Hi again, sorry didn't mean that you should just clear codes and go on your merry way. Definitely address the issues but that being said, I've owned a D2 for 8 years and I've had my ABS lights pop on many times, with no real issue. It's happened from squeaking breaks etc. Thats why I suggested buying an ABS Amigo. That way you can clear the code and also check if something is truly at fault.

 

It seems like you have two problems. The gear 4 ratio code is often an MAF issue. The MAF never really throws codes unless you unplug it. It just causes other mysterious codes to appear. I've had the gear 3 and gear 4 codes thrown and a new MAF fixed them. Another cause can be poor or low transmission fluid. The code theoretically compares the values of two sensors. And if your torque converter is turning faster than your transmission it will throw the code. At least, that's my recollection.

The three amigos is very common and probably unrelated. You need an abs amigo or an icarsoft scanner to know just what is wrong and clear the codes. The scanners cost about the same as a diagnosis from a mechanic and he hasn't even bought or put on the parts yet.

One thing that can cause both issues is a massive vacuum leak that affects everything.

 

When you turn the ignition key to position 2 all the lamps illuminate for a few seconds (AFAIK 10 seconds) then they should nearly all disappear (it's the system self test) and as soon as you start the engine all should disappear. Any that remain means you have stored fault codes or current faults in the ECU's. In many cases the TC, ABS and HD coming on with the engine running is the '3 amigos' which depicts battery, alternator or ABS faults, often ABS and the faults usually are the ABS wheel sensors or shuttle valves.

Best is to clear all of the codes with a diagnostic tool and then drive the truck a short distance and then see what codes re-appear, of course unless the oil pressure lamp is illuminated and then don't even start/run the engine.