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Dodge Ram Radiator Fan Cycling to Keep High Side between 270 and 310

94RX-7 on Mon May 12, 2008 5:04 PM User is offline

Year: 2005
Make: Dodge
Model: Ram SRT-10
Engine Size: 8.3l
Refrigerant Type: R-134a
Ambient Temp: 80
Pressure Low: 35-40
Pressure High: 270-310
Country of Origin: United States

Today, out of curiosity, I hooked my gauges up to my SRT-10 to see what the pressures looked like since the performance of the system has always been mediocre when sitting still or moving along through traffic slowly. Things seemed OK except for the fact that the hydraulically driven radiator fan is being cycled by the PCM to keep the high side pressure between 270 and 310 psi. This seems awful high to me. Hosing the condenser down brought the high side down to around 140. It would seem to me that improving the airflow through the condenser would help improve cooling performance. Is this a fair assumption, or will I be still be hampered by the amount of BTUs the compressor can put out at low speed?

TRB on Mon May 12, 2008 6:11 PM User is offlineView users profile

I doubt you will ever see 140PSI on the high side. If you could get it down to around 210PSI I think your cooling would be fine.

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94RX-7 on Mon May 12, 2008 6:58 PM User is offline

Maybe I misread the high side pressure while I was hosing it down...I was standing a little ways back with the water hose trying to read it. Wouldn't surprise me if I was 10 or 20 psi off.

TRB on Mon May 12, 2008 7:03 PM User is offlineView users profile

Sorry I missed you were wetting down the condenser. You can get 140PSi while the condenser is wet. Doubt you would see 140PSI just by improving air flow is the point I was trying to make.

-------------------------

When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com

bohica2xo on Tue May 13, 2008 2:39 AM User is offline

Well, the bad news is your system is well within specification.

@ 80f ambient, the center vent temp is 55f. The high side range is 250 to 350 psi.

@100f ambient, the center vent is 64f. The high side range is 350 to 430 psi.


The pressure transducer normal range is 0.451 volts to 4.519 volts. Anything outside of that range will shut the compressor down. Someplace on that curve the PCM decides to turn the fan on. Above 4.519 volts is excessive pressure, and shutdown of the compressor occurs. Voltages below 0.451 volts indicate a temperature below 50f, (or a nearly empty system) and the compressor is inhibited.

Other than checking the calibration of the pressure transducer it looks like you are stuck with the operating paramaters of the PCM, unless you find a way to re-program it.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

94RX-7 on Tue May 13, 2008 10:43 AM User is offline

Yeah... that's what it's looking like...I'm at the mercy of the PCM unless the transducer is bad. I was reading up on the pressure transducer in the shop manual last night. Shop manual said that the fan should be triggered at, I believe it said 240 psi. I don't know if that means the fan will be called upon for a slight breeze at that head pressure, or if it should be really moving some air. This fan is one of those hydraulic jobs that runs off the power steering pump. It could also be that they forgot to edit this section of the shop manual for the SRT-10....I could see how it could be overlooked since that tidbit was buried in the middle of a paragraph discussing the transducer.

There are options for reprogramming the PCM on these, but I haven't heard of any of the flashes being able to control the fan on/off set-points based on head pressure. They can do it based on engine coolant temperature, however.

Looks like I'm going to have to ask the SRT engineers at Chrysler next time there's a chat session.

bohica2xo on Tue May 13, 2008 11:35 AM User is offline

There is a minimum flow on that fan to keep the oil moving. I believe it is 100 to 500 rpm with the engine running in the Viper. I wonder if the bleed orifice for that condition could be enlarged... Or perhaps a manual bypass valve for the control installed.

B.



-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

94RX-7 on Tue May 13, 2008 12:13 PM User is offline

So, if I were to put an electric fan on the condenser, would it be possible to control the fan operation based on the temperature of the condenser outlet line, or is that temperature pretty much constant? I'm guessing the temperature would vary since the volume is fixed, and the pressure increases. If it could be controlled based on temperature, what would an optimal temperature be?

bohica2xo on Tue May 13, 2008 8:46 PM User is offline

The condensor inlet tube is where I usually place my thermal switch for an aux. fan. I use a 70c switch, so it will come on anytime the underhood ambient goes above 160f...

You could use a higher rating, they are available in 5c steps up from 70c. I use the Cantherm F11-E06 type switch, it is rated at 220c max case temp, and will switch a 1 amp inductive load like a relay. Same size as a TO-220 transistor. The Digikey part # for 70c is 317-1019-ND.

Be sure you use a relay, and a diode across the coil to extend the switch life. I have one installed like this that is over 5 years old now, and it still works great.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

94RX-7 on Wed May 14, 2008 12:37 AM User is offline

Quote
Originally posted by: bohica2xo
The condensor inlet tube is where I usually place my thermal switch for an aux. fan.


Thanks for the info. One question....why the condenser inlet tube rather than the outlet? I would think that the outlet temperature would vary more based on airflow through the condenser. No?

bohica2xo on Wed May 14, 2008 11:49 AM User is offline

You can put the sensor in either location, but why wait for the condensor discharge to get hot? 158f is the lowest setpoint available in that switch type, and the aux fan should be enabled if the underhood ambient goes that high even with the A/C off. Attaching the switch to the compressor discharge line gets the fan going as soon as possible.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

94RX-7 on Wed May 14, 2008 1:21 PM User is offline

Quote
Originally posted by: bohica2xo
You can put the sensor in either location, but why wait for the condensor discharge to get hot? 158f is the lowest setpoint available in that switch type, and the aux fan should be enabled if the underhood ambient goes that high even with the A/C off. Attaching the switch to the compressor discharge line gets the fan going as soon as possible.

I was hoping to arrive at a setup which would only run the fan when it was absolutely needed (mainly trying to avoid it running while on the highway, getting plenty of air through the grille), and I would think that the condenser discharge temperature would be a good indicator of such. I'll have to take some temperature measurements at idle and see.

bohica2xo on Wed May 14, 2008 1:44 PM User is offline

Like I said, you can use either location. What is the issue with the fan running on the highway?

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

94RX-7 on Wed May 14, 2008 4:46 PM User is offline

Quote
Originally posted by: bohica2xo
Like I said, you can use either location. What is the issue with the fan running on the highway?


It shortens the life of the fan and the electrical system, and it causes the truck to burn slightly more fuel. It isn't a HUGE deal, I would just prefer that it not run when it isn't necessary.

oznznut on Wed May 14, 2008 8:35 PM User is offline

Let me get this right. You have an SRT10 truck, and you are concerned that an electric condenser fan will cause your fuel mileage to go down?
You have got to be kidding. The stereo probably pulls more juice than that fan.

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