Figure 1: Car view
Figure5 & 5A: Fluke 88 voltmeter taped outside on hood so we
can monitor while driving. Snap-On Vantage in dual voltmeter mode resting
on dash for easy view while driving.
90 Lincoln Town Car
5.0 liter 4spd/at
Very Intermittently-Cranks OK Won't Run
About a year ago the owner of this
Lincoln reported that the car cranked and would not run. They wanted us
to "check it out" during an oil change service but we were unable to duplicate
the condition. The car was running fine for us at that time.
Needless to say, now, approximately
a year later this same car found its way back to us via tow truck. The customer
reported the same symptom-cranks ok won't run.
After the wrecker left we went over to the car and hit the key. Guess what? The car cranked but wouldn't start. As we began our diagnostic, the car started. Now what? Here are the results we accumulated during the "symptom window," or in other words, when the car wouldn't run:
A. Spark ok.
Although the car was running fine
now, we knew this car had an intermittent problem with the fuel system. Since
the car was still running fine, we advised the customer that we would not
be able to continue with our diagnostic until the car was symptomatic again.
The customer decided to take the car.
The customer decided to leave the
car with us so we could spend some time test driving it. We checked for service
bulletins, there was a load to look through as usual, but none that matched
the symptoms. We decided to print the wiring schematic for the fuel circuit
for analysis. Studying the circuit, with test light in hand, yielded some
useful information. On this model, Fig. 4, the fuel pump relay receives
power at all times from a 20-amp fuse down the orange/lt. blue wire, relay
pin 30. This is the power source for the fuel pump when the relay is energized.
The pink/black wire, relay pin 87, carries this voltage to the fuel pump
when the relay is energized. The red/yellow wire, relay pin 86, brings power
to the relay coil providing that the inertia switch is not open and also
providing that the EEC power relay is energized. The light blue/orange wire,
relay pin 85, is grounded by the ECU during: key on two second prime, cranking
or running, and at key off. This grounds the relay's coil which activates
the relay. At first glance it would appear that the fuel pump would run at
all times the key is off since the ECU also grounds pin 85 at key off. Remember
though, that when the key is off the EEC power relay will power down and
prevent power from going to the power side of the relay coil, pin 86. This
prevents the fuel pump from running at key off. On a hunch, we decided to
check the voltage drop across the contacts of the fuel pump relay by connecting
a voltmeter between terminals 30 and 87. With the engine running the voltmeter
showed 0.00 volts so we knew the contacts were fine.
At this point we knew the ECU was
grounding the fuel pump relay's coil but there was no voltage out the relay
to the fuel pump. Before condemning the fuel pump relay we wanted to make
sure the coil side of the relay was getting power at pin 86 and that
the constant power was still available at pin 30. Both were fine and we still
had no power at pin 87. This proved the fuel pump relay was faulty. It's
a good thing we had a long enough "symptom window." If this car had started
during testing, we would have been back to road testing again.
Mark Giammalvo M.A.T., S.A.E., L1
Glenn Giammalvo M.A.T., L1