With this 90 Ford Escort, the question was, "who's fooling who?" We finally found the fibbing component.
|90 Ford Escort 3dr
1.9 Liter 5sp/MT
Bucks heavily under moderate to full acceleration accompanied by a blinking MIL lamp.
Anyone who repairs today's modern vehicles will surely agree that
automotive driveability diagnostics can be both challenging and aggravating
but usually for us technicians it's a labor of love. This time the vehicle
in the shop was a 90 Ford Escort. The vehicle is owned by one of our employees
who works at our shop as an automobile reconditioner & detailer. The employee
was complaining that suddenly the Escort would buck violently when accelerated
moderately and that during the bucking the check engine lamp would flicker
on and off.
Our Escort's Powertrain Control Module (PCM) operates a central
fuel injector (CFI) and Fords well known Thick Film Integrated (TFI) ignition
system, to control fuel and spark delivery. A Manifold Absolute Pressure
(MAP) sensor and an Air Charge Temperature (ACT) sensor are used to monitor
engine air requirements and a Throttle Position Sensor (TPS) is used to report
throttle position. A Coolant Temperature Sensor (CTS) is used to determine
engine temperature and a heated O2 sensor is used to control fuel trim. Onboard
diagnostics is limited to a self test function that lists and stores three
types of trouble codes. Key on/engine off (KOEO) codes, key on/engine running
(KOER) codes, and continuous memory codes.
We all know how secretive the manufactures are about computer strategy.
Ever notice that there's always someone out there in our industry that has
that little piece of "secret strategy" information that you could use? Today
it was the tec-line specialist that was telling secrets. As soon as we gave
him our sensor data and explained the "mysterious" lean command, Jerry, our
Ford Specialist at Diagnostic Hotline, was pretty sure he knew the solution.
Alas! We had been chasing a wild goose! Jerry stated that a new idle control
motor would cure our bucking problem. We really didn't believe it at first.
How can an idle control motor cause a bucking problem? We've replaced these
idle control motors before for the "hunting idle" problem but we've never
had one cause a driveability problem. For those of you who don't know,
the idle control motor used on this model is fairly common. It is simply a
reversible motor that moves an idle control shaft out to push against the
throttle lever to raise idle. When the PCM reverses the motor power and ground
the control shaft is retracted therefore reducing the idle speed. In addition,
the motor incorporates an "idle tracking switch" or " nose switch". Basically
the idle motor has an internal switch that senses when the throttle lever
is resting on the motors shaft. When the throttle is closed the throttle lever
pushes against the idle motors control shaft. Inside the motor this moves
a switch which sends about 10.5 Volts down the motors LG/W wire to the PCM.
When the PCM sees voltage here it knows the car is at idle. When the throttle
is pressed, the switch opens and the PCM sees 0V on the LG/W wire indicating
throttle not closed. Well, we know this information and still fail to see
how this relates to our problem. Listen carefully.....here comes the secret.
Jerry went on to explain that when the PCM sees voltage on the LG/W wire,
it assumes the car is at idle, and alters the fuel delivery program and goes
to "lean idle strategy mode". Basically, Jerry was telling us that
the idle switch is probably stuck in the idle position at all times. In essence
the idle switch is lying to the PCM telling it that: "were at idle...go to
lean idle strategy." The vehicle is not designed to be driven when the car
is running on the "lean idle strategy" in the first place. That's what happens
when the idle switch lies to the PCM. That explains the lean injector
command regardless of all the sensors that were reporting a load. The PCM
in effect ignores all other sensors when the idle control switch is reporting
closed throttle. We confirmed 10.5 Volts at all times on the LG/W wire as
Jerry had suspected and advised our employee that we would have to replace
his idle control motor after all.
Shame On You
About three months after this repair, the Escort was back in the
shop again. This time our employee was complaining of an intermittent hesitation
on acceleration. We were only able to duplicate the condition once. We wondered
what demons possessed this car now. The car had code 22 set KOEO indicating
MAP sensor out of range. A quick test with the DVOM on Hz showed 159 Hz KOEO
and 108 Hz at idle. A test of the MAP sensor's wave form on the Fluke 97 also
revealed a healthy sensor. Now what? On a hunch we moved the lab scope over
to the TPS signal wire. Bingo! Sometimes, during the throttle sweep, the
TPS would drop low while the throttle was actually 70% to 80% open! A new
TPS switch cured the problem. But why did the PCM have a MAP code stored?
Why not a TPS code? We theorized that when the PCM saw the faulty low TPS
signal during the actual open throttle it set the MAP code. We think the PCM
actually believed the TPS sensor over the MAP sensor. Normally when you are
applying heavy throttle the PCM sees high voltage on the TPS wire and high
Hz (low vacuum) on the MAP wire. When the TPS lied the PCM saw low TPS voltage
combined with high Hz from the MAP sensor. Evidently the PCM believed the
TPS instead and thought "the throttle is closing therefore MAP Hz should have
been lower than the reported value". At that point the PCM must have flagged
the MAP code. Evidently when this condition occurs the PCM believes the MAP
sensor is the faulty component. How's that for a tough driveability problem!
Don't always believe the code you see as gospel truth. I'll show this article
to the next person that tells me "cars must be easy to fix now, the computer
actually tells you what's wrong." If a less experienced technician had put
more faith in the computers MAP code, rather than in his own diagnostic of
the circuit, he may have replaced the MAP sensor to no avail.
Mark Giammalvo MAT, SAE, L1