Land Rover Repair: D2 1999 130.000 miles -3 amigos smiling after a flat tire., obd ii scanner, obd scanner

Question
Hi John,
When I turn the ignition key without starting the engine, the 3 amigos, TC,HDC and ABS show steady amber after the self check. The MIL is on as well what it supposed to do. Under the hood the idle air control valve hums. When I start the engine the MIL goes of, however the amigos do not. The ECU gives me a P1590 pending. Once driving the P1590 is there and can be cleared after shutting down the engine. As the 3 amigos show up even before starting the engine (the OBD scanner does not detect any faults) I thought about a short circuit or harness problem. the 4 speed sensors resistance shows ok on the SLABS connector telling me there is no harness issue. measuring each sensor against the ground does not give me any readout either what exclude a short to the ground. The hill descent signal coming from the ECU and used by the SLABS is a question mark as I do not have a scope to see the wave length. Maybe you know how to check this one...Once started the engine runs nice at idle. To be sure I measured the TP sensor, looks good. The IACV as mentioned before hums but seems to work well. Applying the brakes while driving does not pull the car to one side. There is no brake fluid leak.I did not measured the shuttle valve switches yet as I doubt this could cause the system fault. Could it be that a simple scanner like I use can clear a code like P1590 but is unable to clear a system fault? No clue but I want to go till the bitter end before calling a dealer. Can you help me stopping the 3 amigos laughing at me...

Answer
You've certainly gone a long way with a test meter on your own.  The three amigos store fault codes in the SLABS processor, and those codes are not readable by a generic OBD II scanner.  That's why you can't see them.  You need a LR test system.  There's no workaround I know of.

You've read the ABS sensor resistances . . . you can try looking at them on a scop.  Maybe you'll find a bad sensor or hub.

The 1590 code means there's a problem with the data stream from the SLABS computer to the Motronic computer.  I don't know of any home remedy for that.  Our T4 and Autologic systems allow us to watch that line to see what's wrong.

Sorry I don't have easier answer, but there it is.