Author Topic: RPO C60 vs C61 AC system  (Read 3463 times)

Offline Henry

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RPO C60 vs C61 AC system
« on: August 16, 2018, 12:37:29 PM »
Hi:
Can anyone tell me what is the difference between the RPO C60 and C61 AC systems?
Regards,
Henry

Offline bd

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Re: RPO C60 vs C61 AC system
« Reply #1 on: August 16, 2018, 01:51:28 PM »
Look at page 50 of your parts book:
« Last Edit: August 16, 2018, 01:53:02 PM by bd »
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Henry

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Re: RPO C60 vs C61 AC system
« Reply #2 on: August 17, 2018, 10:47:46 AM »
Hi bd:
Yes, I have seen that...and it is in fact the wording of "Automatic" in C61 option that raised my curiousity...is this the AC system that has the brand name "Comfortron"? Is it a complete ac system alternative to the C60, which I believe has the brand name "Four Seasons"? in my 76 service book there is no reference to the "automatic" temp control system. I am also wondering if it was offered for the light trucks and if there was any differences in the ac components in the engine bay.
Regards,
Henry

Offline bd

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Re: RPO C60 vs C61 AC system
« Reply #3 on: August 17, 2018, 02:20:02 PM »
An educated guess, but still a guess, C61 is automatic temperature control, which I have never encountered in real life in a truck, only in El Caminos and Caballeros.  My memory of marketing terminology from that period (the mid-70's) is too dusty to be certain, but Comfortron was likely RPO C61, an option that never actually came to meaningful fruition in the light-duty truck lines, remaining restricted to the car and car-like GM models.  RPO C60 was the manual control Four Seasons version.  RPO C69 was the overhead A/C used in Suburbans.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Henry

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Re: RPO C60 vs C61 AC system
« Reply #4 on: August 19, 2018, 12:15:21 PM »
Hi bd:
That sounds logical. Do you have any knowledge of those other 3 ac options?...the C62, C65, and C68...I would guess that they may not have been options for the light trucks either in the mid-70s...later years maybe?
Regards,
Henry

Offline bd

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Re: RPO C60 vs C61 AC system
« Reply #5 on: August 19, 2018, 01:09:54 PM »
You realize, of course, you're tempting me to crawl further out on a treacherously thin limb - lol. 

Honestly, based on what I recall repairing during that period, I am surprised to see those three RPOs listed in a light truck book.  Drawing further from sketchy recollection/speculation, RPO C62 (front, floor-mounted A/C) may have been used in some delivery/utility vans whereas RPOs C65 Temp[ramental]Matic was a semi-automatic temperature control and C68 were restricted to the car lines.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Henry

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Re: RPO C60 vs C61 AC system
« Reply #6 on: August 20, 2018, 12:54:35 PM »
Hi bd;
Thanks for your thoughts and speculation on this...the list of options listed for these trucks is so large it makes me think maybe it was optimistic enthusiasm by  the GM marketing guys and in reality a lot of options just were not ordered or available for practical reasons...or maybe the pubs guys just copied all the GM options available to all vehicles in the parts manual because it took too much time to figure out which ones were only for light trucks.
Regards,
Henry

Offline 75gmck25

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Re: RPO C60 vs C61 AC system
« Reply #7 on: August 21, 2018, 07:38:58 AM »
I have the repair manual for my '75 GMC, and the terminology there is a little confusing.   My option sticker shows a C60 system in my '75 pickup truck.  However, the GM repair book says I have a Four Season system, and that the C60 is the system in the vans. It also shows pictures of the controls for both systems, and I definitely have the Four Season system.  It also explains how to fix the C69 overhead system, which is only available if the C60 system has been ordered.

My A/C converted R134a system has had a lot of modifications, but it works well.  I have the original evaporator and controls, but everything else is non-stock.  I have a parallel flow condenser, Ford blue orifice, and a Sanden compressor and mounting bracket from Vintage Air.  I had to have custom barrier lines made up to connect it all together.

During my junkyard dives I've also found some quirky things about A/C parts that may or may not interest you. 
- My '75 was the last year they used an evaporator that has a large lower fitting (#12?) for the orifice tube.  The tube has a large brass filler/plug inside, and then the small orifice tube fits into it.  Starting in '76, and through '87, the evaporator has a smaller diameter tube for the orifice (#8?) .  They also moved the low pressure switch from the evaporator fitting up to a port on the accumulator.

- This is significant during junkyard searches because you need to make sure the A/C lines and evaporator fittings match each other.  When I had my system converted to barrier lines it also complicated the conversion because the fitting size on that line was very mismatched (#12 on the evaporator end, and #8 on the condenser end).  He had to braize fittings together and make an adapter so that a #8 line size would work with a #12 fitting. 

- I also found that that where the upper evaporator fitting connects to the accumulator they changed the angle of that fitting when they went to the smaller lower fitting.  This means that the angle of the fittings on the accumulator changed to allow for the new angle of the evaporator fitting.  If you mount an older accumulator model on a new evaporator, the upper line running to the compressor may be at the wrong angle.  My accumulator is a model used on other '80's GM vehicles, but I used it because it fits the brackets, but has the right angle to line up well with my new barrier hoses.

Bruce

Offline Henry

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Re: RPO C60 vs C61 AC system
« Reply #8 on: August 21, 2018, 01:29:28 PM »
Hi Bruce:
I see what you are saying...the 74-75 service manual definitely does describe the "Four-Seasons" AC as a different AC system from the C60 system. My 76 service manual does not refer to the "Four-Seasons" or C60 AC anywhere, but does describe a lot of little differences to the "C-K Truck System" starting in 76. It does not mention any changes to the C60 system but does mention some small changes to the C69 system. So my RPO sheet says I have the C60 AC in my 76 C-20, and the control panel does look like the changed 76 control panel. You know, I think the 75 and earlier "Four-Seasons" AC for C-K trucks simply morphed into the C60 for C-K trucks in 76.

Your notes on what you have done to your system are noteworthy to me as I plan on servicing my system myself in the future. One point of annoyance is that many years ago a AC shop serviced my system and replaced the evaporator and I dont think it was the right one because the discharge pressure switch at the lower evap connection is now moved a few inches to the front of the truck with the wires as tight as banjo strings....it appears that the lower pipe from the evap core is longer than the original one I had.

Regards,
Henry