Code 28 QDM 3
Thread Starter
7th Gear
Joined: Dec 2004
Posts: 7
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Code 28 QDM 3
93LT1 2800 Vigilante -4.10 Rear - headers - Random Cat
Service engine light comes on whenever climbing a hill. Goes away when the engine is stopped and started again, but comes back when climbing hills again. Does not come on when operating normally only when climbing hills.
I checked the plugs and plug wires, they are OK.
Where should I be looking.
Service engine light comes on whenever climbing a hill. Goes away when the engine is stopped and started again, but comes back when climbing hills again. Does not come on when operating normally only when climbing hills.
I checked the plugs and plug wires, they are OK.
Where should I be looking.
Le Mans Master
Joined: Jan 2003
Posts: 8,596
Likes: 19
From: Portland Oregon
Originally Posted by vetter3596
93LT1 2800 Vigilante -4.10 Rear - headers - Random Cat
Service engine light comes on whenever climbing a hill. Goes away when the engine is stopped and started again, but comes back when climbing hills again. Does not come on when operating normally only when climbing hills.
I checked the plugs and plug wires, they are OK.
Where should I be looking.
Service engine light comes on whenever climbing a hill. Goes away when the engine is stopped and started again, but comes back when climbing hills again. Does not come on when operating normally only when climbing hills.
I checked the plugs and plug wires, they are OK.
Where should I be looking.
DTC28 is for the quad driver number 3 circuit. It controls the relays for both cooling fans and the ac clutch relay. Does you cooling fans operatre? That is the first thing to find out, then turn on the ac system and see if the ac clutch pulls in.
Another way to check this is to ground the diagnostic connector, pins A and B this should make the ECM ground the relays to enable the devices. Meaning that both cooling fans should be running and the ac clutch would pull in. If any of the three devices does not operate, then disconnect the relay, using a test light connected across the relay coil connections, and with the diagnostic terminal grounded, the test light should light up. If not make sure that there is 12v on one of the coil terminals of the relay socket, if that is OK then then problem is a defective relay. If the light does not light up and there is 12v on one side of the relay coil connection then the problem is in the ECM.
On both fan relays the 12v terminal of the relay is pin D, on the ac clutch relay it is pin 85.
