I have a Caterpillar 320DL excavator (serial prefix KGF). When we turn the key to the ON position, the “Switch On” fuse immediately blows (short circuits). Our technicians initially suspected the ECM, since the fuse does not blow when the ECM is disconnected. We replaced the ECM, but the problem still persists. They checked: All relays, wiring, and fuses – no visible issues. Connected Cat ET – no active fault codes were present. However, the display shows an error 36: 91-8, which we believe relates to the engine speed sensor (this sensor has not been replaced yet). At this point, we’re unsure if the speed sensor could be causing the short or if there’s another hidden issue. What could be causing the “Switch On” fuse to blow when the ECM is connected? Could a faulty engine speed sensor or a grounded sensor wire be responsible? What else should we check?
Do you have an electrical schematic.? If not do you want one.?
Can i say you should not keep saying you have a short circuit because that infers a wiring issue. You might have a short circuit but that is not established and is more likely a sensor. What you have is current draw from the ECM exceeding the Fuse Rating. The excessive current draw could be the ECM or any sensor or logic circuit connected to the ECM that is being powered by the ECM. The reported speed sensor fault may not be a faulty sensor because in the few moments before the fuse blows the voltage can be dragged down from the overload current and the ECM reports the sensor as faulty caused by low voltage. I would suggest you pull all the plugs off the ECM except for power and establish that the fault is not there. Ideally if the wiring permitted you could reconnect sensors or plugs one at a time. Often this is not possible so an auto electrician would install something like a Low Wattage bulb across the fuse to restrict the high fault current which can damage things and it gives him more time to disconnect sensors and measure voltages at the sensors as a test. Because of the cost of damaging things you may have to use two people, one person at the fuse and one out checking sensors. The person at the fuse can coordinate with the other person so as to limit the time the circuit is powered with the globe. The ECM may not function correctly with the bulb across the fuse but is only purpose is to give more time in locating the fault. That Schematic from Nige would help me if you post it. Cheers
Hello i do have the schematic
In that case I suggest to identify each individual circuit downstream of the key switch as a first step, followed by disconnecting every one of them. Then put the key ON, does the fuse still blow.? If not, reconnect the circuits one at a time, testing each time. When the fuse pops you know which circuit to focus on.
Main power relay. Does it show the speed sensor code when you have et just hooked to it not running. If it does that’s normal.