Buckin’ Bronco II

By Richard MCuistian

1987 Bronco II

2.9L Engine

A4LD Auto Transmission

160,384 miles

Unit skips while driving down the road

It had apparently been everywhere twice and it still bucked and snorted when she dug in her spurs on takeoff.

A Vacation From Teaching and A Week in the Service Bay

            I had two weeks of summer break right in the middle of the summer semester and I decided to spend a week back at the Ford dealer.  I’ve been teaching Automotive Mechanics  and I’m constantly working on ways to remain current in the field.  

    I put a few tools together in a toolbox that was more portable than my rollaway and clocked in first thing Monday morning.  I wanted to help out without stealing labor hours from the two regular driveability guys, so I started out slowly and worked as a consultant and advisor, helping out where I could.  By the third day I was there, the service advisors were throwing tickets at me like I was a regular, but there was plenty of work for all of us, so the other guys didn’t mind.  I saw some interesting stuff I could really sink my teeth into.  

One example was a 1989 Crown Victoria that popped and skipped in a really nasty fashion when the engine was warm.  Plugging the Service Bay Diagnostic System between the TFI module and harness mysteriously repaired the skip (it happens sometimes, why I don’t know), so I couldn’t use the SBDS Power Balance or Spark Duration screens to determine the cause of the problem.  If I had connected the ignition oscilloscope right away, I would have found that the ignition coil polarity was reversed, but as it was, I found out the hard way.  The wires had become unlatched from the connector shell and whoever had reinserted them had transposed the wires.   I found the reversed polarity after an hour or so of doing one thing and then another, but the coil had to be replaced before the Crown Vickie would run right.

Buckin’ Bronco

            Before I tackled the Crown Victoria, I ran afoul of an interesting little Bronco II that had a relatively simple writeup. 

“Unit skips while driving down the road” is a fairly common complaint, but this was no ordinary problem.  The little SUV ran better than I expected on the test drive and it was smooth and responsive on hard acceleration, but when it got warm enough to drop into closed loop fuel control and started using Oxygen Sensor input to adjust the injector pulse, the tailpipe started belching soot and I experienced the bucking and jumping the owner wanted to correct. 

Closed loop fuel control is as old as oxygen sensors (which first appeared in the mid seventies in Europe), and one of the interesting things about a vehicle entering closed loop is that you should never feel it happen. If you do feel it, something is wrong.  And if you punch a Wide Open Throttle (WOT) and feel the system drop into open loop, the engine will come to life again instead of running like a slug.  When you level out to a more sensible throttle angle once more, the engine will generally run well for a second or two until you feel it drop back into closed loop once more, then the sluggish bucking will return. 

Armed with a scan tool, a guy who understands Electronic Fuel Injection can watch the PIDS and determine when Closed Loop takes place. The problem was that Fords didn’t have datastream capabilities in 1987, so I headed back to the shop to rendezvous with my old friend, the Service Bay Diagnostic System.

SBDS

Mental Analysis

            I was thinking about my first course of action on my way back to the shop.  I could check fuel pressure; older fuel pressure regulators fail fairly regularly and cause the pressure to bounce up near 100 psi, but when that happens, a WOT kick won’t clear the problem up.  What I was feeling on my test drive was more likely to be related to the relationship between the PCM and the Oxygen Sensor.

FORD VIP TEST CONNECTOR

Connecting the SBDS to the VIP test connector, I ran a KOEO test (Key On Engine Off) and pulled a code 41 from memory; there were no hard faults.   Next I ran a KOER test (Key On Engine Running) and got another code 41.  This didn’t necessarily mean the oxygen sensor was bad.  There could be an Oxygen Sensor heater circuit problem, a shorted or open signal wire, or, most interesting to me, a bad oxygen sensor ground.

