless they have a four-wheel dyno. The truck may get damaged in the process.

Dave Breggin
'95 Diesel Wagon