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Thread: How To: Diagnosing and bulletproofing your A/C

  1. #1
    Join Date:  Nov 2016

    Location:  UK

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    Post How To: Diagnosing and bulletproofing your A/C

    Hello owners!

    I thought I will write little article about Air Con (A/C) system on D as it is not the best design from electrical point of view. I will focus on electrical part as I'm Electrical Engineer working in automotive, so I know how it should be done properly.

    Let's assume that your A/C system is filled with gas to correct pressure, engine is running, but no cold air is coming from vents, neither compressor is kicking in. I assume also that your fuses and relays are OK.


    I suggest diagnosis in reverse:

    compressor wire.jpg

    1. Your compressor is not kicking in when you turn on A/C on (engine running, your fan at speed 1 minimum). That can be easily diagnosed - your compressor pulley is not engaging - this can be seen/heard.

    To check if fault is with compressor (electromagnetic clutch) you disconnect RED wire that is on top of it and bypass it straight to +12V on your battery. Good practice is to use wire with fuse (in this case I would use 15-20A fuse and 1.5mm^2 wire) as in case of short on your clutch you will blow fuse, not melt wire!

    REMEMBER TO SUPPLY POWER TO COMPRESSOR ONLY FOR FEW SECONDS! If you leave power permanently system will generate too high pressure and pipe will explode and release gas! You doing this only for diagnostic purpose. When A/C is on, compressor goes on and off all the time to keep certain pressure! It is not working all the time.

    If you compressor is working once you supply power, it means problem is somewhere else. If compressor is seized you will see some serious shit happening with your belt, so be prepared (Always were adequate protection, in this case Safety Glasses)!

    Once you done with diagnosing, remember to reconnect wire to original terminal!

    2. Low pressure switch is your next suspect! Why? Unfortunately it is related to overall system design. When you energize compressor clutch, there is huge current spike as electromagnetic clutch is a big coil. This is causing sparking on all contacts (working contacts) all way down starting with your A/C Instrument Panel mode switch, through pressure switches. Sparking is causing contacts corrosion which after X amount of cycles will stop conducting current and results in your A/C not working at all. I would like to believe that whole system was designed to be operating witch such current spikes, but I have my doubts. To make it bulletproof you can make one small improvement that will let you forget about A/C electrical issues, but I will talk about this on the end of my post.

    Now let’s go back to low pressure switch:


    This switch is in open state when there is too low pressure in system. When pressure is correct, it should be closed.

    Disconnect wires from it and check terminals for corrosion. Check with multimeter if switch is closed state (Remember, I assumed on beginning that you have correct gas pressure in system) . If switch is showing open state, then there is possibility that :

    a) your gas pressure is too low to operate and you need to re-gas it (my assumption was wrong! You did not check it in first place! :dodgy: )


    b) your pressure switch is faulty!

    This can be replaced even with gas in system, however sometimes valve might be stuck and when you will be removing it, it will release some portion of gas with oil (ON YOUR FACE!) so please use all protective equipment like safety glasses/mask to cover your face and gloves. I would not remove it until you have replacement.

    If you sure that your gas pressure is correct (as your system was just regased and checked for leaks!), you can short that switch and check if your compressor will kick in (engine running! fan speed 1 minimum). You short it using fuse (15A-20A). Basically, push on connectors on fuse. If it starts, you got faulty low pressure switch. Shorting this switch is only dangerous if you have no gas/oil in system as you will seize compressor. Remember, you are shorting this only to diagnose the system.

    If switch is in closed state then go to next point :

    3. High pressure switch


    This switch is normally in closed state. Will be open once Air Con system will reach some certain value (preventing your system from blowing up. That is why in section 1. I warned you about not running your compressor for more than for few seconds as by-passing this switch is dangerous for longer than few seconds).

    Diagnosis - remove connectors from switch and check for corrosion. Check using multimeter if switch is showing closed circuit. If no and you know that your gas pressure is correct you can short connector for few seconds using 15A-20A fuse to check if your compressor will kick in. If it will kick in, you got faulty high pressure switch. If nothing happens, go to next point.

    4. A/C Panel

    AC Panel.jpg

    Your MODE SWITCH contacts might be worn due to sparking, as I said before, whole load to compressor goes through switches. Here is example of contact corrosion:

    mode switch.jpg

    That deposit on your contacts prevents conduction. Good thing is that this can be rescued with some contact cleaner and really fine sand paper (800+). Removing HVAC panel takes a bit of time, so I would leave it for the end as I did in this tutorial.
    Once all those elements are checked and in working condition your system should be in working order, unless wiring is damaged.
    To diagnose that you will need multimeter with buzzer option to “ring” wires. If you own a DeLorean you possibly have wiring diagram, if not, it can be easily found on the internet. Here is snapshot of important bit:


    High pressure switch is between number 90 and 91 where you have a bump on diagram. Not sure why it is not marked there.
    To make system bulletproof from electrical point of view you need to make little upgrade. Basically, you need to introduce a relay that will take a load from switches contacts. It can be added in the cabin or in engine room. Decision is up to you.

