Red Tek

[DMC1890]

Thanks Brandon. Perhaps I made some mistakes here. I'm basing my determination on page N:07:01, which gives the total R12 charge as 2.5 lbs. Then I'm using the Red Tek weight equivalent chart:
RED TEK - Google Chrome_2018-09-04_11-20-32.jpg
This give me a Red Tek weight of between 13 and 14 oz.

Possibly the R12 capacity is actually only 2 lbs?

[Morpheus]

The R12 charge was reduced to 2.2lbs in one of the service bulletins, I can't remember which one at the moment.

Edit:
Nevermind, found it.
http://www.dmcnews.com/bulletins/ST-29-11.81.html

[DMC1890]

Well, that just confuses things even more! First, it says the previous spec was 2.75 lbs (not the 2.5 lbs in the manual), but then it also says that the spec is being changed with VIN 4080. So, does that mean that compressor and other component changes at >4080 prompted the update, or that it applies to all, including previous VINs. BTW, mine is 1890.

So, I called DMCH, and they say that the Red Tek charge should be 11 oz, regardless of the previous R12 charge spec. And, the only reliable way to get the right charge is to vacuum it back down and start over.

[DMC-Ron]

Well, that just confuses things even more! First, it says the previous spec was 2.75 lbs (not the 2.5 lbs in the manual), but then it also says that the spec is being changed with VIN 4080. So, does that mean that compressor and other component changes at >4080 prompted the update, or that it applies to all, including previous VINs. BTW, mine is 1890.

So, I called DMCH, and they say that the Red Tek charge should be 11 oz, regardless of the previous R12 charge spec. And, the only reliable way to get the right charge is to vacuum it back down and start over.

There seems to be a dependency in the R12 charge. As you note the Workshop Manual lists 2.5 lbs. (N:07:01) However, the DMC Technical Manual lists 2 lb. 12 oz (2.75 lbs.) on page 139. Perhaps the Technical Manual is including the oil?

Ron

[Ron]

$.02

The reason the charge was changed is because some compressors were knocking when starting the engine with the AC on. IF yours does/did this, then you would use 2.2 lbs R12. Converting R12 to 134a calls for a 10% reduction in the charge (because of higher pressures etc). So, you would use 2.0 lbs 134a. Which means you would use 12 oz = 2 cans Red Tek 12a, according to their chart.

Note:
2.75 lbs R12 => 2.475 lbs 134a => 14.85 oz Red Tek 12a
2.50 lbs R12 => 2.250 lbs 134a => 13.50 oz Red Tek 12a
2.20 lbs R12 => 2.000 lbs 134a => 12.00 oz Red Tek 12a

So, 14 oz RedTek may be too much (if yours knocks), but probably not the problem.
I would guess replacing the cycle switch with one that cuts out at 25-32 psi (as per WSM, N:09:01) instead of 40 psi would be the next step. Then confirm that the high side pressure falls within the given range when the clutch cycles out -- If it's high/knocks, reduce the charge...
Double check on a very hot day!

[David T]

If you put the correct amount of refrigerant in and it isn't cycling the way it should (assuming everything else is perfect, clean coils, etc) the low pressure switch CAN be adjusted by turning the screw in between the two electrical terminals. Ron is correct, under some unusual conditions you can get knocking so DMC's 'fix" was to reduce the amount of refrigerant but that reduces the A/C's cooling capacity. Refer to St-29-11/81 where the charge was reduced from 2.75 lbs to 2.2 lbs if knocking noises are observed during starting with the A/C on!. You can take that to mean always use 2.2 lbs or use 2.75 if you can as long as you don't hear knocking. Regular and consistent knocking will damage the internals of the compressor so you don't want to do that. DMC reduced the factory charge to 2.2 lbs after vin 4080. The label on the compressor says 2.5 lbs.

[Ron]

...His is ~30% off, so if it was factory set, I'd pitch it.
(Old switches are often flaky after adjusting them anyway....Random -5 to +10.)

[DMC-81]

After the comments of RedTek being flammable, I thought I'd share this warning about R134-a that was on the instructions for the OEM spec AC compressor for my daily driver. I had a chuckle about this.

image.jpg

I guess I'll have to be extra careful when I swap this out!

[David T]

While R-12 isn't flammable, if you oxidize it in a flame you create Phosgene gas. Very deadly. Propane is a VERY good refrigerant too but it is also flammable.

[lazabby]

When using Redrek, what should be the high and low pressures on the gauges?