I want to make sure so I don't make a costly error: on a 1990 F-150 XLT Lariat 5.0L, is the low side A/C fitting the larger fitting near the firewall on the passenger's side, and the high side the smaller near the front on the driver's side? I know the refill will only fit the proper fitting, but I hear too many people saying the low-side is always the smaller one, and it kind of confused me.

 

Well on a 1990 they were both the same size the last time I looked.
But yes the one by the big black round can looking thing on the passenger side safety wall and by the heater box is the low side.
This if the system is still R-12 and has NOT been converted to R134a.

 

Whoops, I forgot to mention that — it was converted to R-134a many years before I had it. It has threaded valves and not quick disconnects, but yes, the valve near the firewall is the larger one.

 

This is not right I don't think (man I know I am out on a limb here disagreeing with subford). Manufacturers starting using a smaller fitting on the high-side in the late 70s to prevent shade-tree "mechanics" from buying the little cheap fill-up rigs from KMart and then having the can blow up in their hands because they put it on the wrong port. I *know* GM did this but my truck has been converted to R134a for so long I don't remember for 100% sure. I will look at my Bronco tonite to make sure, it still has the old fittings on it.

You have it right, DocSavage, the bigger port over by the firewall is the low-side. It is attached to a big black cylinder called the accumulator. The smaller fitting over close to the radiator on the driver's side is the high-side. Whoever is telling you the small fitting is the low-side is wrong.

 

Thanks very much!

 

When I bought my first R12 gauge set in 1967 for my 64 Buick they had the same size on the high and low side.
I have now used this same gauge set on my 1967 Pontiac, 1971 Ford SW, 1982 E350, 1983 F150, 1985 E250, 1986 Bronco II, 1988 E150 & 1989 E150 and both high and low sides fit.
So I was guessing the 1990 was the same.

I did hear of a Ford with a different size so I bought the adapter for them back in the 70's but never found one to use it on, still have it.
My Snap-On R-12 recovery machine also has the same size hose fitting for the high and low sides.

If you do not have quick disconnects on a converted system then somebody did not do the job of converting right or it has some other refrigerant in it..
It sounds like your system was converted to some other refrigerant than R134a.
It needs to be recovered, pumped down and the proper conversion done.
Their should have been a place card installed under the hood telling what was put in your system.

 

It's funny you say that because I was gonna tell a similar story, my Robinair gauges I bought in 1979 came with an adaptor for the high side. I still have the gauges and I leave it screwed in there because I don't ever encounter an r12 system that doesn't have the smaller high-side fitting. I checked my 91 Bronco and it has the smaller fitting on the high side. So the question is when did Ford start using this? I used to have a 90 F150 but I honestly don't remember whether it did or didn't. I know GM made this change in the late 70s, tho. My 77 Can Am has the smaller fitting on the high side.

 

Interdynamics used to sell an adapter called AD1.
3/16 inch to 1/4 inch adapter.
Fit GM 76 and up and Ford 81 and up.