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I recently purchased an R134a A/C manifold gauge set. As I glanced through its Quick Reference Guide, I noticed bullet point #6 says that I need to turn off the vehicle's engine before attaching the gauge. I thought I was supposed to put on the manifold gauge set when the engine's running and the compressor working so I get the pressure reading at that time. Am I wrong? For safety, must one turn the vehicle's engine off before attaching A/C manifold gauge sets to top off the A/C system?

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It's not necessary, your engine won't explode and your AC system won't burst if you connect the couplers while the engine is running, in fact it will make no difference whatsoever. If the AC system is running the high pressure will be higher pressure than if the AC system is not running, but the worst that could happen would be a more gas could escape if you push the coupler on with the valve closed - a minimal difference.

It is prudent though. You typically have three long hoses to deal with, and it's easier than you may think to get the filler line wrapped around the high or low side line and drop it into your engine. If your engine is running then hilarity ensues when it gets wrapped into your serpentine belt or the end gets smacked into your radiator and punctures it, or gets kicked up into your face and punctures that instead.

So, if you have a good reason to keep your engine on it's fine as long as you are deliberate and careful, if not then turning your engine off is no big deal, you probably do it several times a day.

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I, MTA, some random guy on the internet, give you my permission to do repairs in any manner that you feel is consistent with your own personal safety.

Joe Lawyer, who wrote the instructions, has only one goal: to prevent his client from getting sued after someone uses his client's product in some stupid, unsafe manner that no thinking person would ever do.

A multitude of things can go wrong when you work on a running engine, none of them really the fault of the maker of the A/C recharge kit. But the lawyer / instruction writer has to consider all possibilities and as much as possible, remove all opportunities for the plaintiff's lawyer to tell a jury, "See this? The A/C kit maker never told my client's husband to turn off the engine before attaching the kit. If they had just made that simple instruction, his necktie wouldn't have been grabbed by the serpentine belt, and he would still be here with us today."

So ignore safety instructions at your own peril, like we all do from time to time when it makes sense to do so. In this case there is no technical reason why the kit must only be attached to a non-running engine.

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    Anyone wearing a tie while doing car repairs deserves what they get.
    – GdD
    Commented Jun 21 at 13:34
  • @GdD, please note any/all car maintenance from the mid-1960s and earlier, particularly in Britain.
    – FreeMan
    Commented Jun 21 at 15:14
  • @FreeMan these days it's more likely to be a badge lanyard - not all break free easily. As well as machinery, I have to make sure people don't wear dangly shiny things round their necks working with high power lasers; at least ties only smoked a bit.
    – Chris H
    Commented Jun 24 at 14:23
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Do yourself a favor by following guidelines/friendly advice/legal liability issues. As someone not 100% familiar with vehicle refrigeration, be extra cautious; when ac runs, normal operating pressures can be hazardous - low side pressures vary from 29-35 psi while high side pressures vary from 125-250+psi. Not wearing safety glasses/goggles and releasing refrigerant/oil/dye can injure anyone when connecting/disconnecting fittings. Depending on manufacturer, some/all may have dye added to systems during factory assembly for easier leak detecting with an inexpensive uv blacklight. Dye, usually inside one or both service valves when caps are removed, can help diyers find leaks with a uv light in the shade or night time. Find the leak and determine if you can make repairs followed up with using a loaner vacuum pump from Autozone or other store with a free tool loan program, before refilling a properly repaired, sealed system that won't leak. If you can, buy only r134a without sealer, maybe one can of r134a with dye if your system does not have dye in it. Don't add more dye to a system having it. Dye circulates in a system along with refrigerant and oil and permanent without a 'freshness date'. If you buy and use a uv light, you're likely saving on long term repairs if you can prove a leak exists or not before topping off a leaking system. Service valves can wear out and leak; replace the valve cores if necessary.

When using refrigeration gauges; engine off, connect quick couplers, purge hoses, turn on engine and ac system and monitor pressures. Some manufacturers requires above idle rpm for pressure measurement. After servicing, shut off gauge valves, close coupler valves, shut off engine, disconnect couplers with a loose rag over fittings to minimize high pressure refrigerant, oil, and dye dispersal. Put caps on and tighten; they seal the service valve fittings.

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