i got the same problem here.

Trouble Code 26 on a late 93 LT1

Air Pump works and shuts off after about 2 minutes, soon after i get a SES TC #26 and fan comes on because of SES light.

i get a correct 12V / 14V reading on the EVAP and EGR Solenoid connectors on one pin to ground (Engine Body), also when the Trouble Cods is set.

Resistance on each Solenoid is about 41 Ohms.

As i'm not in the US i don't want to order all solenoids and EGR Valve together, but i guess this is probably the best way to do it as i 1) get an EGR Trouble Code when using cruise control in 6th gear for some time and 2) have high shipping costs per order, so ordering an egr solenoid and egr valve together with the evap solenoid should get me rid of the EGR Trouble Code i get twice a year when going long distances in 6th gear and hopefully rid of the # 26.

what do you think about my situation here, is there a way to further test what part is causing the # 26 here other than the evap solenoid without having to take off a lot of parts in the back at the tank ?

is a 40 Ohm Resistance of the solenoids a clear sign that they are really working properly ? Or could an open Vacuum Line at the evap stuff back at the gas tank also be resonsible for that 26 Trouble Code ?

could the EGR and EVAP Trouble Codes be related and how ?

 

On my '92 I had a few codes, including the quad driver one you have. I also had other random issues. In my case the issue was cold solder joints on the ECM that I had repaired by SIA Electronics in southern Illinois. My car has been running great for about a year since their repair of the ECM.

 

i've only got this code 26 (and that egr code coming on when driving in 6th gear and cruise control).

i guess ecm is working fine.

got all other TCs sorted the last year (ICM Module, Opti) and it's running great besides this #26 code right now.

so i wouldn't bet on the ECM right now, but thanks for the advice.

any other options here to find out whats wrong other than replacing/repairing the ecm ?

 

Not having a Tech 1 Diagnostic Scanner, this is a "Lay Persons" way of testing for the Quad Driver Module 1 DTC26:
I have a 92 LT1. I received the "Service Engine Soon" notification which came up as a DT Code 26. The notification came on consistently "only" after the engine had warmed up to operating temperature.
Using the posted DTC26 Diagnostic procedure, I first checked voltage to engine ground at the PNK/BLK pins of the Evap Canister Purge Solenoid, EGR Solenoid Valve, Air Pump Relay and Air Bypass Solenoid Valve with key on and received a voltage of 12.9DC Volts. While the engine was cold, I checked the resistance of the Evap Canister Purge Solenoid, EGR Solenoid Valve and Air Bypass Solenoid Valve and read between 43.0-51.7 ohms. I drove the car around until the the "Service Engine Soon" came on and rechecked the resistances. I found the resistance on the EGR Solenoid Valve was open (infinity). I removed the valve from the car and re-metered it, and verified infinity. As it cooled down, the resistance popped up to 51 ohms. So I found a heat related failure in the EGR Solenoid Valve.
A few notes about misinformation. It was previously stated in this thread to check the devices by removing them from the circuit and see if the "Service Engine Soon" goes away. I believe this to be a waste of time (at least on a 92 LT1). The ECM is sensing a difference in voltage potential and by removing the device from the circuit, you are creating the maximum difference in voltage potential. I did try this test and it did not turn off the "Service Engine Soon" light. Disconnection of the Air Pump Relay from the circuit in it self will display the "Service Engine Soon".

 

i found the fault:

repair shop forgot a plug, EGR plug on driver side next to wheel, beneath the ASR. i plugged it in, no trouble codes anymore and runs fine.

 

FWIW- Had the code 26 quad driver fault onmy 93LT1 , mechanic told me I needed a reflashed computer .He tested the individual components and they checked OK. Problem was he checked components cold. I drove the car till the idiot light came on then tested the individual components turned out the EGR solenoid was bad.