So, my brothers van has been getting poor gas mileage. His is a '97 AWD and so I figured I could plug in my scan tool to determine if a tune up could help. I plug in my scanner and did not find any codes, for my scanner did report a closed loop fault, a LTFT of -100%, and an upstream o2 sensor voltage of a steady 0v. I'm confused that you could get a 0v sensor output wthout a sensor code. The downstream sensor is doing the same thing. Any ideas on what might be wrong?

 

Don't know what to tell you Bear.

If you find out why, please let us know.

 

blown fuse?

 

do the O2 sensors have 12v to their heaters? powered from fuse link E dk blue 20 ga
primary front O2
gray/lt. blue is sensor signal to PCM, gray/red is signal ground
heater=red from 12v source PCM relay, red/white is line grounded by PCM to turn heater on.

there is a PCM inline fuse in front and below the relay group.

are the O2 wires shorted together?

is the PCM powered up and running?
may be in default drive home fail mode
would be my only guesses as to why no codes.

http://www.motorcraftservice.com/vdi...f/obdsm973.pdf

from Ford Svc cd
Hardware Limited Operation Strategy (HLOS)

This system of special circuitry provides minimal engine operation should the PCM (mainly the Central Processing Unit or EPROM) stop functioning correctly. All modes of Self-Test are not functional at this time. Electronic hardware is in control of the system while in HLOS.


HLOS Allowable Output Functions:

l Spark output controlled directly by the PIP signal.

l Fixed fuel pulse width synchronized with the PIP signal.

l Fuel pump relay energized.

l Idle Speed Control output signal functional.


HLOS Disabled Outputs To Default State:

l EGR solenoids

l No converter lock-up

NOTE:
In HLOS mode Self-Test DTC's will not be output.

 

I think i'll take it to a shop that can do good electrical diagnostics. They should be able to quickly determine what is going on. it just seems to me that if you have no sensor voltage, that the LTFT should be +100% and a code should set, not -100% with no codes.

 

an aerostar with a crazy ecu? thats unpossible!

 

It sounds like both signal output wires are shorted to ground or there is an ECU fault. Another possibility is that both o2 sensors are shot (unlikely). I agree that 0v would send a lean signal to the EEC & + LTFT should be commanded, which if does not correct the fuel ratio this would result in a code. However it also sounds like the system is running in fault mode,
If you pull off the electrical leads does the o2 sensor then read properly with just a DMM connected?( .1-.9v) if so there is a circuit or ECU problem.
You could check the output pin for shorts to ground at the EEC plug with it disconnected. I dont know the pin Nos though!
ATB Aeroman.