I changed out the alternator on my 1992 Pickup Tuesday and it has run great. But today (Saturday) when I tried to start it up, it just cranks normally without starting. The only difference about today is that it rained. I have never had problems with rain before. The battery remains at 12.6V.

This truck is my daily driver. Any ideas about what could be going on?

 

I would check fuses first, then check for spark.

I had a civic that wouldn't start when it rained, the ecu was bad. However i would not dig to deep into that until you know why is not starting.

Fuel, spark, etc

 

I was able to push start it after many attempts and it turns on just fine now. This would have me believe that it is sparking properly, but I'm no mechanic. Still need to diagnose it so it doesn't happen in the future or later today.

 

If it works "fine" now, you probably have same bad insulation ... somewhere ... that is shorting out when it gets wet. This is not going to be easy to find.

Of course, you have no idea why it didn't start. Next time, put your timing light on when you crank it. If the light doesn't flash, that plug isn't firing. Move back to the coil lead to see if the coil is firing.

 

The above is true. First you have to find why it didn't start. Then find what caused it.

If it truly was related to rain, good luck that is so stupid hard to find sometimes. Took me months on that civic. ..of course it was during a drought

 

Next time it or even before it happens, inspect area around the ECU, is there any leak around it? How about ignitor? Some rain water may splash on it can cause shorts.

On first-gen 4Runner, ECU is in right-side kick panel, and if grommet for antenna cable that runs through the body to the wheel wheel is not well-installed or well-sealed splashes in the wheel well may come into the body and get ECU wet.