Electrical Issue Re: Cooling Fan And Ac - Need Insights

[mbsl98]

I finished a successful evaporator swap ( with receiver) over last weekend, and had the system recharged yesterday. It has been empty since last fall (end of cooling season in NE) when I stopped the near weekly recharges. At that time, all worked great, exceprt for the massive leak. Around January of this year, I had a check light/ code for "problem in low speed cooling fan circuit", although fan came on and seemed to operate properly. Being winter, fan is seldom called for, so I cleared the code and waited. I have had it come back twice, each time after long idling, and fan operated while throwing up a code at same time. I don't know if it went to high speed when it really wanted to operate on low, or if it was just telling me that there was excess resistance, etc. Anyway, speed on to yesterday, with newly recharged AC system. It was done outside, in direct sun, at around 97 degrees. When all was buttoned up, tech turned on AC to see how it performed. It started to cool, and then the compressor went into on-off cycling just as if it was low on freon. Apparently this is the same cycling it does with excess pressure (it was fully pressurized with proper 1.63 pounds). The cooling fan never came on (at any speed), and no check engine light either. Since that shop was not ideal for chasing electronic gremlins, I settled up with them and headed on to work. About a mile or less from shop, I turned the AC on again, and it worked just fine. Note the fact of cooling wind generated by road speed, in lieu of fan. However, when I got to where I park, another 5 miles or so, I left it running and got out to see what I could, and found the fan running away - couldn't tell if it was high or low, or if it was triggered by the AC system or by coolant temp (still in high 90's) - nop check engine light either.

This morning, in low 70's, AC again turned on and cooled, but no cooling fan at on all. I expect that the low air temp allowed the compressor to work without overheating the freon to point of cycling compressor off, but not proven. Again, no fan and no check engine light. Note also that the coolant would not have risen to temp to call for fan, and whole trip to station is only 3 miles.

background: new ECT, thermostat and fan relay this winter, cooling system in fine shape. I tried testing the fan this winter and was able to jump it to run two separate speeds, although I was surprised that there was not more difference between the sounds of the different speeds (a possible clue?).

questions: Can anyone say definitively which sensors or signals trigger the high speed versus the low speed? Or how the high versus low speed is otherwise selected? I have studied the wiring diagrams, and it looks as if the ECU gets a single temperature signal from ECT, and another one from AC high pressure sensor near the fill port. This suggests that one calls for high speed, and the other for low? However, that would suggest that either the coolant or the AC should end up triggering the fan on one of its settings. The two inputs to ECU are completely independant of each other, so a problem with one sensor should not interfere with the other's ability to turn on fan, so those two sensors are probably OK. If the ECT sends a variable voltage signal, that could explain how ECU decides, and how the fan was triggered in the very high temps of yesterday (ie: called for high, versus the earlier lights coming when calling only for low). However, that would not explain why the AC system didn't call for the fan and no light came on either. Also, if the sensor from the AC high pressure calls for the fan, what comes into play cause the compressor to cycle on and off at high pressure (only one signal into ECU from pressure sensor that I can see). That could also be sending a progresive signal that turns on fan at fairly low pressure and turns off compressor if presure and signal get high?

Any insights or suggestions for diagnosis HIGHLY appreciated.

PS: just a side not on swap. The first step I took was to get teh glove box out and some other interior stuff, and teh very first thing was that teh glove box latch decided that that was the time for it to break in the classic 850 manner. My first steps in swap wound up being teh hacksaw opening of the box - set a poor standard for the rest of the project, but it was the only particular glitch. Special thanks to Prasmin, however, for sending me the tip on how to actually get the AC quick-disconnects to release - that took me more time than the glove box ( but will tke me only a couple of minutes on the next try).

[Ozark Lee]

Can anyone say definitively which sensors or signals trigger the high speed versus the low speed?

That decision is made by the ECU based on inputs from the ECT and the A/C system pressure. By system pressure I mean that the high pressure switch indicates that the A/C compressor is actually running.

new ECT, thermostat and fan relay this winter, cooling system in fine shape. I tried testing the fan this winter and was able to jump it to run two separate speeds, although I was surprised that there was not more difference between the sounds of the different speeds (a possible clue?).

There should be a fairly pronounced difference in the fan speeds between low and high. What procedure did you use to test the fan?

I would try to test the fan and the relay by unplugging the middle connector on the fan relay (The one with the yellow and yellow/white wires) and individually grounding each of the pins. Off the top of my head I can't remember which is high speed and which is low but if the relay and fan are working properly you should get high speed on one and low speed on the other.

If that test works OK you will need to ring out the wires back to the ECU. I had a bad connection on one of my wires on the '94 right at the connector to the ran relay.

The fan should come on with the A/C and stay in as long as the A/C compressor is running.

...Lee

[mbsl98]

Hi Lee,

Thanks for the input. I jumped the fan with 12 volts direct from the battery to each of the leads going direct to the motor (at the connector off the output side of fan relay). I could easily tell that there were two speeds, but neither seemed to me to be that much different. I sort of remembered the fan sounding much louder on high, but I never had a reason to pay close attention to the difference before. Our '98 T5M has a different fan, and is much louder, but I suspect that has to do with the reason that the '98 fan costs about double the '95's ( Something like a bad connection does sound like the most likely issue at this point, since the relay replacement didn't change the operation at all. After that, my thought is that it might be internal within the ECU itself, but I have more testing to do this weekend before going for that answer. I should be able to prove if there is a signal being sent by the ECU to the yellow or Yellow/white wires going into the fan relay. If current is there, then it must be in the wiring between relay and fan or actually in the fan, and if no signal there, then either wiring between relay and ECU or in ECU itself. I am just up in the air understanding how the fan could operate and send a code earlier, probably on low speed, and then yesterday operate (without a code) for coolant, yet not operate at all for AC and no code. Seems to me that if there was a fault in the fan itself, then a signal by either the coolant or AC to turn it on would generate the code. Anyway, all thoughts still appreciated. Mike