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Suction line Frosting and Pressure check

Merc450 on Wed April 16, 2008 3:00 PM User is offline

Year: 1979
Make: Mercedes
Model: 450SE
Engine Size: 4.5
Refrigerant Type: R134
Ambient Temp: 78
Pressure Low: 30
Pressure High: 140
Country of Origin: United States

Just retrofitted with 134a Nippondenso compressor and Large parallel Flow condenser with big fans. Changed TXV. receiver/dryer and flushed system and pulled a vacuum for 25 minutes, added oil and started filling gradually to allow for system stabilization and measuring temperature at vents.

At Idle 750RPM

started adding 134a little bit at time till LP25 and high of 140. Vent temperature came down to 50F.

@ 1400 RPM LP 16 HP 150 Temp vent Temp. down to 45F stable for 10 minutes suction line frosted all the way to compressor, at this point auxiliary fans kick in, most likely not by true gas temperature but by engine temp since I had to relocate reciever dryer with attached Gas Temp. sensor inside engine bay, sensor temp activates auxiliary fans @125.6F.
at this point I got:
LP16 hp135 vent temp down to 41F, still suction line white frosted.
I decided that was enough for the day.

Today, same operating and ambient Temp,
Static pressure 79
@ 750 RPM. I thought someone on this great forum would suggest that system was undercharged, I decided to add some 134a. little at a time and stand for stabilizing , LP30 HP180 but vent temp. started climbing gradually to 55F no frosting.

@1400RPM LP19-20 Hp155 vent Temp.46F Frosting suction line.
auxiliary fan kicks in, LP stable , HP140, vent temp 45F.

Please help with these results.
Should I worry about suction line frosting ?
I am surprised at these very low operating pressures and result. which are better than operating parameters of an R-12 specification for this car.
I did not have the time to check the TXV on my unit to see if its frosting or if the cappilary is closely isolated to the return line.

Any suggestion


Edited: Wed April 16, 2008 at 3:02 PM by Merc450

TRB on Wed April 16, 2008 3:17 PM User is offlineView users profile

How much refrigerant do you have in the system?


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Merc450 on Wed April 16, 2008 3:39 PM User is offline

I really can not tell, but I am sure its just a little bit over charged. The process of filling was really gradually and I gave enough time, two hours, for filling to what I think was optimal at the vents. I am very sure that adding more will increase low and high pressures, but vent temp will also go up. I would guess to say that it is charged at least 30% less than what Would be charged when I had the A6 compressor and R-12. that required a charge of 2.2 lb.

thanks for your help.

TRB on Wed April 16, 2008 3:55 PM User is offlineView users profile

Looks under charged to me. If you have the good PF condenser you should be able to charge the system to the OEM specifications.


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Merc450 on Wed April 16, 2008 4:51 PM User is offline

Since I tested the system with all windows open, would you think that if the capillary of the expansion valve is not attached or insulated to the evaporator return line, thus expansion valve is open or stuck open, that I could have the same symptoms ?.

If I add more gas gradually, and bring pressures up to 25 LS and 170 HS @1400 RPM but notice a drastic increase in temp vent, can I revert to previous status by freeing some gas out ?

thanks again

TRB on Wed April 16, 2008 4:58 PM User is offlineView users profile

Capillary tube has to be attached to the suction line at the evap covered for the valve to work correctly.


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Merc450 on Tue April 13, 2010 6:06 AM User is offline

Well It was a defective TXV, that caused this problem.

mk378 on Tue April 13, 2010 5:56 PM User is offline

Frosting of the line back to the compressor means that the whole evaporator is also that cold. Which is a good sign.

Realize that a TXV is not designed to regulate the evaporator to any particular temperature. If the system is really cranking the BTU's, the evaporator will get cold enough to frost up. Then the airflow is blocked and the unit becomes ineffective. There needs to be another control (usually a thermal switch on the evaporator) to stop the compressor before that happens.

Edited: Tue April 13, 2010 at 5:57 PM by mk378

Merc450 on Wed April 14, 2010 4:46 AM User is offline

Thank you MK378 for your reply but I have to differ with you. Frosting Does not mean the whole evaporator is that cold. I believe in my case, the TXV was partially blocked, So in medium to high demand the TXV is letting , lets say, half of the amount needed to cool all of the evaporator as rated. Suction is taking place with limited amount of refrigerant in the evaporator, which pulled the pressure down to 10 PSI at high RPM. My compressor never cycled at medium to high demand, it only cycled when ambient is around 65 F and on low fan speed. I monitored the vent temp and the thermo switch cuts out at 35F and cuts in at 44 just like it supposed to do.

Frosting of low side could also happen at low charge and thats why TRB suggested that its undercharged, but When I attempted to very gradually add 134a, as I stated before, vent temp started to go up. This suggested that adding more is becoming burden on the system on the high side and not helping on the low side because of the partially blocked TXV.

Edited: Wed April 14, 2010 at 5:05 AM by Merc450

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