VEHICLE: 1991 Ford F150 pickup truck

DRIVETRAIN: 5.0L engine with AOD transmission

MILEAGE: 142,453 miles

CONCERN: Unit cuts off after driving a short distance and won’t start for a while.

Under the Shade Tree

I don’t like doing shade tree work, but this “quick look” at a friend’s vehicle got its hooks into me, and I wasn’t about to stop until I had figured it out.

 Richard McCuistian

Sometimes I stop to help stranded motorists. There aren’t many serial killers and axe murderers in rural Alabama, so it’s still pretty safe. More than once I’ve seen vehicles smothered by a choke that never opened because of an inoperative choke heater. Those aren’t hard to get going.

If a vehicle is overheated, it’s time to pull out the cell phone and call a relative or a friend for the distressed motorist. Usually the engine is toast in those cases, and there’ll be no fixing the car beside the highway. Once in Savannah, I saw a dead battery on a Lincoln because the alternator couldn’t get juice to it. Someone had disconnected a wire from the starter relay.

For somebody who enjoys troubleshooting as much as I do, those dead-in-the-water vehicles are fun to toy with. This month’s story is about a dead and neglected F150 that had been put in mothballs because, in the owner’s eyes, it had become untrustworthy.

The F150 looked healthy enough, but was already showing signs of entropy: It was sitting near the corner of my friend’s yard with one half-flat tire. His wife Shelia had been using it as a work truck, hauling equipment and supplies back and forth on their farm.

“When I try to drive it to the chicken houses, it just quits on me,” Shelia explained. “I haven’t driven it in a couple months. The battery’s probably dead now.”

She and her husband had already purchased another truck to replace the high-mileage F150. Aside from the stranded motorist jobs – which usually they take about 60 seconds to solve – I don’t make a habit of working on cars on my off time. However, this man and his wife are good friends of mine, so I decided to take a quick look.  I really didn’t expect to find the problem with the few tools I carry with me. But I also thought that I might buy the truck if the money was right, and I wanted to get a handle on what might be wrong with it.

This was to be shade-tree mechanic work at its best. Shade-tree work is challenging, and I’ve never been particularly fond of it. Having worked for so many years in the field, I’m accustomed to having the right stuff to do the job. But in this case, I tackled the problem armed only with my humble years of experience, a digital multimeter, a logic probe and a few hand tools. I have some shop manual information stored on my laptop, but nothing for a vehicle this old.

Under the hood: start and die

Just about any car or truck today has a tendency to drain the juice from the battery if it sits a few weeks because of the keep-alive power feeding the ever-increasing number of electronic gadgets on these vehicles. This unit was no exception. The battery was nearly new, but it was too dead to even illuminate the warning indicators when the key was switched on.

With the jumper cables attached, the truck started easily, and it actually ran well for about 90 seconds. After the engine stalled, I could hear a vigorous buzzing over near the master cylinder where the Evaporative Emission Control (EEC) power and fuel pump relays are located. A closer examination on that side of the engine compartment indicated that the fuel pump relay was the buzzer. The EEC relay was solidly latched. Investigating the relay terminals, I found good, solid EEC power to the hot side of the relay coil and good secondary circuit power. But while checking the control signal to the relay coil with my logic probe, I found that the Powertrain Control Module (PCM) was firing a rapidly oscillating ground into the control side of the fuel pump relay coil – from pin 22 on the 60-pin PCM.

To ground, or not to ground?

We all know poor or disconnected grounds can cause these computerized systems to do some weird stuff. Furthermore, it feels rotten to change parts and then find that the ground was the problem all along. We’ve all fallen in that trap at least once.

Loose ground

You don’t have to be a master electrical technician to know that a ground connection like this doesn’t get the job done. Because this ground is so important to the PCM, I fixed it first, but to no avail.

I immediately checked the battery-to-body ground and found it loose and poorly attached. The stud is like a bolt that has an 8mm hex and secures two grounds to the body. A 6mm nut connects the battery-to-body ground to the stud protruding from the top of the 8mm hex. The star-washer-style wire lug had become distorted to the point that it was dancing around on the 8mm hex part of the stud, instead of being trapped between the nut and the hex.

When I pulled on the ground wire, the buzzing would stop, then change in its pitch and frequency. Screwing the nut off the stud, then screwing the stud out of the radiator support with the 8mm hex, I scratched the radiator support metal clean of paint around the threaded hole, polished the terminals, and put all three ground wires under the 8mm hex for a good solid ground connection. Well, the engine started and ran for about five seconds before it stalled again, and the buzzing continued unabated. Because the battery was still as dead as a hammer, I could perform some tests with my jumper cables to determine if there was a ground problem somewhere else.

I had seen noisy battery-to-engine grounds cause strange starting problems on EEC IV systems. In these cases, sometimes the starter will spin just fine, but the engine will start hard unless the Spark Output (SPOUT) connector is removed. In the days when we had an operational Service Bay Diagnostic System at the Ford dealer, I could do a 60-pin recording of all the grounds while spinning the engine and find one or more scratchy ground signals at the PCM. Cleaning the battery-to-block ground generally took care of that. In the old days, a slightly scratchy ground didn’t cause the problem it does now.

