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AC shuts down after 10 miles

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New User

Jun 7, 2017, 5:57 AM

Post #1 of 7 (715 views)
AC shuts down after 10 miles Sign In

I have a 1960 MGA coupe that we've added aftermarket a/c to (it's like driving around in a goldfish bowl without it).

We're using a Sanden 505 compressor and the Vintage Air 61005-VUZ-A Gen II Super - Heat, Cool & Defrost system.

My problem is after driving 10 miles or so the system stops blowing cold air and just blows the ambient temperature. I've bypassed the thermostat and I've rigged up a light from the compressor voltage to see if it was shutting down but it doesn't...and I verified that by stopping the car and looking at the was still merrily spinning away...BUT there is no load on the engine as there normally is when the compressor (a/c) is turned on...idle speed stays the same.

I'm a/c illiterate so I really don't know much. Is it possible for the compressor to spin without actually doing anything? If so, what else should I be looking at? I'll try any suggestion.

Ultimate Carjunky / Moderator
Discretesignals profile image

Jun 7, 2017, 6:23 AM

Post #2 of 7 (706 views)
Re: AC shuts down after 10 miles Sign In

When the ac is blowing warm and the compressor clutch is engaged, what are the high and low side pressures?

Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jun 7, 2017, 6:27 AM

Post #3 of 7 (704 views)
Re: AC shuts down after 10 miles Sign In

You said you were A/C illiterate so just save the grief this will be both ancient and aftermarket will need a pro for sure to diagnose and fix hopefully not a huge deal.

It part works and at that age my guess is probably correct that your part of this that makes cool (no proper terms needed but ask if you want) is icing up from a low charge of the gas used in this unknown right now but would do that and leave a puddle when you just stop if humidity is high enough and must be.

In short - A/C is containing a compressed gas and the #1 reason for a problem begins with a leak. Being able to find where is a skill and the fix unknown.

This is so antique for a car now and aftermarket too you really want someone who can figure it all out with what's there for a fix or yet another whole aftermarket A/C set up if you must have working A/C in this,


New User

Jun 7, 2017, 7:13 AM

Post #4 of 7 (694 views)
Re: AC shuts down after 10 miles Sign In

This is a recent problem and I haven't had a chance to check the pressures on the low and high side...

By the way, I forgot to mention...this is a fairly new installation...within the last three years and we're using R-134.

New User

Jun 7, 2017, 7:15 AM

Post #5 of 7 (692 views)
Re: AC shuts down after 10 miles Sign In

So the compressor continuing to run without loading down the engine is common?

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jun 7, 2017, 7:47 AM

Post #6 of 7 (686 views)
Re: AC shuts down after 10 miles Sign In

OK - newer aftermarket fitted in. It's still some guessing for capacity so you would use pressure when knowing compressor is engaged. You basically said you knew nothing about it so left it at that. Sorry.

You need a gauge set and thermometers at least to begin.
If not I'll just bet you have a TXV set up and it's low such that it works but is evaporating before the expansion valve causing frost you can see now if you just pull over when it quits blowing cool air you might see that on hoses.

It could have a sight glass to see the charge of 134a is close or correct. You should just look up what sight glass should look like (clear) and the forms it takes when low but partially working should show foamy when low as it's already evaporating.

What can't be known so easily is if charge is correct with pressures at a raised RPM in conjunction with the ambient temp if it failed almost certainly a leak the charge is off of the best area to be in.

So again you'd see frost under the hood even on lines. That's some evidence so combine that with pressures the expansion valve could be adjustable and set wrong for the conditions you are in. Can't know what you have for that aftermarket?

Then set it to a flow that can't allow a freeze up if your pressures show close you would be tweaking up or down some to hit the right spot to work at all for the widest assorted weather you are likely to use this.

I don't think this car even had A/C available new so it's a creation with a lot of thought to fit with A/C from the beginning.

Said - leaks are the most common reason for performance problems but need to know it's engaged and pressures as well. Finding a leak is it's own trick. Dye can help, sniffers just for that and even soapy water on connections of lines especially.

Aftermarket? How do hoses go thru firewall? Are they rubbing there that you can see. Oil evidence is also a clue to where a leak is that couldn't be from something else.

The most important question is do you want to buy stuff to do this at all which will add up for the least amount of stuff possible to do some checks. What needs fixing remains unknown as you said it can work and quite blowing cool air such a strong clue of a low charge. Just adding more is not the fix and too much a disaster so beware.

It's one of many things of automotive work that doing something wrong sets you back much worse off than not touching it yourself so any of it AYOR if you take it on.

Up top is charging procedures but that whole read is for a system already known leak free and operable.

What do you wish to do. It's a lot to understand what is happening where from not understanding it at all,


Veteran / Moderator
Sidom profile image

Jun 12, 2017, 12:13 PM

Post #7 of 7 (639 views)
Re: AC shuts down after 10 miles Sign In

No you should notice a difference with the a/c engaged....
You are going to need to get some pressure reading for this problem.....Ideally when the system is working and when the problem is happening...also the outside temps when you are taking the readings....
If the readings are equal when the problem is happening, you may be looking at a defective compressor....also since there would be no load you wouldn't notice the engine idle down.

Im assuming you can tell by looking at the clutch when the system is engaged and when it's not......with the engine running...and the clutch IS engaged when the problem is happening....

(This post was edited by Sidom on Jun 12, 2017, 12:15 PM)

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