1993 E150 w/302 engine
 

I just transplanted a factory rebuilt longblock into my 1993 Winnebago conversion van and the Rear ABS light is now on and will not go off. I read the EEC-IV and tried to erase the code shown, 512, with no success. I understand that code 512 is a failure to communicate the information from the EEC-IV to my code reader. Does this mean my PCM is toast? I have never had that code shown on any of my cars or others that I have worked on. Response??

Well, patience is the thing here, as I went out this morning to move the van out of the shop and it started right up, rear ABS light went out and stayed out and it shifts normally now. I ran it about 40 miles and everything seems to be back to normal, can't wait to see what it is going to do tomorrow! Must be some number of starts that resets the PCM because nothing was touched on it. So, now a new serpentine belt and it (hopefully) is ready for a trip to Yosemite.

Engine Codes, ABS Codes
 

EEC-IV is pretty "forgiving". For "Check Engine" light problems, the code "thrown" is kept in force after the engine is shut down.

ABS codes seem to be less worrisome to the designers. If, subsequent to getting an ABS" warning light, the fault has disappeared, so will the warning light, generally.

As the engine computer (PCM) scans it's program, continually and very, very fast, it compares acceptable data values over and over. Should it "see" an unacceptable value, repeating itself for a fairly long time, it "throws" a code and turns on the CEL (Check Engine Light), more properly called "MIL" (Malfunction Indicator Light). The unnaceptable condition must be "seen" several hundred times in succession, in order to cause a code to be thrown. This is to prevent repeated and often unnecessary codes being thrown, due to non-repetitive functional "glitches".

In order to run a 4-cycle engine at say, 6000 rpm, each fuel injector and spark plug, must fire 50 times every second. So, the PCM is operated at a very fast scan-rate, much higher than 50 scans per second. In reality, if a "fault" occurs, it takes very little time to accumulate the required repitition of the fault to necessitate throwing a code.

Fun, ain't it? imp :)

Rear ABS light
 

Just as easily and quickly as it went off, it came right back on this morning and has disappeared and reappeared 3 times since, each time lasting less than 5 miles in each cycle. One noticeable thing this morning though is that at 55mph, the speedometer needle was oscillating slightly, about + or - 3 mph but when the light comes back on, it remains steady. Guess I will check the ring gear to ensure the magnet is secure and clean. Probably time to change the differential lubricant anyway, its been about 3 years and 40K miles since I changed it last.