Automotive Air Conditioning Information Forum

Since Arizona has already had a few 100F days, I noticed that the AC in my 1998 Frontier wasn't as cold as it had been, and the AC compressor clutch was cycling way more than it had in the previous 20 years (I have a dashboard light in parallel to the AC clutch so I can monitor this). So today I went to read its pressures, and found the high side service valve cap was loose, and a piece stuck inside the recess. I unscrewed that broken part, and I had an R134a high side cap that did fit. So I added 10 oz. R134a, and now the AC behaves as it should.

Photo of broken cap, I have removed the rubber seal.


But I also had several pink/red R134a high side valves that looked similar, but they wouldn't screw in. So what gives? Are there slight differences in the protruding male end inside the cap? And if so, why so different????

Apparently, there are differences/two types of red high side R134a caps and also differences/two types of blue low side caps, see the photo below from a replacement cap assortment, see the differences. But I don't know what the internal differences are - anyone know ???

Cusser, I believe that auto manufacturers make their caps with proprietary threads and size differences inside because I've had the same problem as you with replacements not threading on. Why they do that is anyone's guess. One would think that the caps would be standard across the industry with only size and color differences between the high and low sides, but apparently not.

By way of comparison, the photos (which apparently didn't post; I'll try posting them again) show OEM caps taken from my '23 4Runner, which is an R-1234yf vehicle. Notice that both caps are black, the smaller one being from the low side and the larger one being from the high side. Notice also that ridges are molded into the outside surfaces of each cap, ostensibly to enhance gripping them. But I discovered that a 17mm socket fits perfectly onto the smaller cap, while an 18mm socket fits perfectly onto the larger cap, both sockets being of the 12-point variety.

Curiously, my 4Runner also has a viscous-drive fan instead of the more typical electric fan(s).

JohnHere wrote: ↑Tue May 07, 2024 10:46 am Cusser, I believe that auto manufacturers make their caps with proprietary threads and size differences inside because I've had the same problem as you with replacements not threading on. Why they do that is anyone's guess. One would think that the caps would be standard across the industry with only size and color differences between the high and low sides, but apparently not.

Thank you SO MUCH, John; really helps to know others more knowledgeable and experienced than I have encountered this themselves.

I thought I was going nuts - which isn't a really long path for me !!!!

M8 & M10 on the red caps.

Cusser, Here are two out of the five photos that failed to post with my earlier comments. Might have been too many at once. Three more to go

And the three remaining photos.

I set a limit per post. Some like to post a ton of images for their essays.

Thanks, Tim. I thought that might be the reason.

My 1998 Frontier uses a red high side cap and a blue low side cap, no H or L on them. They look pretty much like the ones shown here.


My 2004 Frontier with essentially same engine, transmission, and AC has two black caps of same shape as the red and blue ones. Both Frontiers were purchased used, 20 and 13 years ago respectively.


I do have some of the shorter light blue caps that do have H or L on them, from the parts yard.