Pontiac Repair: Pontiac Transport Radiator fan, coolant temp, engine coolant

Question
Yesterday I noticed my engine running hot. It occured to me that the radiator fan wasn'nt coming on. I switched relays, and turned on the AC, now it turns. No AC, no fan. Assuming the relays and fan motor both work, is this simply a case of a bad temp sensor? I have a '91 transport without the big fancy engine-it only has 3 relays rather than 4. Thanks for your time

Answer
well it sounds to me like you found the problem if switching the relays around made it work then that's the case a bad relay. On the other hand on those older vans when the a/c is on the fan ran all the time not so on the newer cars. The ECM "engine control module" controls the fan relay/relays depending on wether it has more than one fan or not. basically what happens when you turn on the A/C the HVAC control head sends a signal to the ECM and #1 tells the computer to turn on the a/ac clutch relay and and also the fan relay as well. in some of them the fan relay will not turn on until the engine hits a certain temp. that van is new enough that it more than likely doesn't have a separate sensor to turn on the fan relay it uses the engine coolant temp sensor for the ECM and when the ECM sees the engine at say 218 degrees It's usually around 220-225 degrees when it decides to turn the fan on but every car/van is different, it then turns on the relay for the fan. So it's a bit more complex then just a relay or a temp sensor. the best thing to do it find someone that have a scan tool so you can see what is going on there wether the ECM is not commanding the relays on or the ECM is not getting the signal from the hvac head to turn the relays on etc.. I'm rambling on here but you can see what I'm getting at. Good luck :)