Yes but I have a Robinair 34788ni and rechecked the readings when I got home. It is no big deal for me to recover and recharge at will.
When I jump 30 to 87 at the relay it will engage and blow cold air.
30 is usually the "Hot" side of a "Bosch Relay" (those cubes, now actually produced by TYCO, who therefore now makes the best ones, most are cheap knockoffs) and 87 is the Normally Open contact, meaning circuit isn't made up till the coil closes 30 to 87.
The other two terminals (unless your particular relay also includes a Normally Closed 87A terminal) power a coil and have to get +12V and -12V (Ground) to close 30 to 87. Those coil terminals can work by being powered, by being grounded, or both, but they need both + and - to make up 30 and 87.
So when you connect 30 and 87, and the Compressor runs, you might be bypassing a bad relay. You might be bypassing a bad control, like a low pressure cutoff (LPCO) switch, or a temperature control device. OR... your system may be correctly NOT telling that Relay to run the Compressor. Like say the refrigerant charge is below low limit.
Haven't worked auto HVAC for a long time Thank GOD, but LPCO was in the low 20-psi range with R134A. If charge is inadequate, it'll exceed that till the compressor starts, then immediately fall below, opening the switch and stopping the compressor.
Trick with Refrigerants... They'll have "Static Pressure" (meaning the system is not running) whether fully charged or nearly empty. That's why putting gauges on a system that's NOT running is meaningless unless they tell you it's completely empty. It should show roughly ambient temp at static. Running, should be at a low side pressure that translates to an Evaporator temperature (right on the blue low side gauge dial) say 35-45-degrees F. About 35-45-psi. That'll give a good cold air flow out the vents but can't be below freezing or the evaporator core will freeze over from condensation. If the LPCO cuts off, low 20's, pressure may float up into the 50's before switch closes and compressor pulls it back down to that 35-45-psi. High Side, running, has a spec, but ballbark is 2.2 x ambient, so if it's 100F out, High can be in the low 200's. Hopefully not too much more, but R134A is very sensitive to charge level and air flow over the condenser. Manufacturers added a High Pressure Cutoff (HPCO) switch to keep things from getting too crazy.
Thank you, current update I junk yarded a control unit 2002 year only and when I swapped it out the original unit has scorching on the back case plastic, instead of being nice and white one corner is brown.
I took it apart and found the circuit board scorched also and two of the 100 ohm resisters are not reading correctly.
Now we wait until a hot day should be about Sunday this weekend.
Hot day yesterday and the outside temp was 81 but the vents were blowing at a cool 42.
The head unit from the junk yard seems to have fixed the issue. A used $30 part.
Thanks and I hope this helps others.
Good for YOU! It's always good to do a thoughtful visual inspection. You probably don't know how that module controls the compressor. But you COULD tell that it was scorched and maybe badly enough to cause a problem.