I have a diesel pickup with a “shut off solenoid” which I have replaced with a push pull cable. Now when I pull on the cable it pulls a lever on the injection pump to the “run” position. I can then start the truck. If I then turn the ignition off, the truck continues running until I push in the cable and put the injection pump in the “off” position. My question is, if I shut the ignition off and continue to run the truck, are there potential electrical problems due to the fact the alternator is still spinning, etc., with the ignition off? Is electricity being produced with no place to go (battery or elsewhere) and could this damage the electrical system?
I have no idea the year of you pickup so I will give some general information that probably applies. Most alternators are always connected to the battery. The only necessity needed to charge the battery is field excitation. That is usually fed by the ignition switch. Some Dodge products activate the alternator field using the PCM which is powered by a relay activated by the ignition switch.
If your pickup is a recent model, there may be emission control devices that are driven by the PCM which would not be active when the ignition switch is ‘OFF’.
The burning question is "Why would you want to run the truck with the switched systems ‘OFF’ and the charging system deactivated?
The alternator could lose it’s voltage reference and become very confused…Kill the engine first, THEN turn the key off…
My truck is a 1994 Dodge diesel Ram 2500. I only want to know whether shutting off the ignition before pushing in the cable can cause an electrical problem. If so, I will make sure to push in the cable first. Actually, I’ll probably just do it anyway to be safe. A buddy with a 1997 Dodge diesel wanted me to ask because he wants to pull the key out of the ignition, lock the truck, and run into a convenience store or something. He has “big trucker” issues.
Why did replace the solenoid with a manually operated cable in the first place? I mean if it went bad and you had simply replaced it with another one, this question wouldn’t even be an issue.
I looked at the schematic for your truck. The battery feed voltage for the alternator field comes through the Automatic Shut Down relay. The ground for the field is provided by the Power Control Module. So when you turn off the ignition key the the PCM will drop power to the ASD which will drop power to the generator field. With no power to the PCM the field switching transistor will go to open. Therefore the alternator field is cut off from both ends and the rotor will not produce the magnetic field necessary to develop voltage that the alternator diodes can rectify. The alternator will just spin electrically inert.
I checked and there doesn’t seems be any emissions outputs of the PCM that the diesel engine needs to run. Although if this has an automatic transmission the TCC lock and possibly the OD will not engage. Of course, the cruise control is definitely not going to work as well as anything run by the Body Control Module.
Let us know how your experiment is going.
If your buddy wants to leave the truck running while he is in a store, why don’t he use a spare key?
I drove tractor -trailer for many years and sometimes to get a rig back to the terminal it would be necessary to set the pump to manual and push start it with another rig. It never had any ill effect on the electrical system. They will run like this even if you take the batteries and alternator out.