new crate motor, now no A/C and just one fan speed

Discussion in 'Tech Discussion' started by krcaddis, Oct 22, 2018.

  1. krcaddis

    krcaddis Junior Member

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    '85 C20 350/400, A/C- Thought I saw a similar post but didn't find it. I just plugged in a multi wire plug to a relay on my firewall and now I have a only low fan speed (in all run modes), but no A/C . At least it's some progress over nothing at all. A/C worked great before the swap after a R134A conversion.
    I've got power to a pair of light green wires on one end of a coupling that also has a single dark green that goes to the A/C clutch and apparently he accumulator/drier. but no power. There's a multi wire plug on the black plastic fan(?) box and all colors show power with a test light. Looks like no power at to the clutch is part of the problem. I don't think that is related to having only a low speed on the fan. Since all work was in the engine compartment and fuses are ALL good, the problem must be under the hood, electrical, and maybe vacuum?. Second photo shows light and (dead end)dark greens' plug, the first shows a dark green wire (by left fender. near booster) that doesn't seem connected to anything. Should those two connect?

    Wiring diagrams I have for an 85 don't show any light green wire and only one dk grn from "panel lps 5 amp" fuse to "Panel & interior lamps control switch" (WTF?), not to A/C fuse at all. IDK what "convenience center" is. Don't know where "ACCotroller is at. Haven't seen any "gra" wire at all. Is BLK a ground? It has no power.

    lt grn connect.JPG

    plug 1.JPG
     
  2. krcaddis

    krcaddis Junior Member

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    Sorry both same photo, (There isn't a dead end by fender/booster), but here's a better shot of greens plug w/dead end light green. Dark goes to A/C clutch (no power in any switch position) Sorry if prior description is confusing.
     
  3. SquareRoot

    SquareRoot Full Access Member

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    That green wire should come from the control unit and go to the binary pressure switch BEFORE going to the compressor clutch. If you have power into the binary switch but not out you either have a bad switch or you have no pressure in the system. You obviously had to mess with the compressor, lines and harness if you did an engine swap? If you don't have power coming from the control unit then you blew a fuse, fried a relay or have a cut/loose connection.
     
  4. krcaddis

    krcaddis Junior Member

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    Thanks for the reply. Your diagnosis sounds very likely, but quickly went over my head. What's a binary switch look like and where is it?
    What is the "Control Unit" (- under dash, on dash, under hood?)

    I mostly suspect a cut or incorrectly connected wire (s) and/or vacuum line(s). The wiring and vacuum schematics that I have (days on Google) label almost everything in secret code apparently. Like "12034482" "200402 C222", "S281", "12034481" with no indication where it's at (under dash, or between bed & spare tire (rant)), or what it looks like (relay box, dash HVAC Control, under dash on heater, IC board on firewall, connects to receiver, etc). Color codes mostly don't jive with what's there, although the truck was unmolested, only 40k miles. Small hoses/tubes broke easily.

    I had one guy pull the old 350 another install the crate- long story related to our recent CA fires). AC wasn't discharged, just moved out of the way. Reconnections were made mostly by me later and based on how far wires reached, color, and puzzled out what seem like it belonged. (same with vacuum lines). My '72-84 C10 Haynes book is useless crap ("assembly is reverse of disassembly")- photos don't label parts, too vague and not enough.

    [OT: Don't want to rant - I wrenched at a British & BMW dealership for years, am generally considered pretty smart and mechanically inclined. Rewired cars, ground up builds, resto-mods, so on (I'm 70) Don't know/trust any current local mechanics (never saw anything old, from the 80s...) I've had '54,58, 91,& 85 C & K trucks and did my own work mostly but too stiff to crawl around for a major R&R. The 85 has complex and cross connected emissions tubing and wiring].
     
  5. 75gmck25

    75gmck25 Full Access Member

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    The binary pressure switch just stops the A/C compressor from running if the pressure is too low (below 20?).

    The pressure switch on an '85 is probably screwed into the far right side of the receiver/drier, although on earlier trucks like my '75 it was in the hard line that comes out of the evaporator and has the orifice tube. The one on my '75 is similar to this one https://www.oreillyauto.com/detail/...-terminal-pressure-switch/35757/4517230?pos=4.

    The wire connector for the switch is round (fifty cent piece size), has a black rubber body, and has two female terminals inside that are connected to the green wire for the compressor. The connector just pushes onto the switch, which has two male terminals. If you pull the plug off the switch and connect a jumper between the two female terminals, this bypasses the pressure switch. Its very common to bypass the switch when charging A/C with the small cans of refrigerant.

    Bruce
     
  6. SquareRoot

    SquareRoot Full Access Member

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    I have an 85 k20 that I recently put an entire aftermarket ac system in. I eliminated EVERY factory wire and vacuum line associated with the factory ac. Do yourself a big favor like I did an get a wiring diagram for your truck from LMC. It's cheap (like $10 if I recall) and shows every circuit and electrical component in your vehicle.
     
  7. 73c20jim

    73c20jim Supporting Member Supporting Member

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    I believe wiring diagrams are in our reference section.
     
  8. WHEELMAN

    WHEELMAN Full Access Member

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    compressor-6-re.jpg There are two wires going to the compressor right high side.low side

    Low pressure and a high pressure cut out right.one wire to the clutch.one on the back.round switch.and then one in a

    Gas line right.

    compressor-3-re.jpg

    compressor-1-re.jpg

    chevy bigblock.jpg
     

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