Where There’s Smoke…

One day I encountered a plain old ‘88 Ranger that morphed into a “fire truck” before my very eyes!

 

The little pickup looked clean and healthy. It was a 1988 Ranger with 126,000 miles on it.  The repair order had the name of a local auto parts chain store on it.  I wanted a clearer picture, so I made a trip to the service writer’s desk:

           “This guy bought an alternator at the store down the street, and replaced it himself.  Then he saw smoke coming out of the center of the dash. He took the alternator back to the parts store and demanded that they pay to have his truck checked, since he believed their alternator had shorted out and burned up some wires somewhere.  The parts store told him to bring the truck to our shop to have it checked and agreed to pay for any repairs we said were caused by any defect in their alternator.”

In The Service Bay

`           The first thing I wanted to have a look at was the wiring under the instrument panel.  Lying in the floor with my pocket flashlight, I spent a few minutes looking for blistered wires.  They’re not always so easy to see in situations like this, but I was hoping to find something I could identify as a problem.  I didn’t on that inspection. 

One thing most mechanics know about smoke jobs like this is that it doesn’t take much wire-burning to make an awful lot of smoke.  A six-inch piece of 14-gauge wire can fill the whole cab on a pickup with smoke if it heats up fast enough.  And while the melted wire could have been running along the top of the big harness under the dash and out of sight, I was having problems understanding how an alternator with only one small circuit leading through the dash panel to the inside the truck could cause the sort of smoke he was talking about. Opening the hood on the Ranger, I found the wires disconnected from what appeared to be the original alternator. 

Just for grins, I hooked up the charging system tester (a MAC BCH 35) plugged the wires in and started the truck.  The alternator wasn’t charging, but it got hot enough to fry an egg almost immediately and began producing a sharp burning smell detectable from eight feet away. 

First Things First

            The first thing we’d need was an alternator. The customer was contacted, and he delivered one from the parts store in question.  Installed, it was cranking out a nice fifty amps but the battery voltage was mysteriously low, about 11.8 volts at idle and 12.3 or so at 2000 rpm. Further checks revealed that the regulator was driving the alternator to full field all the time. 

So far, I had seen no smoke in the cab, and I wondered if there was even a problem in that area. I should have looked under the dash a little closer. 

Meanwhile, the customer, disgruntled because we hadn’t managed to place any blame on the parts store, came by the shop to discuss the situation. I showed him the alternator output and we went over the wiring layout in the manual. The heavy wires which carry the charge to the battery lead from the alternator across the top of the engine, then to a splice in the harness about halfway down the driver side fender. From there the circuit takes the long way around the engine compartment getting back to the battery, but never makes it to the inside of the cab.  The customer was satisfied after looking at the schematic that the alternator couldn’t have burned the wires under the dash.

Only the green indicator lamp wire (which also “turns on” the voltage regulator) leads from the alternator to the inside of the cab, and it wasn’t likely to be the problem, since that circuit was working fine.  The battery light would have been illuminated constantly if it had shorted, and inoperative if the wire had been cut somehow.  Neither was the case.

After the customer left, I took another look around under the dash in the instrument cluster area but saw nothing.  I decided to let the truck run for a little while and keep an eye on it to see what happened.

Fire!

            I sat down in the driver’s seat after the truck ran for a bit and was alarmed to find the cab filling up with a lazy cloud of smoke.   It appeared to be coming from around the radio, just as he had indicated.  Somebody had installed an aftermarket radio, so I quickly removed it, but I saw no sign of burned wires.  With my pocket flashlight, I quickly followed the smoke.  It was coming from the passenger side floorboard, a place where I hadn’t looked previously!  Duh! I must have been fighting brain fog or something when I did my first inspection…

I had switched the truck off, but I saw that the carpet was scorched near the Inertia Switch. (NOTE: The Inertia Switch on Ford vehicles is supposed to deactivate the fuel pump in case of a collision. It has a red or white reset button on top to reactivate the fuel pump). I found that the Inertia Switch had apparently developed resistance and melted its connector at some point in time.  This condition isn’t unusual; many replacement Inertia Switches come with a new connector shell and terminals for this reason.  But in this case, somebody (we may never know who) had discovered that problem, cut the wires with a pocketknife, and wired them together, bypassing the melted Inertia Switch.

(Note: In many cases, the melted Inertia Switch will be a result of resistance in the switch itself.  For that reason, most replacement switches sold at the dealer contain terminals and a connector sleeve.) 

The wires where the switch had been bypassed had become very hot, melting all the insulation off the copper, then started to burn the carpet, which was producing the smoke. 

