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Bypass temp control resistor '87 Suzuki

mzerega on Sun September 14, 2014 3:12 PM User is offline

Year: 1987
Make: Suzuki
Model: Samurai
Engine Size: 1.6L
Refrigerant Type: 134a
Ambient Temp: 75
Pressure Low: 25
Pressure High: 225
Country of Origin: United States

Greetings from San Diego CA. I'm considering bypassing the factory "temperature control resistor" on my '87 Samurai with a fully rebuilt 134a system. The compressor turns off what appears to be far too early, and everything in the system is either new or thoroughly cleaned externally and internally -- including a brand new expansion valve....compressor, hoses, evaporator core..etc...etc..

State of the system - basic operation, on this 86-degrees, 53% humidity day:
Low side: 25, High side 225 at about 1,500 engine rpm...and 20 low side, 200 low side at idle.

The problem: when driving around town...right about the point the cabin starts to feel nice and cool, and dry...the compressor cycles off and the air from the vents starts to feel warm and muggy about 30 seconds later. Then, about a minute later...when it starts feeling uncomfortable in the cabin, the compressor clutch engages again...and things start to cool down; the system -- although it seems entirely capable -- never keeps a constant, cool, dry cabin!

The thing that makes me suspicious is the fact that pushing the "COOL" dash lever hard to the left makes a relay inside the Denso logic box click on/off at fairly high frequency. When pressure on the lever is released, the relay stops clicking...and the system starts functioning like I explained above.

My theory is that the "temperature control resistor" is not operating properly. My understanding is this device duplicates the function of the expansion valve -- that it's an electrically-controlled means of keeping the evaporator from freezing up...yes?

Since the expansion valve is brand new, and everything else is new or thoroughly cleaned... why not remove the resistor from the system, by bypassing it entirely. I'll let the expansion valve do its job. If I were to do this, what is the appropriate method? connect to the two wires the currently go to the resistor, removing all resistance....or do I have to put a resistor in there with some specific value? In other words, do these Denso boxes want to see zero ohms in order to keep the compressor clutch engaged?

Any help here would be greatly appreciated. I love this little truck, and the new AC system seems like it "wants" to operate properly!

- Matt Z

Edited: Sun September 14, 2014 at 3:27 PM by mzerega

mk378 on Sun September 14, 2014 3:19 PM User is offline

First take the amplifier apart and resolder all the solder points on the circuit board-- the solder can crack and cause bad connections leading to improper operation.

There may be an adjustment inside the amplifier for how cold 'cold' is. You could also experiment with resistors in parallel with the thermistor. That will make the amplifier think the evaporator is warmer than it really is. It looks like the switch on the control just shorts out the resistor and makes it zero at full cold. You can test that with an ohm meter.

Get a good mechanical thermometer (not an IR gun) and check what your vent temperature actually is. Also make sure the heater is being fully shut down when you move the slider to cold.

A common misconception is that a TXV is intended to regulate the evaporator to a specific temperature. It does not. It tries to make the evaporator as cold as possible consistent with the low side pressure. If the stuff under the hood is working well, the evaporator will keep cooling to below the freezing point. That will cause the condensed water to freeze on the fins instead of running off and block the airflow, and the A/C stops working. Thus the thermistor and amplifier to cycle off the compressor just before that happens.

Edited: Sun September 14, 2014 at 3:22 PM by mk378

mzerega on Sun September 14, 2014 3:51 PM User is offline

I did open the Denso box, because it looked 'grungy' on the outside. I just wanted to see what was in there, and clean anything that looked dirty or corroded. Going from memory only, the only thing I saw in there that isn't solid-state is one relay, and the contacts looked shinny and clean. If I understand what you're suggesting...the "temperature control resistor and micro switch" is simply full "ON" when in the full COOL position, which makes intuitive sense. In other words, the resistor component (installed in the dash) is no longer relevant with the lever at full COOL. (?) Assuming this is so, and since the "TXV" doesn't regulate temp... then the "thermistor" must be inside the evaporator core, yes? And if the dash-mounted device is simply an on/off switch in full COOL position, then *only* the thermistor resistance value is the only thing being considered by the logic box - for regulating temp/clutch cycling -- yes? If this is so... it's starting to sound like I'll have to evacuate the system, remove the evaporator assembly, and start fiddling with the thermistor. Does this sound right? (bummer)

- Matt Z

94RX-7 on Sun September 14, 2014 7:56 PM User is offline

then the "thermistor" must be inside the evaporator core, yes?

More than likely, yes.

And if the dash-mounted device is simply an on/off switch in full COOL position, then *only* the thermistor resistance value is the only thing being considered by the logic box - for regulating temp/clutch cycling -- yes?

The dash mounted lever device doo-hickey (LDD-H) sure looks like a potentiometer (squiggly line with an arrow across it) in the wiring diagram. I suspect the logic box considers both the resistance of the LDD-H and thermistor to determine when to cycle. Since both of these are resistance based, I would suggest you pull apart all the electrical connectors you can find and make sure the pins are flawlessly clean. Dirty connectors = higher resistance which will throw off the behavior of the system.

mk378 on Mon September 15, 2014 9:09 AM User is offline

You don't need to remove the thermistor. You can fool the amplifier into getting different readings and making the compressor stay on longer by connecting resistors into the thermistor circuit anywhere between the thermistor and the amplifier.

AC_Doc on Mon September 15, 2014 4:33 PM User is offline

Sometimes you must accept things at faith value!

AC_Doc on Mon September 15, 2014 4:42 PM User is offline

To: mk378,

Thermistors are NTC (negative Temperature Coefficient) devices.
As the temp goes DOWN, the resistance goes UP.
So, the thermistor resistance needs to be LESS to indicate the evaporator is warmer.
How does adding resistance into the evaporator thermistor circuit make that happen?


Sometimes you must accept things at faith value!

mk378 on Mon September 15, 2014 6:55 PM User is offline

Parallel connection, not series. The combined resistance is going to be less than either one alone.

Combined resistance = (R1 * R2) / (R1 + R2)

Since you only need the resistance to come out as expected at one temperature, the non-linear nature of this approach is not a big problem.

mzerega on Tue September 16, 2014 11:45 PM User is offline

I do understand the series vs. parallel principal associated with using resistors...and thanks very much for the suggestion. I will mess around with try to get the "amplifier" to maintain a lower temp. Note that over the last few days -- driving around with a digital thermometer stuck in the dash -- that it appears my heater core gets hot a few minutes after I start driving. In other words, it seems that the brand-new heater valve allows enough hot water through that the heater core gets hot....heating up the nice, cold, dry air that the evap is making!!

Since the valve is new... and the cable is adjusted to 'full close' on the valve.... I'm going to install a manual shutoff valve on the *outlet* side of the core (my theory is that hot water [heat] is finding it's way in via the outlet side of the core). Hopefully, after installation of the manual shutoff... the heater core will stay cool.. and the cool, dry air from the evaporator will stay that way!!!

I'll provide an update after I do the test.

Thanks for the attention and suggestions. Your/everyones' expertise is very much appreciated.

- Matt Z

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