: A/C blower: Mystery or Misery ?

07-07-13, 02:50 PM
Hello dear Cadillac enthusisats !

I'm back with my 1978 EFI fleetwood brougham d'elegance, and I after a small restoration in the last 4 months (winter time), with checking the EFI system, restoring the body floor panels, changing the carpet, and the head liner, the battery, and refurbishing the ignition module, I still have a last (and may be not least) unsolved problem with my A/C system.

I've changed the A/C blower relay and ckecked the fuse on the fuse panel. The blower speed is OK for any selction from "off" to "Deff". At "off" it blows slowly. At "Auto hi" it blows at high speed...and so on. everything seem to be complinat with the idication sof the shop manual. But when I operate the speed selection on the A/C control panel, the light "generator" at the control panel blows immediatly, whatever is the selected speed (enven if it is the off position) . After 2 minutes, the relay is hot, and when I put the relay out of the relay center, the generator light goes off slowly. At any selection from auto low to def, the A/C compressor does not operate.

The voltage at the battery (brand new optima battery) indicates 13V when engine idle. Then goes down 12,10 when the A/C blower speed is at "auto hi" or "deff". and goes 13V when I accelerate.

I've changed the brand new relay with another brand new one without fixing the problem. I've checked the electrical connections to the compressor clutch, and everything seems to be OK. I've also checked the thermistor in the blower box, without noticing anything bad or not compliant with the shop manual.
no electrical issue to note. everything is OK in my lack. All the fuse are Ok at the fuse panel, and every accesory is operating.

Can anybody help me please ? It's summer time and It gets hot in my cadillac !!!

Have a great day, see you soon, and many thanks for your help.

07-11-13, 01:23 PM
If by blows immediately, you mean "glows" I think you have a faulty diode or voltage regulator in the alternator. The battery should be 13 volts car off, tester across the battery terminals. Once you start the car the voltage should go to to 14 volts. You also need to have a "load test" done on the alternator. A Voltage reading is only one of 2 tests. The load test confirms the amperage output of the alternator. Amperage is the "power" the alt. can produce. If yours is rated at 85 amps for instance, then the load test should produce an amperage reading of close to 85 amps. A failed diode will not allow the amperage to go close to the stated rating and usually show as a glowing generator light. One other thing, The alternator belt wraps around only the water pump pulley. If the smog pump belt is loose, the alternator will slip and not be able to charge fully. Check your alternator, can you rotate its pulley by hand, engine off? If you can, the smog pump belt is loose, glazed, or worn out.

07-11-13, 10:58 PM
There may be a bad AC power module , The biggest problem is finding a fix without being able to get the correct replacement part .I have the 81 deville The original owner never replaced the $16 blower motor so the Electrical amplifier and resistor circuits became toast . I cannot believe with all of these cars having the same kinda defect with the AC blower The have not designed an affordable replacement part that can operate off the voltage coming from the dashboard ECC .I am contemplating switching the ground for the blower to run at full speed I may get fancy and install a 2 speed dial switch for the blower . now I located in the 2 wires in the clip feeding the clutch to engage I a put another seperate switch that jumper to close the clutch for AC to operate . I attempted to just use a 6 volt relay off the thermostat call for Fan and clutch but the clutch engages and causes a voltage drop from thermostat releasing the relay shutting off blower circuit with clutch still locked in. This is why the AC power module was a failure from the get go I took all the silicone out and was in awe of the immense amount of Resistors capacitors transistors and capacitors which all create heat encapsulated in 1/2 inch of silicone then placed in evaporator chamber of the AC system It was as if it was built to fail.

07-12-13, 08:59 AM
Howdy! Carnut and Nyshark,

Thanks for your answer.
I think I will check first the generator, in order to verify that the diodes are OK. I've check the pulley and belt without seeing anything bad.
Concerning the A/C power module, i've checked the wiring and cleaned the electrical connexion, but that's all for the moment. I will let you know after next week end.

Thanks so much and see you later !