I am helping my neighbor across the street with a problem on his 1994 Chevrolet Caprice Classic with 94k miles. His Check Engine light came on the other day while driving home from work. He did the paper clip trick with the car and read the code. Code 26 was stored. Looked online and it says it has a Quad-Driver Error. We know that probably means a new PCM. What we are trying to figure out is what is a quad-driver error.

Hmm... I thought DTC 26 on the LT1 motor was the Evap Canister Purge.

Anywho, to answer your question, a quad-driver is an integrated circuit inside the PCM that sinks current to turn a device on outside of the PCM (kinda like a solid state relay.) Called "Quad" because there are four of them in each IC package.

The PCM turns one of the drivers on, then watches the voltage on the input line. If it doesn't see the voltage drop from the controlled device turning on, it flags the error and stops trying to activate the device to prevent damage to the PCM's internal circuits.

Usually, any driver error mean a short or open circuit on the device being controlled. The way GM wires the cars, positive voltage is provided to one side of the device. The path to ground is provided when the driver in the PCM turns on. So, while the PCM is looking at the wire going to the device, it sees 12VDC when the device is off, and expects to see ground when the device is turned on (circuit complete and the voltage drop provided by the coil within the device being controlled.)

Within the PCM, there are only four types of control signal circuits. Drivers that turn things on & off in the car, Debounce circuits that read on/off switches, A/D converters that read the changing voltage of a sensor, and serial digital data from/to other digital devices. Of the four, the quad drivers are the heavy-hitters in terms of current flow...they turn on analog devices via solenoids.

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Bye Bye 1990 Bonneville LE... Now it belongs to my daughter
In the Garage: 2009 Subaru Outback, 1987 Camaro, 2006 SV650S, 1995 Regal 182 "ASANAGI", 1962 Ford Galaxie 500, 1995 Ford F150 XL 4WD, 1953 Farmall Cub