I have an 87 sunbird, GT, non turbo, but ac is aftermarket type. problem is without ac on, rad fan will not run even with rad temps approaching danger (red) zone. I have gone through most checks including ECM terminal grounding with ECM connected, and fan ran then. Also replaced the relay (its cheap). Aftermarket ac power line ties into the rad fan relay (guess this is normal for this setup? not sure if this matters....
Should I be looking more at the ECM, maybe with a scan tool? I can borrow an actron OBD 2, but need to but an adaptive cable for it.
Any tips for this hair puller would be appreciated.
beware of the arrival
If it all works through the ECT sensor driving the ECM, the sensor could be out of spec and telling the ECM the engine is not as hot as it really is. Sensors that are off but not completely open or shorted may not set a trouble code. Like most GM parts, those sensors were nothing to write home about in terms of reliability.
OK...I may be onto something now, just started the car with temp sensor clip/connector yanked off, and the computer did NOT set a code. What should I be chasing now?
beware of the arrival
If sensor ohms are in spec, check the wiring from the sensor to the ECM by unplugging ECM and measuring sensor resistance at the ECM plug (sensor plugged in).
I emulate the ECT sensor with a resistor substitution box. It's just a thermistor with a negative temperature coefficient, the hotter it gets, the lower the resistance. Liked my Honda and Toyota manuals, showed a curve of resistance versus temperature, can go crazy with domestic cars just trying to find the resistance at a single temperature.
Like my 92 DeVille, can sit comfortably in the drivers seat and play with the climate control and look at every thermistor in the vehicle, takes getting use to like buying a new cell phone that I also hate, but if the temperature in the garage is say 10*C and the car was sitting all night, every thermistor should show 10*C so I am not crawling all over under the hood trying to find these little suckers. Understand the GM Tech II does the same thing as other way overpriced scanners, but haven't spent the 5,300 bucks to buy one. The circuitry for this stuff only cost pennies.
Typically the connectors get a film of corrosion, doesn't take much, but this adds to the resistance of the thermistor making the PCM think the engine is cooler than it actually is. Another question is, does the ECT double as the temperature gauge sender, on some vehicles it does, others use a separate thermistor, so you dig though the circuit diagrams, if it was a separate sender for the gauge you know it's not doubled, if it doesn't, can assume the ETC doubles for the gauge. Using an IR thermometer, if your thermostat housing is right at 195*F and your gauge is low or pegged, you know it's your sender.
Most important job of the ECT is to tell your PCM, your engine is warm enough to switch into closed loop mode where the O2 sensor takes control, can get very poor economy if stuck in open loop mode.
Using the ETC for fan control is stupid, stupid, stupid, fan doesn't kick on until the engine temperature reaches around 230*F where your coolant is near the over boiling point and switches off around 210*F, so in city traffic driving on a hot day, you have this additional thermal cycling that is tearing your engine apart. Proper place to put fan control is in the radiator tank to kick on the fan if the radiator temperature exceed 160*F, this keeps your engine at a constant 195*F to avoid that additional thermal cycling. Have converted some vehicles, but too much work. You just buy a new car, drive it a couple of miles, toss it in the garbage can and buy another.
Thanks, Nick. Yeah, I do have 2 sensors, one for rad and one for sending unit on dash gauge. That one is new also. I heard you NEVER take resistance on an ECM, just voltage. Is that true? (going outside in 92 deg heat, mosquito infested to try some readings.......whew!)
beware of the arrival
OK, heres what I got: cold engine (92 degrees here!), resistance is about 4200 ohms on end of wire harness going to temp sensor. Doesnt that indicate wiring from sensor going to computer is good?
Also, Ignition on "run", engine off, voltage shows 4.98 (book says about 5 V from ECM). Doesnt that indicate all good from ECM to sensor ?
Semsor cold about 2250 ohms, about right according to spec. Didnt test ohms hot yet.....
Thanks again, peolpe!
beware of the arrival
Edited: Tue October 13, 2009 at 5:20 PM by pippo
Different world, automotive electronics, try to tell you how to check things without the proper test equipment. Same old crap you find in electronics everywhere else, microcontrollers, A-D converters, sensors, resistors, capacitors, transistors.... But run into perfectly idiotic statements like, "Solid State, Do Not Measure" or if in doubt, try a known good component when they don't even give you the specifications to know whether that known good component is good or bad. Most scanners are a joke, can only test if a component is open or shorted, but not way the hell out of tolerance. Would be so much easier if they gave that information. Circuit diagrams of the components is out of the question, lucky enough to get a pin number and really lucky if they describe what that pin is for.
You walk into a dealer or an auto parts store to see a big sign, no refund on electronic components. Well this guy did, you sold me a piece of worthless crap and give the technical reasons why. Really bad now with all the crap from China.
Just from your reading, your ETC values seem high from what I have encountered. One trick is to get a vehicle identical to yours that seemingly runs perfect, and trade parts. Have found ECT resistances at two different temperatures in shop manuals, but may take you two days to find them, could jot that down right on the schematic, that is what everybody else does.
I was a component designer, typical to write over a hundred page document on each component loaded with specifications and test data, that would be turned over to the tech manual department and reduced to half a sentence, don't want to confuse people with facts.
Abbreviations and acronyms, hate those too, ECT stands for Engine Coolant Temperature sensor, the thingy that tells your PCM, also an abbreviation that is called different things by different manufacture that stands for Powertrain Computer Module so it knows the engine temperature to run the computer in closed looped mode, switch on the radiator fan if things get too hot, can even disable the AC compressor if things get too hot, or even shut down the engine, but avoiding that lately as that can cause an accident.
We've updated our forums!
Click here to visit the new forum
Copyright © 2016 Arizona Mobile Air Inc.