Having a Look at the Graph

            The SBDS machine has a breakout harness that connects between the PCM and its connector, and since I had entered the Bronco II at the beginning of my diagnostic session, the machine knew which pin was what.  Snap-on’s  new DDC unit does basically this same job on most vehicles from 1981 – 1999.  I selected several pins and let the machine build a graph, which is pictured in the first screen photo.  The O2 Signal appears in yellow, with voltage levels on the left and a timeline across the bottom in standard graph style.  Note the odd O2 voltage displayed on the screen.  Instead of nice humps between zero and 1 volt, we have a negative voltage reading of  –1.2  volts raggedly making its way across the bottom of the graph.  I had seen strange figures like this before and they generally indicate a bad ground somewhere.  The body-to-block ground can cause screwball negative O2 readings which will clear up when the alternator is disabled, but unplugging the alternator did nothing to correct this particular reading.  I decided to check the O2 sensor ground.

First Graph

Well-Grounded Inspection

            When the 2.9 Liter engine had been around for awhile, I began noticing a recurring problem, particularly on Rangers and Bronco II units with automatic transmissions.  There is an orange ground wire that leads from Powertrain Control Module (PCM) pin 49 to the back of the passenger side cylinder head.  It should be clamped between the dipstick tube bracket and the head.  The circuit inside the PCM depends very heavily on this ground in order to properly read the Oxygen Sensor Signal, and some technicians would forget to reconnect that ground when reinstalling the transmission.  Black smoke problems and odd O2 sensor voltages were common with ground loose or disconnected. 

Incidentally, the Ford 60-pin PCM connector had five grounds leading to it until 1989.  Pins 20,40, and 60 were connected to the negative battery post or the vehicle body near the PCM.  Pin 16 was a ground from the TFI module mounting plate at the distributor, and finally, Pin 49 was the oxygen sensor ground.  In 1990, when the three-wire Oxygen Sensor was supplanted by a four-wire sensor, PCM pin 49 fed a ground out to the Oxygen Sensor from the signal return circuits inside PCM.  Note: The ground on four-wire sensors was going out to the sensor instead of coming in to the PCM from the back of the cylinder head. Late eighties V8 engines with two O2 sensors actually had two O2 grounds, one to each head.  Older oxygen sensors had just one wire, with the ground provided at the point where the sensor screws into the exhaust.

            What I found on my visual inspection of the Bronco II was an Oxygen Sensor ground wire that had been connected to the wrong spot, which was a bolt on the upper intake (see photo).  There are simply too many gasket and bolt points between this spot and the O2 sensor (which is in the exhaust header pipe) for the PCM to get a good O2 sensor ground reading.  Another ground wire is supposed to go under that particular bolt, but that ground had apparently been attached somewhere else.  A lot of guys will put the orange Oxygen sensor ground wire here to avoid fighting with the confined space at the rear of the head, but it can be accessed fairly easily through the wheel well (see photo), and I took the necessary measures to move the O2 Sensor ground to the right spot.

Wrong Spot

Right Spot

Better Readings, but…

            The SBDS 60-pin graph looked a lot different after I moved the ground, but there were still problems.  The nice switching humps I had been looking for were there now, but the sensor was switching in the wrong voltage range (see photo).

Second Graph

Notice the switch on the yellow O2 sensor trace was switching between –0.2 and +0.3.  The PCM liked the signal a little better, but the engine still ran ragged because the O2 voltage never broke into the rich range above +0.5 volts.   A quick look at the oxygen sensor revealed a fairly new-looking sensor, so I decided to look for further ground problems.

More Ground concerns

 At the negative battery terminal, I saw a nasty universal-style terminal of the type that I have no use for.  Twenty minutes later I had replaced it with a good solder-on terminal and the secondary grounds were securely soldered into the terminal with the battery cable, but the SBDS graph kept looking worse and worse. 

Another KOER test produced yet another lean code and I decided it was time to replace the oxygen sensor.   I’ve seen sensors get out of sync with what’s proper and pull that stunt.  They’ll read within a range that would be acceptable if it were higher on the scale, but the whole pattern of sensor operation will be out of line with what the PCM wants and the engine runs accordingly.

Finally Fixed

The new O2 sensor made the little Bronco run like a new horse.  Pulling up a new 60-pin graph cinched it.  The range was proper and the switching was normal.  This is the pattern that a driveability guy likes to see.

Third Graph

            My final test drive went very smoothly. The Bronco II performed like a Mustang compared to the old nag feeling it had given me when I first drove it and it gave me some satisfaction to know that I had managed to surgically make the repair without changing several hundred dollars worth of hardware.