Battery voltage increasing (failed board component?)

[turbodoug]

Started doing this out of nowhere after a year of running fine...

When I first power on the Megasquirt the battery voltage is correct and stable, but after about 15 seconds it starts climbing real high (18-19v). The actual input voltage at the DB37 connector is stable. This happens on both the car and on the stimulator. Swapped in another MS2 chip and it does the same thing.

What components on the V3 board could possibly cause this to happen?

Any advise would be greatly appreciated.

Thanks,
Doug

[turbodoug]

Update... when the battery voltage displayed in Megatune starts increasing, the 5V on MS board starts decreasing...

[turbodoug]

D19 gets blistering hot immediately... and F1 gets hot after about 20 seconds...

[turbodoug]

Replaced D19... problem solved!

I replaced the 5.6V ZD with a 5.1V ZD because it's all I had available. Do you think it will be okay to leave it in there permanently?

[Matt Cramer]

Should be fine like it is now.

[Bernard Fife]

Doug,

If this was mine, I would replace it.

The reason is that D19 is a dead short to ground if the voltage rises about the Zener level, and it will burn out very quickly if that happens for any length of time. D19 is intended only to handle very short spikes of high voltage, not sustained periods. Your replacement is a 5.1 V, and the supply is 5.0. However, the voltage rating is generally ±5%, and the voltage rating bit changes with temperature, as can the regulator output. In some circumstances it is entirely possible that the 5.1V Zener will burn out again if the regulator voltage rises about the effective Zener voltage for any length of time (tenths of a second).

In fact, if this was mine, and given the fact that you have already had trouble with a 5.6V Zener, I would replace it with a 6.2V Zener, as described in 14.b) here: http://www.megamanual.com/ms2/V3trouble.htm

Lance.