Truck Repair: Electrical, speed sensors, reluctor

Question
I have a 2000 FL60 with a cunnins B Series engine with a Asia automatic transmission. The problem I am having has something to do with when it rains or throws water up underneath the truck is shorts out the speed sensor contol(speedometer quits working), ck engine light comes on and truck loses power.  We have replace speed sensor contol(3 times) and something sorted out the ECM(they said it was something to do with transmission).  I have spent thousands of dollars.  The last time after replaceing the ECM is ran good until it rain.  Then it starts all over again.......speedometer quits, ck engine light comes on, and engine losses power.
Looking for some anwsers........David

Answer
Hello David,
Most speed sensors just send a pulsed signal, weather a refference voltage is used, or just completely from a magnetic reluctor. At any rate, I don't think there is anything in the sensor that water shorting across the contacts would cause an ecm to burn out. Water would cause loss of signal, and make it do like you say.
I am thinking, that with the history of sensor changes, that possibly just the act of unplugging it and reconnecting it to a new one might be what temporarily fixes it.
I assume you checked for water in the connection?

If there is a refference voltage, and someone used a test light to check for it, rather than a good voltmeter, that could damage an ECM.
Or if the wires to that sensor are chaffed somewhere back along the frame or body, and actually rub against metal, but I wouldn't think it would only happen during rain, unless where it is rubbedis where a big slug of spray from a tire, for instance, may push the wire against a metal edge, when otherwise it would just hang free.

And it may not necessarily be the speed sensor wire, but another wire on the same circuit.

I would look at all the wires near a fender well, or where a splash could hit them.

Any chance you have some rodent chewed wires along the frame or firewall, or on the engine, that are laying there bare, and getting touched together by a spray from a tire?

Van