The sensitivity of a vehicle’s computer to impure ground signals can be downright annoying. This 60-pin Ford PCM has major grounds at pins 20, 40, 60 and 16, which is the ignition ground that is fed to the PCM through the harness. It originates at the Thick Film Ignition (TFI) module mounting screw. Pin 49 once was a ground input, but in 1991 it had become a signal return output to the O2 sensors. I disconnected the PCM and checked the ground at these pins and the power to pins 37 and 57. With my logic probe and multimeter, the grounds and powers all appeared to be fine, but a low impedance test light actually works better for this type of test.

It doesn’t take much current on an unloaded circuit to illuminate the LED in a logic probe or fire up some decent looking numbers on a high impedance meter. The test light, on the other hand, requires some current flow to heat up the bulb, and if the light won’t burn, you can bet the computer can’t do much work with the ground or power being checked. I’ve actually been led astray on pinpoint tests by decent looking numbers on a multimeter, only to find that the circuit in question wouldn’t even burn a low impedance light bulb. Enough about that. This time around, with no trusty test light in my holster, I assumed the powers and grounds were fine.

On this truck, however, in order to check the integrity of the main engine ground, I connected my negative jumper cable directly to the engine block because the battery was dead. The relay buzzed just as aggressively, so I wasn’t about to lie in the dirt and fiddle with the block ground.

What’s buzzing?

Disconnecting the fuel pump relay connector, I discovered that there were other components buzzing as well: most notably, the Thermactor Air Bypass solenoid and the ignition coil.

TAB and TAD Solenoids

Here are the air control solenoids: The TAD solenoid was buzzing along with the fuel pump relay and the ignition coil, but the TAB solenoid was silent.

Disconnecting the coil wire from the distributor cap, I found that the ignition coil was firing with a strong and steady buzz. All that lightning was fun to watch, but useless to an ignition system that needs a carefully timed spark to fire an equally carefully delivered fuel mixture. Glancing at the tachometer in the instrument cluster, I saw the needle reading about 800 rpm with the key switched on and the engine dead.

This was getting weird, especially because the PCM doesn’t directly tell the TFI module to fire the coil. The PCM does modify the spark signal via the SPOUT circuit, but the TFI module doesn’t depend on the SPOUT circuit to fire the coil. However, if the SPOUT circuit is shorted to ground or power, it can kill the ignition system.

I disconnected the SPOUT connector, thinking the computer might be sending some kind of renegade signal to the ignition module through that circuit, but to no avail. The coil kept up its infernal buzzing.

Terminal ignition

Disconnecting the six-way TFI module connector, I removed the red wedge from the connector and unlatched the Profile Ignition Pickup (PIP) signal wire, which happens to be the uppermost wire in the connector. The PIP wire carries crank speed and position  to the PCM as a square wave, and it would have to be the circuit that was causing the TFI module to fire the coil. Ordinarily, the distributor stator produces this signal with its Hall-effect hardware and delivers it to the TFI module within the confines of the distributor. The TFI module will, however, fire the coil regardless of whether the signal is fed into the module from outside the distributor through the PIP wire or directly from the stator output circuit in the distributor.

91 Ign

Notice there are three wires leading from the TFI module to the PCM. The Profile Ignition Pickup (leading to PCM pin 56) was the path taken by the trigger signal that was causing the module to fire the coil. The wire leading to pin 16 carries a ground path that originates at the top bolt securing the TFI module to the distributor.

The lowermost two wires are ground and coil trigger (TACH) respectively. The next two are start and run circuits, and the one right below the PIP wire is SPOUT. I was checking all these circuits from memory, and it had been a couple of years since I had tackled a TFI system.

TFI Layout

At any rate, with the PIP wire removed, the TFI module immediately stopped firing the ignition coil, but the Thermactor Air Bypass (TAB) solenoid was still chattering away. The renegade signals were all coming from the PCM and on three fairly unrelated circuits, but why? There were other PCM outputs that weren’t chattering, and I was troubled by this as well. The problem appeared to be within the PCM itself.

Disconnected PIP

Deliberately disconnecting the PIP signal wire from the TFI module, I found that the coil immediately stopped firing.

The heart of the problem

This was getting uncomfortable. I didn’t have a spare PCM in my back pocket, and we’ve all plugged in our test unit or a new PCM to find that the problem was a circuit we didn’t think to check. I didn’t have a parts department from which to borrow a PCM, and I didn’t have a cache of test PCMs.

Furthermore, plugging a known-good PCM into a truck that might have a nasty problem is extremely unwise. A bad vehicle can fry a good part. Have you ever been there? I have.

I thought, if I had another F150 of this vintage… Suddenly, the answer presented itself: There was another 1991 F150 just up the road that belonged to my friend’s father. I removed the PCM from the crippled truck and drove up the road for a quick A-B-A swap. When I plugged the suspect PCM into the good truck, the good truck immediately started buzzing the same three components. The mystery was conclusively solved.

For a guy like me who’s used to working with breakout boxes, scan tools, o-scopes and all the rest of it, this was a hair-raising exercise, but having a known-good truck made all the difference.

I called the local parts supplier, who was able to have a PCM delivered that very afternoon. With the 1991 F150 running so good, I left my friend and his wife wondering if they actually needed the replacement truck they bought. They sold the 91 F150 to my uncle and he has been driving it for awhile now.

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