I have purchesed a 92 d250 ext cab Dodge Ram and have come across a slight problem. The alternator, overdrive and grid heaters do not work when its cold outside, like 32 degrees or colder. I have checked wiring, connectors, grounds etc. Only thing I come up with is the ecm. Now I have found out if I take a hair dryer and warm the ecm up first for maybe 5 minutes then everything works perfect. By thhe way, I have cleaned the contacts and checked the voltage on #3 and #9 on the ecm and they are ok.
Do you suppose it needs to be replaced or should I just carry a hair dryer till the weather warms up around here?

 

I think it will only get worse and it sounds like the PCM is the problem so you might as well change it or have it rebuild before it leave you stranded somewhere.

 

I have a'93 that is set up with a snow plow so you can guess when it gets started! My alternator, lights, fan etc all "die" when I first start the truck when it is below 20 degrees. It takes about 5 minutes of running for the systems to come back on but once the truck warms a little every thing works and it has been doing this for years.

 

Probably moisture has gotten into the circuitry. The polymer type coating that encases the ECM "brains" is prone to crack and fissure over time, allowing moisture to seep in.

When it gets too cold the water freezes and probably cuts out the connections between several systems. Not until it returns to a liquid state do things start working again.