1990 240sx: Intermittent AC.

[RamonetB]

Hey guys,

I'm still trying to fix my intermittent AC from over a year ago. Same problem still exists. SOmetimes it comes on, sometimes it doesn't. Also, sometimes the AC light comes on or doesn't come on. This is not indicative, however, of when the AC will actually turn on.

I don't have a leak, as my pressure in the lines remains the same. When the AC is on, it isn't terribly cold but cool. Sometimes, while driving, it'll simply stop working. Other times I can get it to come on by turn the the fan off, press the AC button to on, and then turn the fan back on. Then it works. This, too, however, is not consistent.

I've checked the relays and they all work.

So, I was wondering, aside from possibly having to replace the whole system, what about the Thermo Control Amplifier and Thermistor on the condenser by the glove box? Would a faulty/malfunctioning one of these cause many of these symptoms?

-kirk

[RamonetB]

And why I can't seem to find a place to purchase this part from, at least under the name of "Thermal Control Amplifier" and similar variations?

Is this something only available from the dealer?

[boriquaS13]

Could be your a/c compressor clutch is going bad, also does your condensor fan (electric one ) run when a/c is on?

[RamonetB]

The condensor fan does run when the AC turns on. As for the clutch on the compressor, it engages without fail every time I "jump it" forcing it on. I assume if the clutch was going bad, even doing this would mean intermittent engaging?

I'm ready to take it to my mechanic (a great guy whom I trust) but the part of me that feels if I can fix this I should is hesitating.

[ChuckNC]

Ah, what timing. I was trying to find my earlier post on this subject to update it when I came across yours.

I have had the same problem for about a year and a half now. I narrowed the problem to the thermo control amplifier behind the glove box, but I thought the problem was with the wiring harness, so I would "jiggle" the wires by sticking my hand under the dash until the compressor came on. After this fix started to fail, I found that the problem was actually with the circuit board. Take the box out, open it up from the back, remove the circuit board and the two screws holding the plastic wiring harness receptacle. If the receptacle is loose, then your three solder joints have come loose. If you are handy with the soldering iron, the problem is easily fixed.

[RamonetB]

Oooh. Now this possibility I like! I'm very handy with a soldering iron. I'll look into it promptly.

BTW: Is the thermo control amplifier visible behind the glove box or is it inside the airbox / condensor housing?

-kirk

[RamonetB]

So, I've been able to remove the amplifier box and do some repair work on it.The three solder points for the wire harness where indeed broken. This was quickly remedied.

however, I also noticed some heat damage to the solder points near the thermistor wires and the adjacent resistor. I resoldered these points as well. Problem is, the resistor seems to be bad:

The bands of color on the resistor appear to be (appear, since, being an old resistor, the colors have faded somewhat) red, orange, violet, yellow, brown / gold. If I'm correct, this means this resistor is a 2.37 Mega Ohm resistor. A quick test on the desoldered resistor gives me a reading of 14.7k Ohms. The coloring bands for this resistance are nowhere near similar to the ones I found.

This tells me the resistor is bad.

I've replaced it with a 2.3M ohm resistor. Not exact, but far better than what was in there. The AC now turns on all the time (so far) but turns off after a few seconds. Then, after a few seconds (6 - 15 depending on fan speed) the AC turns back on, as if cycling. This is VERY repeatable.

Three questions:Can anyone confirm the resistance of this resistor? Perhaps you still have your old amp laying around or a spec sheet or..?

Is this cycling supposed to be what happens if I need a recharge? Would it be this predictable?

Can anyone point me in the diretion of a place to buy a new AMP? The dealership here wants $75 for it. I'm not fond of the price when I've heard of others getting it far cheaper.

Thanks for reading!

[SamLittle]

Thermo Control Amplifier is an Evaporator Freeze Protection Device

From what I gather, it is a device that protects the evaporator in Nissan and some other automotive AC systems from freezing up. The device is a transistor switch component driven by voltage or current derived from an integral thermistor.

Colder/Freezing temperatures cause the circuit to open and through the ECM and Air Conditioner Relay stop the compressor.

The unit used by Nissan works as follows:As the temperature is decreasing the circuit opens around 38 to 37° F to turn off the compressor. There is a one to two degree hysteresis in the unit to prevent rapid cycling of the compressor. The device closes the circuit when the temperature reaches 39 to 41°F.

Depending on the current requirement it looks like you could make one out of a surplus thermistor, a pot, a LM 239 and a couple of other low cost components.

-Sam

[ChuckNC]

I've seen this part left on many a car at the junk yard because it's easy to see and remove since most of the glove box doors have been taken.