1992 Lexus SC 400 A/C

Yes, the 60A fusible link is providing constant power to the heater relay and the ignition circuit is providing power to the #1 terminal. The replacement A/C Control arrived. Let's keep in touch and I will advise the results of the A/C Control swap. We are also replacing the A/C / Radio switch and the Cooling Fan Control unit. Any other ideas?

Just a brief follow-up to our earlier correspondence. We jumped #2 to #10 on the cooling fan control but, no change. In looking at the electrical schematics we did notice that the A/C Pressure Switch wasn't grounded but, grounding that line (#5 in connector A14) made no difference. Looking at the 3 connectors to the rear of the A/C control (A12, A13 & A14) the charts imply 2 power leads to connector A12 (#1 & #15), one ground to connector A12 (#7) and one ground lead to connector A14 (#5). These are all engaged per spec. But, the control unit is still dead. This result is consistent even though we replaced the cooling fan control, A/C control unit and Temp./Radio switch with others that came from vehicles where the system was working. We are able to control the heating/cooling fan by jumping the Heater Relay (using a rheostat to adjust RPM). Same for rear window defroster with a toggle switch. It should be possible to control the servo motors with a toggle switch and the A/C output with some form of thermostat. I mention circumventing the Automatic Climate Control because we have installed an older reworked Chevy 5.7L V8 with a six speed double over drive manual into this 1992 Lexus SC400 (we call it a "Chevus"); meaning, we are applying 1960's technology to a 1990's style vehicle (no computer, no FI - just a Holley 750, etc.). Accordingly, some things have changed. Everything works regardless of which drive train is installed but, for some unknown reason the A/C control is dead. I should add that although the original A/C compressor and system have remained intact, the compressor is not yet electrically attached but, it doesn't seem that the lack of this component should cause the remainder of the HVAC system to fail. Or, are we missing something? We noticed that the A/C compressor's connection has three leads whereas the clutch probably only needs two leads (a + and a -) to be engaged. Can you explain the third lead? Also, if we convert the system to manual HVAC controls (we have already installed a manual heater control valve for the heater core) can you recommend a Toyota vehicle(s) from which we could obtain a suitable (manual) temperature control for this A/C unit? Have a good day.

Thanks for your continued interest. At first blush, I'm not sure that we are on the same electrical schematic page. I'm referring to the 1992 Lexus Electrical Wiring Diagram Section "K" page 23 entitled "Automatic Air Conditioner" which doesn't present your junction block. I can send a picture of what I'm viewing if you can specify the proper procedure. On the schematic I'm viewing, I can identify a two blue/red wires connected to the A/C dash mounted control unit; one (identified as ACC) feeding pin #15 for connector A12 provides power from the ignition switch to the A/C control and the other (identified as MGC) attached to pin #4 for connector A13 and leads to Engine and ECT ECU. Of course, since the engine is no longer computer controlled, that box has been removed. The only white wire that is visible is connected to pin #8 of connector A12 (identified as MF ACE) and connects to the Air Vent Servo Motor. Are we on the same page? Thanks for your help. Have a good day.

Thanks for your continued interest. I apologize for any inconvenience. Attached, please find the Section "K" which I mentioned earlier, much of which probably doesn't apply since we are focused on the A/C dash mounted control unit for which the electrical schematic is displayed on pages 36 & 37 of this file and which documents the connections and wire covering colors for each (Blue/Red being L-R) pin. My assumption is that we are trying to fool the system into believing nothing has changed and that it should function as normal. I've included the entire Section "K" thinking there might be some short cut which isn't obvious when just focusing on the A/C control pages. By the way, you mentioned the "PCM" but, I wasn't able to immediately find any PCM reference that remained in the vehicle. Can you help me out? Lastly, you've already spent more time on this issue than I thought you would (thanks for the courtesy). How do I respectfully add more $ to the kitty? Have a good day.

Thanks for your continued support. As you suggested I shorted the #1 Red/Blue lead and the #2 White lead at the A/C Magnetic Clutch Relay but, no change; meaning, the A/C dash mounted control is still dead. Since the schematic stated that the #2 White colored lead was to go, "To the Engine and ECT ECU<2-7>, I grounded the #1 to #2 short but, still no change. I've scanned the total of our previous correspondence Have a good day.

Thanks for your support. As you suggested, I maintained the short between the #1 R/B and #2 W lead at the Magnetic Clutch relay (noted above) plus applied 12 volt positive power to the White wire that was originally (when the original Lexus 400L drivetrain was installed) directed "To the Engine and ECT ECU<2-7>" but, still nothing. The solution is probably something simple but, it isn't obvious. Have a good day.

Sorry for any inconvenience. Sometimes the pin numbers are different on your schematic and mine. So, I will speak in terms of the wire colors. On my schematic, the Magnetic Clutch Relay is fed by the Red/Blue wire off of the 10A Heater circuit. For this relay the Red/Blue wire feeds both the relay coil and the internal switch which ultimately feeds the Magnetic Clutch coil (with a Blue wire) and the A/CIN pin on the dash mounted A/C control unit with a White/Green wire. Adjacent to the Magnetic Clutch relay is the Heater relay wherein the Red/Blue wire only feeds the relay's coil which engages the internal relay switch that connects the 60A Heater circuit to both the blower and a White/Black wire that attaches to a (EB) ground (front side of left fender). To see if energizing the White/Green wire that ultimately attaches to the A/CIN pin on the A/C control unit (D connector #2 pin), I shorted the positions #3 & #5 on the Magnetic Clutch relay but, nothing happened. When I checked further, the Red/Blue wire wasn't providing any power. I then attached a lead directly from the positive side of the battery but, still nothing. I couldn't immediately find the 10A Heater circuit fuse. I thought it might be in the left kick panel but, the manual does not show any 10A HTG in the picture but, shows a 10A HTR within the Schematic. There are two 10A in the picture one labeled MIR-MTB & the other MTB. Significant? I will check them out although since I provided direct power to the White/Green wire with no change, it doesn't seem that's the answer although it may be part of the answer. If it is possible to determine all of the leads which bring power (plus all grounds) to the A/C controller when it is operational, I could adjust the wiring accordingly and see if that resolves the issue. Somehow we are either not feeding the unit with the power it needs or we are missing a ground. As noted earlier, when I originally tested the 3 A/C control connectors, I only found 2 power sources (connector A12 pin #1 (+B) & 15 (ACC)) and 2 grounds (Connector A12 pin 7 (GND) and connector A14 pin 5 (LP). Any thoughts? Thanks for your help.