GM vehicles.† Quad driver fault;
is the computer really the cure?

By Roy, BAT Auto Technical

Probably not but, if the vehicle is early gm and has a constant check engine light with a quad driver fault code set the computer (ECM) may have became permanently damaged. So if the ECM is replaced without any diagnosis to locate and repair the cause of the fault then the new ECM may soon become permanently damaged as well. On newer vehicles with an ECM that has protected quad drivers replacing the ECM is most often not necessary.

There are many different code charts out with different code definitions. Some are quite vague while others seem over complicated. Some simply state, quad driver fault, while others will state, quad driver circuit fault, sense voltage low when should show high. Or sense voltage high when should show low. No mater how the description reads when a quad driver fault is set, then quad driver circuit testing is imperative before an ECM is replaced.

First letís look at the ECM and its quad drivers. Most ECM controlled devices are either solenoids or relays. Typically, solenoids are used for a circuit when the current draw is 0.75 amps or less, and relays are used in a circuit when the normal current draw is expected to exceed 0.75 amps. GM uses quad driver circuits inside the ECM to control the outputs to these devices.† So quad drivers (QD) are integrated circuits inside the ECM that it uses to close or open the circuit path to ground, turning the device on or off. Depending on the make and model year GM vehicles use different variations of quad drivers.
The following is information I have on some vehicles.

ENGINE

FUEL DELIVERY

YEAR

DRIVERS

1.8 liter

TBI

1986

QDR

2.0 liter

TBI

TBI

PFI-turbo

1986

1987-89

1987-89

QDM

QDM

QDM

2.3 liter

PFI

1988-92

QDM

2.5 liter

TBI

TBI

1986

1987-91

QDR

QDM

2.8 liter

PFI

1987-89

QDM

3.0 liter

PFI

PFI

1986

1987-88

QDR

QDRII

3.1/3.3 liter

PFI

1989-92

QDM

3.8 liter

SFI

SFI

SFI ( vin 3)

SFI ( vin C)

SFI

1986

1987

1988

1988

1989-92

QDR

QDRII

QDRII

QDM

QDM

5.0/5.7 liter

TBI

1991-92

QDRII

       

 

Now lets look at testing quad driver devices and their circuits.

As weíve all ready seen, most quad driver circuit faults are due to a short in a device or its circuit. (Wiring/connectors). However at times a device or circuit that is open will set a quad driver fault code.

Before replacing an ECM, always isolate the problem device or circuit. Replace the defective device or repair the circuit, and retest.

Each quad driver can control up to four devices.† So diagrams will have to be used to find which devices the quad driver at fault is controlling.

Below is a typical diagram of QDM circuits. If this diagram was for quad driver B and that was the code that was set, we now know that this QDM controls, the TCC sol. Purge control sol. EGR control sol, and the check engine lamp.

A real diagram would list all circuit numbers, the wire colors, and connector terminal numbers. The important thing is that each device and circuit is tested, found to be good or repaired as necessary. The proper test on these circuits would be, with key off disconnect all ECM connectors, then with an amp meter set on the 10 amp scale connected to the control side of the circuit at the ECM connector and to ground turn the key on and check that the device turns on.† With the device turned on, the meter should read 0.75 amps or less. Run this test for at least five minutes to make sure the device isnít overheating and shorting.† The TCC solenoid cannot be tested in this manner in a vehicle that has a normally open third gear switch inside the transmission.† A scanner thru an extended road test best checks the TCC circuit.

A normally operating device and its circuit will have at least 20 ohms of resistance and can be checked with an ohmmeter. This will tell if the circuit or device is open however, this will not †place the device in operation. So if the device is only failing due to heat after being in use for a period of time the fault will not be found with a resistance test.

With the proper test equipment itís fairly easy to determine what circuit or device is causing a quad driver fault.

You will need a good digital multimeter and, if this first time buy the best would be one that has a 20 amp fused capability. †The larger amp rating can come in handy when testing other electrical systems of your vehicle.

Also two 10 amp fused jumper wires keeping the in-line fuse as close to the power supply circuit as possible.

The most important thing to have on hand is information! The best would be the factory service manual but these are expensive.† If AutoZone in your location still has a copy of Popular Mechanics automotive repair software (by Alldata) in stock for your vehicle, I feel it would be a good purchase.
†If not, then visit Alldata.com .

And check out their on-line AlldataDIY Single vehicle Internet-based automotive diagnostic and repair information.

Your feedback is GREATLY appreciated!! Please email your comments to:

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