A/C compressor confusion? 79 300SD

[79W116]

I'm sure for a few of you experts out there this will be an easy one, but I am stumped!

I purchased my '79 300SD about 5 months ago, and an indy dealer switched it from R12 to R134 at that time with the PPI and other work before I got it to my house. It worked fine as a quick test when i got it, but was winter of course here in Michigan.

I installed the Hamilton upgrade kit a few weekends ago and now everything works, except the compressor. Although since its been winter and I don't know if its worked in the last 3 months cause why would I have it on in 20 degree weather.

The switch on the main panel in the center console broke recently (spring housing gave way on a corner), but luckily i found out that i can just flip the front toggle upside down and flip the "on" and "off" inserts and ball and spring, and it tests fine with my multimeter.

Next, i had a separtate motorcycle battery that i tapped the leads directly to the A/C compressor and the clutch worked great everytime it got power.

From the receiver-dryer hooking the blue with green wire from the pressure switch to the blue wire from the temp switch makes the fan go on fast and constant, but not the clutch engage. and hooking up the red with green and blue (pressure switch) to the blue wire on the temp switch makes the fan go on in cycles (surging on and off) but again, no clutch engagement.

I tested both fuses that are dangling in the fuse box area, and every single fuse in that little box as well. All good.

I'm sure this is an easy one that I'm missing, please someone tell me what my next step is. I would be very appreciative.

[daantjie]

If you bridge the terminals for the pressure switch at the drier then the clutch should engage, as the pressure switch is a safety feature to not engage the clutch with a low gas charge.  There is an "ETR" switch which also locks out the compressor under a certain temp reading to prevent freezing up, but I'm guessing your current ambient is higher than the cut - out so I doubt this is your problem.

[rumb]

Probably some helpful info here:
https://handbook.w116.org/Climate/

[79W116]

Probably some helpful info here:
https://handbook.w116.org/Climate/

yep, I've been buried here for a few days for sure, just just say test everything in this order. I'm just not the best with a schematic and thier generic list. Was hoping for someone to recognize my condition.

[79W116]

If you bridge the terminals for the pressure switch at the drier then the clutch should engage, as the pressure switch is a safety feature to not engage the clutch with a low gas charge.  There is an "ETR" switch which also locks out the compressor under a certian temp reading to privent freezing up, but I'm guessing your current ambient is higehr than the cut - out so I doubt this is your problem.

Ah OK, I'll try to bridge then and see if that works. So if the clutch does engage then I'm replacing the pressure switch then, correct? Which means having a shop remove all r134, replace switch, then recharge, right?

[daantjie]

Before removing the switch I would take it to an AC shop and have them test the high and low side pressures to see if you're in range.  I would assume switch failure to be an outside chance but I would start with pressure testing first.  You may have a leak in the system which the shop can check for which would have to be tended to before anything else.

[79W116]

Well, this morning I tried jumping the pressure switch together to see if that would get the clutch to pull in, but it did not. I had the car running, full cold on the dial, A/C switch on, and Auto High.

Where is the "ETR" switch located?

[daantjie]

Unfortunately it's very hard to get to without essentially stripping out the dash.  It sits under the dash and probes the evaporator temperature with a capillary tube.

[79W116]

That is unfortunate.

[79W116]

Anyone know any good mechanics around the Howell Michigan area that know these cars?

[daantjie]

I'm not super familiar with the Hamilton unit but with the original unit if you hit DEF that should always give you compressor engagement and max heating coming out of the windshield vents. 

Might still be worth taking it to a reputable auto AC shop and let them check it out. Sometimes you get lucky and they have an oldtimer on hand that has seen it all

[benzjag]

As others have said, a low amount of refrigerant in the system will prevent the compressor from coming on. A quick unscientific check is to press the pin on the low side port where you put the refrigerant in. It will have a blue cap after R134 was used. If you get a weak or no release of refrigerant when pressing the pin, you have a low or no refrigerant in the system.

I just added refrigerant today to my 79  SD. It is very normal to have to add refrigerant UNLESS every component has been changed including hoses and  seals!

[79W116]

Thank you all for your replies.

So I ran the DEF and the compressor doesn't come on with that either.

I did press the low side shrader valve and it sounded like a car tire. So there is certainly some pressure.

My next step seems to be to find what opens the flap to mix heat and cool. It's stuck on heat only. Even with out the compressor i used to get fresh cool air, but now just solid heat, ans i dont see the hamilton servo moving.I don't hear any vacuum leaks, and when I leave the car for half a day the door locks are still strong, so I dont think that's it.