Thanks for the links Dictator, already read all that. Even watched a YouTube video on fan replacement. But I was interested in adding the auxiliary fan rather than replacing the factory one.
If the factory fan cannot cool the temperature once overheated, then the fan is insufficiently designed for the heat load. Lowering the temperature is really just a bandaid if the airflow is insufficient. The fact that it's an airflow issue is validated by the fact that the temperature drops so rapidly once the truck is moving.
I'll let the next owner do (and pay for) any additional cooling system enhancements. Right now, I'm looking for the cheapest solution. I wish he would have bought the thing during that fall or winter!
First, let me assure you that the OEM cooling fan is designed to handle the heat load of the SSR in all conditions. GM engineering did their job. The place where the fan falls short is in reliability. The fan has serious reliability shortcomings. I blame this on sourcing, not engineering.
I'm going to wager that you either have the beginnings of a fan failure (they become intermittent first - - makes troubleshooting difficult). The intermittent comes from the sometimes on and sometimes off connection of the brushes to the commutator. It is an electrical motor issue..... the brush holder are too tight and get sticky when they get crap in them.
You can remedy the brush holder issue by disassembling the fan motor and cleaning the brush holders. I think you can get two cleanings on the motor before the bent-over housing tabs get hard to manage.
- - - OR - - - - you have a burnt connector to the fan.
The connector is easy to troubleshoot...... disconnect the connector from the fan, inspect and test the three female connections in the plug. This is easily done using a 1/4" male slip connector (looks like something you would see on a speaker) by pushing it in and testing the "pin retention" of the female sockets. If someone has shoved a voltmeter probe into the connector during troubleshooting, it's gonna cause trouble exactly like this. The initial onset of difficulties looks like an intermittent connection. If left long enough, it will end up getting hot and melting the connector.
The Auxiliary fan is a good tool, but it was never intended to replace the airflow of the main fan.
For the record...... The only main fan replacement u-tube video that I recommend is from Robert Baller at RetroCarGuy530.com