My local garage ran some tests on my 93 and came up with: code 28, quad driver fault #3 and code 26 high resolution pulse failure (recommend distributor replacement) that was replaced 5 yrs ago with a new GM part. I am not so sure about replacing the ECM and the distributor, huge expense any suggestions? Dan

 

You will get a quad driver fault if one of the outputs is shorted to ground or drawing excess current. You need to determine which quad driver is showing the fault (I guess #3 from your post) and then check out the relays/solenoids/wiring associated with that quad driver.

The code 26 could be a bad connection or wiring problem. Check all that out before replacing the ECM.

If you can find someone with an ECM like yours who is willing to swap it for a test then that is a good test.

 

Yes I have a quad driver fault #3, where can I find out what the circuits are for this and its related parts? THX very much for your help. Dan

 

Although QDM failures can be in the ECM, it is not likely. Here is the circuit description for QDM#3.
CIRCUIT DESCRIPTION
The ECM controls several components through the use of a "Quad-Driver" module integrated into the ECM. Most devices, such as solenoids or relays, receive battery power through a fused circuit to the ignition, and turn "ON", when the circuit is completed to ground. The "Quad-Driver" consists of several electronic switching circuits that provide the ground connection to complete these circuits and turn the devices "ON".

Each "Quad-Driver" has a fault line that is monitored by the ECM. When the QDM switch for a particular circuit is "OPEN", the voltage at the "sense line" will be high (near battery voltage), and when the device is turned "ON" (circuit completed to ground), the voltage at the "sense line" will drop to zero or near zero volts. When a device is commanded "OFF", the ECM expects the voltage in that "sense line" to be high, and when a device is commanded "ON", the ECM expects the voltage in that "sense line" to go low. The QDM fault detection circuit monitors the voltage in the sense line for each device controlled by the QDM. The ECM will monitor the voltage at the coolant fan and A/C clutch sense lines only when an A/C request signal is present. If the QDM fault detection circuit "sees" a voltage other than what it expects for a device in a particular "ON" or "OFF" state, the fault line status will change, and a Code 28 will set. The fault line signal status can be displayed on a "Scan" tool.

Code 28 will set if the ECM detects the wrong voltage potential in the QDM #3 fault line for 25 seconds. This code does not turn on the "Malfunction Indicator Light" (MIL, also known as the "Service Engine Soon" or SES light.)
QDM#3 circuit contains the fan relays, the a/c clutch, and associated wiring . If you need the diagram I can provide that.

 

Black bear Bob THX I would like the diagram, my problem could be the low temp fan switch (195-180) I installed with the Hypertech Lev II power chip. Again thanks for your help. Dan

 

Look no further for the code 28, it IS caused by the low temp fan switch. I had the same issue when I had one on my 93, once I had the chip flashed to lower fan temps I removed the switch and the code 28 went with it. Code 28 will not illuminate the MIL, but will be set when using a low temp switch. What I find aggravating is that switch suppliers do not tell you this will occur leaving buyers to diagnose the code 28 0n their own.

 

My orig factory switch is still in place, can I just reconnect to it? THX Dan