Automotive Air Conditioning Information Forum (Archives)

bettsmlb on Wed August 10, 2011 9:50 AM

Year: 1972
Make: Ford
Model: LTD
Engine Size: 351
Refrigerant Type: 134a
Ambient Temp: 90
Pressure Low: 110
Pressure High: 340
Country of Origin: United States

I removed the complete A/C system out of a 1994 Ford F150 and installed into my 1972 Ford LTD. The system is a factory 134a system and the evaporator fit nicely into the old 1972 case.

I had to fabricate the mounts for the condenser and it sits about 3 inches in front of (away from) the radiator. The system is using the standard low pressure switch which is mounted on the accumulator, a variable valve orifice, and the high pressure cutout switch on the compressor. I have appx 2 pounds of 134a and 7oz of oil per Ford specs on the 1994 F150.

What I see with my gauges is that at initial startup of the car with the A/C on, no radiator fans running yet (dual electric fans on independent thermostats), low side pressure goes into the 100s and the high pressure goes so high that it hits the high pressure cutoff switch which then allows the pressures to drop. This goes on for a while until one of the fans starts then the pressures will come down to 40s on the low and 200s on the high which I believe are correct for the ambient conditions. Vent temps are good too at this point. I'm in Georgia and right now it is hot and humid (testing in sun and 90s).

Does this seem correct or usual? I'm having trouble believing the condenser could heat up that much that quickly to be the problem. Maybe it is too far away from the radiator? Should I have one of the two fans start up when the A/C is in the on position regardless of engine coolant temp?

Thank you for any help!

Cussboy on Wed August 10, 2011 10:03 AM

Quote
Originally posted by: bettsmlb
I'm having trouble believing the condenser could heat up that much that quickly to be the problem. Should I have one of the two fans start up when the A/C is in the on position regardless of engine coolant temp?

I would. I might wire both to run when the AC is on. Can't have too much condenser cooling....

GM Tech on Wed August 10, 2011 10:26 AM

condensers need air immediately upon a/c command--- wire your condenser fans through a relay to come on whenever a/c is requested as mentioned above...

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

DaveMcKenz on Wed August 10, 2011 10:33 AM

I have heard that at highway speeds, when you have plenty of ram airflow, you don't need the fans. In fact they act as an obstruction even when turning.
Any experience? Thats why I wire mine to temp or pressure.
Dave

mk378 on Wed August 10, 2011 11:54 AM

A lot of cars run the fans all the time the compressor is on, other than unnecessary wear on the fans when on the highway, it is no problem.

You can get switches that cut in around 240 psi to control fans by pressure.

bohica2xo on Wed August 10, 2011 12:22 PM

Bett:

You need to start both fans as soon as the compressor clutch hits.

You also should fix the edge gap between the condensor & the radiator. Spacing between the components does not matter, as long as no air can sneak past the condensor on the way to the radiator & fans. All of the air the fans move should be pulled over the condensor before it goes over the radiator.


Dave:
That is a terrible old wives tale. A running fan can only offer less resistance than a stationary one. Run the fans. Waiting for a condensor to get hot, or head pressures to go high just adds to the system load. Start the fan as soon as the clutch hits.

B.

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"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.

Edited: Wed August 10, 2011 at 12:23 PM by bohica2xo

DaveMcKenz on Wed August 10, 2011 12:51 PM

Thanks B,
If the clutch cycles the fans will cycle too. If it's off the HPCO switch that could be a problem. I have a temp probe on my condenser outlet tube, which turns on the fans. I think I will just lower the on point to something like 90 degrees F. That will keep the fans running even if the clutch turns off. Does that sound OK?
Dave

Edited: Wed August 10, 2011 at 12:55 PM by DaveMcKenz

bettsmlb on Wed August 10, 2011 1:02 PM

Thank you for the info. I'll wire the fans to the A/C on 12V and run them all the time with the A/C on. thanks again!

bohica2xo on Wed August 10, 2011 2:28 PM

If you depend on the electric fans as the only airflow across the heat exchanger stack, then two relays should be used. One relay should be triggered by the compressor clutch circuit. The second relay can be triggered by whatever system you want to control radiator airflow when the A/C system is not running.

Two relays can switch the fans on for either event, without backfeeding any control circuit. If you have a temp probe for the fans it can be used to control engine temperature independently of the A/C system.

If your electric fan(s) are only for added airflow to boost the mechanical fan during A/C use, then switch them from the compressor clutch circuit.

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.

bettsmlb on Wed August 10, 2011 3:39 PM

I have two electric fans as the only air flow devices. Temp probes are used to start each fan independently. What I will do is connect the A/C on 12V circuit to one of the fans using a diode to prevent back flow in the circuit. Both fans will be controlled by engine coolant temp via the probes but one of the two will always come on when the A/C is on.

Airstream on Wed August 10, 2011 4:10 PM

Quote
Originally posted by: bettsmlb
... using a diode to prevent back flow in the circuit ...
I would like to get opinions on your approach.

Using a diode is EXACTLY what I would have thought is the right thing. But a diode will impose a roughly 0.7 volt loss on the load. If the load is a relay controlling the fan's operation, then there is no significant current draw to worry about.

But if the load is the fan motor itself, then the motor will draw more current because of the reduced voltage. More current means more heat; more heat may mean shorter motor life.

Are diodes typically used in times like these? I'm running 325 psig nominal on the high-side, and am trying to decide how my auxiliary fan should be controlled.

Tom

bettsmlb on Wed August 10, 2011 4:42 PM

The line with the diode will be to the relay that controls the fan so it should operate fine.

bohica2xo on Wed August 10, 2011 6:59 PM

A diode or two on a relay is a good thing.

I hesitate to recommend it on here sometimes, because it can degenerate into a stack of posts about current flow through a diode... especially when we begin to discuss reverse suppression diodes across inductive loads like clutches. since you know your way around a diode, I do recommend something like an MR854 across a clutch coil - it does extend contact life.

One relay is all you really need. Parallel relays work too, and can be wired as discrete systems with a common load. The only small advantage is a bit of redundancy with two relays.

-------------------------
"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.

Airstream on Wed August 10, 2011 8:12 PM

Quote
Originally posted by: bohica2xo
A diode or two on a relay is a good thing...
I agree. It cuts down on arcing across the contacts.

But a diode controlling power to the circuit is what I was hoping to find out more about. A relay appears best, but I did not know what everyone was doing.

Tom

DaveMcKenz on Wed August 10, 2011 9:07 PM

Hi Tom,
A relay is a switch. It requires a signal, either 12 V+, or ground. That signal switches 12 V+ or ground from a power source to a load such as a fan. The power source should include a fuse before the load. As an example, using the 12 V+ of the clutch wire to switch 12 V+ from a fused wire off the battery, the hook-up would be as follows:
30: 12 V+ fused
87a: fan + terminal
85: ground
86: 12 V+ clutch wire
fan - terminal to ground
These numbers appear on the relay itself and is the most common Bosch type relay, referred to as a "53" or SPST or SPDT type.
The diode may be included in the relay for a little extra money, and it helps protect any ECU from voltage spikes.
Good luck,
Dave

DaveMcKenz on Wed August 10, 2011 9:53 PM

Bohica,
You are so smart! I rigged my fans to come on at 85 degree condenser outlet temp. They are essentially on whenever the a/c is on in Summer. I noticed an immediate vent temp improvement. Yes, I drive around with a small calibrated meat thermometer in my center vent.
Thanks,
Dave