Fuse 41 Ford Ranger

A 2001 Ford Ranger was blowing fuse 41 if the vehicle moved at all. A local transmission shop had the truck because when the fuse blew it caused a loss of power to the transmission solenoid pack. When they didn’t find the problem they sent it onto us for diagnostics. When a fuse blows as soon as you plug a new one in, the routine a technician should use is to substitute a load in its place in order to power up the circuit. A headlight works very well for this.  By connecting the headlight in place of the fuse “the short” simply completes the ground circuit that is created and the headlight lights up. Then to do the diagnostics all the tech has to do is follow the current flowing in the circuit to the problem by measuring the current that is flowing and then follow the wiring harness using a low amps current probe. That’s a high tech way to make easy work of what used to be a difficult problem.

With this Ranger the fuse didn’t blow immediately most of the time, but it could blow so quickly sometimes that the truck didn’t even pull out from starting it up. That meant a different approach needed to be taken for this problem. By installing a circuit breaker via a jumper harness the current probe could be connected to the jumper harness and the current that was flowing in the circuit could be monitored. This first capture was the current that was flowing in the circuit when the connections were made and the spike occurred as the technician was getting into the truck to try and take it for a road test.

 

Here is a low amps probe connected to the circuit with jumper wires and a circuit breaker in place of the fuse.

 

 

 

 

 

For a diagnostic tech, that’s a great result for the first test. Just disturbing the body of the vehicle generated a current spike, that’s a classic example of an abraded wiring harness that is grounding somewhere. Now it’s just a matter of finding the location of the harness failure.

 

 

 

Fuse 41 feeds the transmission solenoids as mentioned previously, it also feeds all of the O2 sensor heater circuits and the canister vent solenoid. Most of the circuit is visible without too much effort, the only part that isn’t easy to see is the part of the harness that is inside the transmission. The goal now would be to prove if the problem is inside or outside of transmission portion of harness if possible. If a visual inspection of the harness doesn’t reveal the cause of the problem, then if necessary the plan will be to use a second current probe on just the transmission harness.  That way the next time a current spike occurs, if both current probes show the spike then the problem would be confirmed to be inside the transmission.

Inspecting the harness and the O2 sensor connections didn’t reveal any problems, but when the vent solenoid portion of the harness was checked the harness was clearly abraded and intermittently contacting the bed of the truck.  The repair required the replacement of the pig tail connector for the vent solenoid and by creating some relief for the assembly the wire could be assured to not abrade again in the future.  While this event was very easy and straight forward, by taking a disciplined approach it wouldn’t have mattered if this would have been a more complicated problem. The solution would have been achieved in an efficient manor and that’s the key when it comes to doing diagnostics. A solid diagnostic routine is one that is repeatable and doesn’t fall for the inherent traps that are based on relying on silver bullets. Fuse 41 has been known to fail because of a bad heater circuit inside of any one of the O2 sensors. It’s been known to fail because of problems with the solenoid pack or its harness inside the transmission. There can also be main harness issues as well as the problem this time of the vent solenoid portion of the circuit like this truck, but none of that matters if a tech can come up with his/her own game plan and again be able to take a disciplined approach, he/she will go straight at the problem and that's exactly what the customer needs them to be able to do.