Initially, I figured that the poor connection at the splice had oxidized, and began to heat the wire along its length like a slow burning fuse as is prone to happen in situations like this.  While I was pondering this scenario, I was alarmed to see smoke coming from under the hood.  By the time I got there, I found that the fuel pump relay had melted to the point of shorting out internally and had just burst into flame.  I grabbed a nearby fire extinguisher and hosed the fire out with the acrid yellow powder.  It was quite a spectacle, and it didn’t go unnoticed by the other guys in the shop.  I set the extinguisher down and yanked the battery cable off about the time the relay burst into flame once more.  This was pretty exciting, and it called for another application of the extinguisher.

 

  After the fire was safely out, I investigated.  At the very least this guy was going to need a couple of relays and connectors (the A/C relay was damaged in the fire), not to mention an Inertia Switch and a section of wire.   He gave the okay for an Inertia Switch, the relays and connectors, and the labor to do the job.  This sort of job should always be done with solder and good quality heat shrink.  In a little while, the work was done and the truck was re-started. 

More Heat

With the engine running, I felt of the wires at the fuel pump relay. The wire leading from the relay to the fuel pump was getting really hot really fast, so I switched the ignition off once more.  Briefly actuating the fuel pump relay, I checked the fuel pump circuit for current draw.  With current flowing and the inductive probe from the alternator tester measuring it, I recorded an alarming load on the fuel pump circuit.  A fuel pump should pull about 8 amps when pumping fuel and only about 1.4 amps when spinning in an empty tank.

I tracked some burned wires in the main harness (see photo) where it passes through the bulkhead to the fuel tank selector switch. The selector switch connector was mildly distorted, but the terminals weren’t oxidized and it was still useable (This Ranger was one of the rare ones with two tanks).

Back under the hood near the master cylinder (see photo), I found the 12-pin connector leading to the fuel pumps. Disconnecting it, I found that side of the connector containing the female pins was melted to the point of no return, at least in my opinion (see photo).

This circuit starts at the fuel pump relay on the other side of the engine compartment, passes through this 12-pin connector inside the cab to the selector switch, then back out through the same12-pin connector, to travel down to the frame mounted high pressure pump at a point under the driver’s feet, then through an electric selector valve on the frame just behind the pump. This selector valve, (controlled by the switch on the dash), switches electrical current and fuel flow from one tank pump to the other.

Disconnecting the 12-pin connector to isolate the circuit going to the pump, I connected a wire to the positive side of the battery and briefly tapped a little voltage into the male terminal in the 12-pin connector. If I detected a short here, it would pinpoint the short to the part of the harness under the truck, since the pin I chose carries current down the frame through the selector valve to the fuel pumps. When I touched power to the terminal, the resulting spark could have ignited a torch.  The wires leading to this connector weren’t melted and this side of the connector shell looked okay, but the high current load was certainly under the truck somewhere.  Disconnecting the frame mounted pump did nothing at all to alleviate the short circuit.  The harness along the frame looked good and wasn’t pinched, so I disconnected the rear tank unit, but the high current load was still there.  Finally, with my helper watching the test light I had connected to the shorted circuit, I unplugged the front fuel tank fuel pump connector and the high current load evaporated.

More Cash Needed

This guy was going to need some more harness repairs and a new pump to replace the faulty one in the amidship fuel tank.  Diagnostic time, labor for replacing the pump and repairing the harness would put his total bill at nearly $800.  As it turned out, his insurance company took care of everything except the fuel pump and the alternator.

While the front in-tank fuel pump wasn’t shorted dead to ground, the 30 amps it was pulling was more than enough to melt wires all along the way without destroying the fusible link protecting the circuit (a fusible link is a special type of wire with a special type of insulation.  It is always a couple of sizes smaller than the circuit it protects).  In many cases, a high current load like this won’t even blow a fuse, but it can melt the plastic housing of the fuse, oxidize the terminals and scorch the fuse panel around the fuse.  It’s likely that this bad fuel pump destroyed his first alternator, and the new, stronger alternator he installed indirectly caused the smoke problem he had noticed. Rather than an alternator problem, however, he had a fuel pump problem that could have burned his whole truck to the ground if I hadn’t been uneasy with the low system voltage.    The 30-amp load was the reason the system voltage had been low. I got a pat on the back from the insurance adjuster for that.

Where there’s smoke, there’s fire, even when it may not seem that way at first.  But in this case, the “match” that started the fire turned out to be on the other end of the vehicle!