On an 87, the OD temp switch (on the intake manifold, smallest single connector sensor - similar to temp gauge sender. to be sure, ground the sensor wire with the switch on. If it's the gauge sender, the gauge will go full hot when the wire is grounded. DON'T leave it grounded too long...) kills o/d when the engine gets too hot, not until it warms up. This was changed in later models for emissions purposes. I think it changed in 88 or 89.

The same temp sensor kills the a/c on an 87 in an engine overheat. It's the ground path for both the o'd and a/c relay coils.