Problem: 1988 n/a HVAC blowing HOT at all times, even after re-charging A/C.
It was working properly before I had my dash apart & before replacing the DME with SDS EFI standalone ECM.
The directional controls & fan speed work great but blows hot.

Note: I have moved vacuum lines around, but the engine bay side items are attached to intake manifold vacuum lines is this possibly the problem? I don't think so because when I click the a/c button the a/c compressor doesn't engage. I might be possible I plugged something in wrong when dash was pulled out...

Your help & expertise is appreciated.

 

Constantly blowing hot is usually either the heater control valve at the rear of the engine or one of the servos that control the heater flaps under the dash. However if your compressor isn’t engaging when you press the a/c switch that would suggest an electrical issue.

When you press the a/c button the compressor should engage and both radiator fans should run. If these aren’t happening then it’s either a hard electrical problem like wiring, fuse or relay or it’s one of the a/c sensors preventing the compressor from engaging like the pressure or temperature sensors behind the lh headlight area.

Time to get a multimeter out and start probing.

 

I had the same problem! Turned out it was a blown fuse.
Ain't nothing gonna work if the fuse is blown!


The cause of the blown fuse was a shorted coil on the magnetic clutch.

Read about it here

Here's a preview:
Check for 12V at compressor.

Yes - replace clutch
No - check low-pressure switch

Check voltage at low-pressure switch

Voltage at only one connector--system has low pressure or switch is defective.

Check A/C relay

Now at this point the manual starts talking about term.3 and term.2, but on my relay, we only have terminals 30,31,85,86,87a.

Check de-icing switch.

There is a switch that is supposed to turn off the compressor if it detects that the coils in the passenger compartment are icing up.

Voltage at only one pin, defective, replace

And the final choice is:

Replace Control Switch

 

Okay... NOW these are great insights! THANK YOU. I'll check it out.