Fault Codes

RON MAYFIELD

MEMBER
JAGUAR

The VCM code is 77, Fuel Failure, on my 1994 XJ12, 100050 miles. 6.0L The manual I have does not cover that code. Can you tell me what it is? There has been a sudden drop in fuel mileage from around 19 city to 13.8 and it won't go up. Car runs and starts great, just passed MD emissions with flying colors. I am considering replacing the air/fuel sensors just ahead of the CATS, they might be original. I just purchased the car about 6 months ago and have no idea of its history. Thanks for any help.

1 Reply

CHEVY22

MECHANIC
1,375 POSTS

The code 77 is related to no rpm signal from ignition, there are a few things this could mean one of the crank sensors going bad or wiring in between them to the pcm, there is one at the front of the engine at tdc if you turn the ignition on and check the red wire for power and have it move to the rear sensor on the flywheel and check the blue wire for power, if no power on either of these wires you will need to find the open in the circuit. The only other thing it could be is a faulty pcm, I cant find ohm spec for the sensors or I would post them, hope its one of the sensors

happy hunting
jim