    Cabin option: you will need to find wire in passenger legroom going back to compressor. That wire will control relay. On top of that you will have to get +12V and Earth, so system can work. Remember that your +12V can’t go straight from battery. You need to introduce fuse (I suggest 15-20A fuse and 1.5mm^2 wire). Earth can be local or straight from negative post of battery. You will have to fix your relay somewhere next to HVAC. You can use relay/relay base with fixing point or introduce relay box (this will help to soundproof relay cycling sound).

    Engine room option: you will have to disconnect compressor flying lead and extend wire long enough to reach your relay (+ return wire to energize compressor clutch). This wire will be controlling relay. As in cabin option, you will have to get +12V and earth, so system can work. This solution is for “purists” as does not involve cutting OEM wiring and can be reversed by disconnecting upgrade kit (BUT WHY WOULD YOU WANT THAT?!), however relay/relay box will be visible in engine room.

    Your circuit should look like this after modification:

    compressor with relay.jpg

    Remember to put your relay in dry place or introduce a relay box to cover it when exposed to moisture. Don’t be cheap and invest in relay base. In case of relay fault, replacement will be easy.

    If you want go step further you can get a relay with fuse. However this can be only used next to battery or when you connecting to already fused line. Reason is that if your wire shorts to ground before this fuse, it will melt wire/boil battery as there is no protection on that portion.

    I hope that somebody will find this short article helpful. Unfortunately, I don’t have photos of switches locations (I do not own DeLorean yet, but worked on them).
    I bet somebody will upload them under my post... (be that guy!)
    If you have any doubt, think there is mistake in what I wrote, please let me know. I will try to explain more or correct my mistake.

    Remember, this tutorial describes only electrical part of A/C system. It assumes that mechanical part is in working order.

    kind regards,
    Last edited by Reinsch; 06-18-2017 at 04:08 PM.

  2. #2
    Senior Member DavidProehl's Avatar
    Join Date:  May 2011

    Location:  Maple Grove, MN (Minneapolis)

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    Awesome write up! Thank you very much for documenting all of this! The next time I have A/C issues I'll be coming back to this. Hopefully that isn't for a very long time.
    David Proehl

  3. #3
    Join Date:  Nov 2016

    Location:  UK

    Posts:    3

    If you add the relay you should be good from electrical point of view!

    kind regards,

    Looking for Project DeLorean. Non runners are welcome.

  4. #4
    Senior Member Dangermouse's Avatar
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    Thank you for understanding the purpose of the "How To" section

    Great write up.
    VIN 2743, B/A, Frame 2227, engine 2320

    I don't always drive cars, but when I do, I prefer DeLoreans

    No-one is to stone anyone, even, and I want to make this absolutely clear, even if they do say "carburetor"

  5. #5
    Owner since 2007 Farrar's Avatar
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    Nice write-up and good idea about the relay!

    Thanks, Marek, for contributing this to the community!
    DeLorean status: It's complicated.

  6. #6
    Dr. Bob Bob635's Avatar
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    Nice article on the HVAC wiring.

    Now if someone could come up with a good reason for all the brown + wires on the alternator, rather than one heavier gauge supply wire, and
    how to replace them to clean up this mess, that would also be helpful.

  7. #7
    Senior Member Domi's Avatar
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    Thank you for sharing this with us

  8. #8
    Join Date:  May 2011

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    Please add: the location of all the components such as Lo pressure switch, high pressure switch. where you installed the relay.

  9. #9
    Administrator Ron's Avatar
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    Here's the switches:

    Quote Originally Posted by DMCMW Dave View Post
    There are four configurations. Let's call it "Very Early", "Early", "Middle", "Late"

    ALL versions have the low pressure switch screwed to the accumulator. This never moves.

    Very Early (500s, through low 1000s) - Uses the accumulator with the outlet at the bottom. No cross in the crossover hose next to the accumulator (Feeds the orifice tube) . No high-pressure switch and no blow-off valve. If the system gets plugged, something blows up.

    Early (up to sometime in the 2000s) - Uses the accumulator with the outlet at the bottom. Adds the cross in the crossover hose next to the accumulator (Feeds the orifice tube) with the high pressure switch and the blow-off valve.

    Middle (the rest of the 81s)- Uses the accumulator with the outlet at the side. Same crossover configuration as "Early"

    Late (82 and 83 but not necessarily at the exact 82 model changeover)- Uses the accumulator with the outlet at the side. No cross next to the accumulator. Has an aluminum block at the condenser OUTLET with a high pressure switch and blow-off valve.

    There are exceptions as many cars have been converted from early to late setups. It is possible to replace the crossover cross-type hose with a late/very early straight hose and completely lose the high pressure switch and blowoff valve, so it would look like the very early config.

  10. #10
    Mr. Pickles-mobile Shep's Avatar
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    Talk about coming in with a BANG, jesus! I love this guy

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