Alrighty, folks. Sitrep: the Disco started*, ran, and drove around the parking lot under its own power tonight.
Now to deal with the *...
To briefly recap, after the truck failed to start on the second morning we owned it, my wife and I have been backwards and forwards through the thing trying to bring it back to life. It would crank, had spark, but no fuel pressure. In the process of diagnosis we have:
- Replaced the fuel pump
- Tested every inch of the fuel pump wiring harness for continuity and resistance, twice.
- Replaced the inertia switch
- Bench-tested both inertia switches... the old one was good
- Replaced the Multi-Function Relay (which in AEL trucks houses the Fuel Pump Relay)
- Bench tested the old MFR... it was good, and in fact, the truck started tonight ON the old MFR... (Figured if it blew up, I wanted a nicer spare. HA!)
- Replaced the Crank Position Sensor
- Verified continuity from the CKP to the ECM
- Cleaned and dressed all the grounds that address the fuel system and ECU
- Verified that the ECU is not immobilized
Nothing worked. Every time we thought we had solved the problem, we were met with the same results - cranking, but no starting. The single exception was when we ran a jumper from the battery directly to the fuel pump, at which point the truck started instantly. Since that time, we have combed every shred of the RAVE, as well as any piece of documentation we could find about the GEMS ECM and how the system works. In the process I've discovered that the ECM commands the fuel pump "on," by opening a ground path for the control circuit of the fuel pump relay, thus allowing the relay to close and energize the system. Prior to doing this, the ECM checks for a signal from the CKP, then powers the pump briefly at position II, prior to switching the pump on permanently once the engine is cranked, referencing the CKP and the MAF for continued operation.
Based on this information, and armed with the knowledge that we have a brand new CKP and functioning circuitry, I am lead to conclude that somehow, the ECM has faulted, and is not opening the earth path to actuate the fuel pump relay. Whether that is a hardware fault (there is corrosion on the ECM case) or something else, I do not know yet. However, in order to test the theory, as well as the rest of the system once-and-for-all, this evening I wired the earth side of the FPR circuit directly to ground. The FPR is now directly controlled by the ignition switch. It is permanently grounded, and receives power continuously from the time Position II is selected, until the ignition is switched off. Lo, and behold, the engine fired up immediately, and settled down to idle at approximately 950rpm. After allowing it to warm up (there was a fair bit of cold-engine noise) we cautiously put it in drive and took it for a loop around the lot (I didn't want to find myself having to push it back home!!!) without issue. We never got out of first gear, but the truck runs and drives, with no hesitation or stumble on gas application or modulation. The engine shut down smartly when switched off back in our parking spot.
So. Here I sit, a whiskey in hand, with a truck that starts, runs, and drives, and yet still no solid answers. Assuming that the ECM does in fact have a hardware problem, I now need to wade into the sticky issue of how to source a replacement ECM or have mine fixed. This seems to be a bit of a contentious point here in Land Rover Land. What I've been able to glean is that yes, ECMs can be replaced, but that they need to be re-programmed to accept the body computer, or jail-broken to run without its input. Is that correct? Are there any GEMS ECM gurus here who can guide me?