diagnosing suddenly-deceased Megajolt Lite Jr

General Topics for configuring, operating and tuning the Megajolt. Also see the <a href="http://www.autosportlabs.net/MJLJ_V4_Operation_Guide">Operation Guide</a>

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diagnosing suddenly-deceased Megajolt Lite Jr

Post by tomicdesu »

my fairly ancient MJLJ died today. its been in service since 2010 or 2012 (don't recall). it's running 4.0.2.

it died while carb tuning; highway cruise speed suddenly got very pingy, and since i had been leaning the carb out i went in with the tuner program to retard spark across the board for carb testing, then saw that the LOAD value was erratic, jumping all over the place; it appears that mostly it returns "0", then various erratic values and maybe once in a while something like the correct value. but not usable. the spark advance value selected was correspondingly nutty.

in the car, all features of the MJLJ are working properly as far as i can tell, only LOAD is erratic. RPM tracks engine RPM. spark advance is the one value (erratically) selected from the matrix.

if i pull the vacuum hose off the MAP sensor, the LOAD is maximum and spark advance is taken from the correct bin (RPM in the max load row).

running the engine but the hose to the MightyVac, smoothly decreasing pressure/increasing vacuum also generates erratic values. set to some arbitrary pressure, that load value is wrong but steady. less or more pressure generates a different crazy, but steady, value.

out of the car and on the bench, with a MightyVac hand pump on the MAP port, i get crazy, erratic "load" values from the S command; the rpm vs load bin jumps around erratically to match. i put my 'scope on pin1 of the MAP sensor (output) and it's steady, voltage increases with pressure (decreases with vacuum).

board is spotlessly clean, dry, no corrosion. it is mounted in the passenger compartment, cool and dry. for good measure i examined the MAP sensor pins, resoldered them, alcohol wash (i know flux is not a problem but wanted it clean to look for cracks, corrosion, foolishness, etc under a bright light and magnifier).

so it appears to be kaput.

any suggestions?

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Post by brentp »

If it is only the MAP sensor operation, then you might need to replace the map sensor itself, or the resistor between the MAP sensor and the processor.

You can put a voltage meter on the output pin of the sensor and see if the erratic voltage corresponds with the reading in the software. You can see the pinout by looking at the datasheet (MPX4250AP).

Good luck, let us know what you find out.
Brent Picasso
CEO and Founder, Autosport Labs
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