Hello everyone

I recently retrofitted an A/C system to my non-A/C 1989 F-150. I sourced the parts from a 1995 F-150, since I wanted a R134a system. It appears that in '95, they had two pressure switches for the A/C, one "Cycling Pressure Switch", which is closed above some minimum pressure, and one "High Pressure Cut Out Switch", which is closed as long as it is below a certain maximum pressure. (This information is from wiring diagrams I found online.) My '89 didn't have provisions in the wiring harness for two pressure switches, only for one. So I followed the wiring diagrams I found online and wired both pressure switches in series with each other and the compressor.

The system works fine and blows cold. However, the compressor seems to be always on when the A/C is selected on the heater control module. I've had it idling for 5 minutes, and the compressor never cycled off during that time.

Is this normal? If not, how often would the A/C compressor normally cycle on/off over the course of 5 minutes of idling?

Also question 2: Is it normal to hear some hissing from the accumulator after you shut the engine off? I've read online that this may just be the sound of pressures equalizing inside the system as it blows through the orifice tube. However, if it was a leak, it might explain why the high pressure switch never triggers the compressor to shut off (if there's a leak, not enough pressure could build is my theory).

Thanks in advance,
André

 

Normal to hear hissing system off. Pressures balancing.

5min at idle with what ambient temps and blower speed? Not much info given. Down south even on low speed fan you pretty much stay engaged at idle b

 

That's a relief!

Blower on high, temperature today here was about 70 F. How does the blower setting affect the A/C cycling? It's not directly wired to the compressor clutch in any of the diagrams I saw.

 

The higher the blower fan speed, the more heat the system is removing from the cabin. So when on low, it cycles easier, especially when shaft speed on compressor is up and more airflow goes over condenser.

I would just bypass the high side switch and verify the low side is fine.

How much gas is in it? What orifice tube?

 

That makes a lot of sense. Thanks for the explanation. I will test and see if it cycles off at a lower blower speed.

Is there no risk in bypassing the high side switch with it potentially building up too much pressure? Just to be clear my system seems fully functional, it blows cold, it just never seems to cylce off. If there's nothing to worry about here I'll just leave it as-is.

I had an AC shop evacuate and fill it with 2 lb 6 oz R134a since I didn't have the tools myself. Orifice tube would be whatever came in a '95 F-150, I didn't change any of that.

 

Of course there is a risk, but I think it only trips over 400psi. If there, you have other issues and early trucks like yours and my '88 did not have these. So I would not stress.

2lb6oz = 38oz aka factory charge on 1994+ trucks. Should have red orifice too. Get you some ceramic tint!

If 4.9, use that seven blade fan to move most air as the five blade is weenie. 5.0/5.8 use a 302HO for most. I never had issues with stock 302 metal fan (well stock on mine) in Houston summers, but I prefer the HO fan as more airflow and less mass on the water pump.

460? No idea any upgrades. Same with 6.9/7.3 IDI. 7.3 DI of this era can do a 6.0 PSD fan.

 

Thanks for the insight! It's very much appreciated.

I was wondering why my truck only had the harness for one pressure switch ... good to know that they didn't come with the high pressure one.

I'll leave the high pressure one in-line for now, since I already made the custom wiring to have it in-line with the other one. But I won't stress about the compressor not cycling off then.

So far no-A/C to A/C is already a huge upgrade. I've got the 302 with stock metal fan. If it ever gets too hot I'll think about upgrades. Ceramic tint may cost more than the truck is worth right now but would look cool ... maybe down the line. I've already put a lot of money into this thing.

 

Post up a pic of your wiring when time.

Any money into these trucks is silly. I have $5k into an 01 Screw (incl purchase) and doubt could sell for that. But hey, everything mostly works!

 

So I tried again with the blower on low and it actually cycled on/off perfectly. Crazy how big of a difference that seems to make. Thanks!

Here's some pictures:


Wiring by the compressor. I spliced in the high pressure switch (red/yellow and green/orange wires) in-line with the black/yellow wire coming from the low pressure switch (which in turn comes from the AC switch on the dash). Also note that the bracket holding the compressor takes 3/8' bolts in my '89, but in the '95 donor vehicle they were smaller bolts. I got four 3/8 in bolts, length 5 in (I believe) from the hardware store and drilled the mounting holes on the compressor to a larger size to accommodate the larger bolts.

Same wiring by the compressor with more insulation to make it look more factory. The wires were a little short, which is why I made it in this triangle shape...

This is by the low pressure switch. The connector of my donor part and my existing wire harness connector did not match up here, that's why I crimped on these red connectors.There are two purple wires coming through the firewall, and both get 12V when the heater control on the dash is set to AC. One of the purple wires goes to the low pressure switch and comes out the other side as a black/yellow wire. I connected that to the black/yellow wire on the existing harness, which goes to the compressor. There's two wires I left disconnected: the black wire on the existing harness (goes to ground) and the other purple wire coming through the firewall (12 when AC on). I don't know what those are for on the default setup and mine doesn't seem to need it.

 

I have converted a couple trucks to R-134a and never bothered to install a high pressure switch. It will be fine without it. R-12 systems didn’t have one.