I was called to a shop for a complaint of a no start on a 2000 Ford Ranger with a 3.0L engine (Figure 1)
.
The vehicle was towed from another shop that just recently replaced a right side head gasket. After the
engine was reassembled it never started again. This is a tough situation for a shop to be in especially if
a car is driven into the shop in the first place. The truck probably ran poorly or had to be towed to the
shop do to a cooling system fault but at least it ran. It would be hard to even collect monies due to a shop
for the time and parts that were put into the vehicle. I would guess the only fair thing would be to default
on all labor costs and just charge a customer for cost on the parts prior to shipping the vehicle out of
the shop. The current problem was a flashing theft light (Figure #2)
and no starter operation. The present shop now involved did not have the ability to work on Ford antitheft systems so I was called in for assistance.
Ford uses a few variations of their PATS (Passive Anti Theft System) and they may store the controller within
the PCM, Instrument Cluster or use a stand alone PATS module such as the one used on this vehicle. There is
an immobilizer chip imbedded in the key that is read by an antenna ring and transceiver assembly which saddles
the ignition lock cylinder. The transceiver sends the key information on this truck directly to the PATS module
which in turn will send a command to the ECM to allow the vehicle to start if all system checks are okay.
When I arrived at the shop I hooked up my Ford IDS and tried to scan the vehicle. The IDS would not communicate
with the vehicle (Figure 3)
indicating an inoperative ECM or a communication network problem. I wanted to
validate the integrity of the ECM so I decided to check the 5 volt ref supply at the TPS. The 5 volt ref is the heartbeat
of the computer and is the quickest way to determine if the ECM is alive or not. I turned the key on and measured
the ref voltage at the TPS. It measured about 11.45 volts (Figure 4)
and was about 6.45 volts elevated above its regulated spec of 5 volts. I next checked the ref ground at the TPS and it too was high at 11.74 volts (Figure 5)
This indicated to me that having both the ref volts and ref grounds elevated high together was do to a ground
failure to the ECM or possibly a problem within the ECM. The ECM must regulate the 5 volt ref based on its ground
level so it is always important to perform voltage drop testing at the ECM when you encounter reference feed problems.
Now I needed to check all the powers and grounds feeding the ECM. The best way to do this is going directly to
the ECM. Checking where grounds reside or even fuses at the panel does not guarantee that power and ground feeds
are reaching their destination. There is always the possibility of an open circuit between the feeds and the ECM.
When I performed a voltage drop test on the grounds at the ECM with just the key on I had a reading of 11.74 volts
at the ECM (Figure 6)
.
This vehicle had problems with the main ECM grounds. The ECM grounds on this car
were located at the left front of the engine compartment on the inner fender well. When I performed a voltage drop test
there I got a reading of less than 100 mV. This vehicle had a wiring problem between the left front fender and the ECM.
I performed a visual inspection of the harness to try to follow the routing of the ECM grounds and discovered a
main engine harness bulk connector the grounds traveled through (Figure 7)
.
Connectors have high failure rates for high resistance and open circuits so I decided to take a closer look at the connector. As I was checking the ECM
ground at the connector I saw something that caught my eye. The ECM connector was 180 degrees out and was forced
together by the center bolt that secured it. This was most likely caused during the reassembly of the engine after
the head gasket was done. Okay so all I had to do was re-index the connector properly and the car will start right?
Wrong? The car would still not start. Now I had good grounds below 100 mV but my ref volts were now at 170 mV.I
must tell you that when ref volts are high you need to concentrate on grounds first but when ref volts are low you
need to check all power feeds at the ECM.
I located the battery and ignition feeds at the ECM and they all checked good but I still was not going to condemn
the ECM yet. Low ref volts could also indicate a partially shorted circuit to ground caused be a bad sensor or a
pinched wire. I pulled a diagram from Mitchell-On-Demand and found that the TPS, MAP and EGR backpressure sensors
shared the same 5 volt ref line. It was not uncommon for the exhaust to work its way back into the EGR backpressure
sensor and damage the sensor resulting in a 5 volt ref line shorting to the ref ground feed internally. I next
disconnected all three sensors and still the ref volts were low. In this business you need to eliminate all
possibilities and leave no rock unturned. I knew that there was prior work done on this vehicle so I had no choice
but to cut the 5 volt ref line at the ECM to see if there was anything else pulling the line to ground such as a
possible pinched 5 volt ref line during the prior cylinder head gasket operation. After cutting the 5 volt line
at the ECM (Figure 8)
I cycled the ignition switch to put the ECM to sleep for at least 1 minute. This
step is important because I have seen a lot of control modules that were unwilling to turn back on the regulated voltage
circuit during the same key cycle when a short was detected and eliminated. When the key was turned back on the 5
volt ref supply was still at 170mV. This ECM took a turn for the worst. It did not survive the improperly indexed
connection. The only resolve here was to replace the ECM.
I really felt bad for the owner of this vehicle who probably drove his truck to the first repair shop for a simple
head gasket problem. It just goes to show you how careful one must be when disassembling and reassembling a vehicle.
You need to pay close attention to what you are doing and not let those around you distract you when you are working
on a vehicle. That even includes not going out that same night drinking after disassembling a vehicle because chances
are that the next day hangover may draw a blank on reassembling the vehicle. Hope this story hits home with some of you
techs out there because I have been there before myself in the early days.