drtidmore

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About drtidmore

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  • Birthday 12/04/1952
  1. FYI, what you thought were thermocouples are typically thermistors. Thermocouples are used in HIGH heat (thing furnaces) as they generate a small electrical voltage proportional to the temperature. That is how the old household heating thermostats used to work as the thermocouple held open a safety valve that kept the pilot lit AND provided the power to open the main valve when the wall thermostat closed the circuit. A thermocouple is not a good fit for vehicle A/C controls. What used to be common in car A/C was a capillary tube thermostat where the expansion/contraction of a working fluid in the bulb at the end of the capillary tube created pressure to move a mechanical piston in the thermostat to make or break a set of contacts. These are still used in some some simple applications but even those applications are moving to thermistors and circuitry as the cost of the capillary tube thermostat has become the more expensive option.
  2. I have thinking about HOW, if possible, to bypass the OEM design regarding the compressor clutch cycling. I pulled out the FSM and studied the schematics closely. First a few things to note. The ECM, NOT the BCM, is actually the one that ultimately controls the compressor clutch relay. The BCM computes the desired state of the compressor clutch and sends a signal via the data network, based on the low pressure and low/high temp sensors, that the ECM reads and then using logic in the ECM controls the compressor clutch relay state (i.e. engaged or disengaged) . The power steering pressure switch, the throttle position, and the engine coolant temp also feed into the ECM as part of the compressor clutch control logic so even if the BCM sends the engage signal along the data network, the ECM MAY decide to NOT engage the compressor clutch based on its other inputs (i.e. wide open throttle, engine too hot, power steering load). The BCM also feeds the A/C programmer with a PWM signal (direct wire) that the programmer in turns converts into a signal to the blower control module (blower speed). Also, the BCM computes the refrigerant high pressure value (there is NO high pressure switch in the system) using the low and high temp sensor inputs and uses that computed value as a safety to kill the compressor if the computed head pressure exceeds safe limits. Of course the compressor itself has a mechanical high pressure relief valve as the avenue of last resort against exploding something in the A/C system. Additionally, the ECM, which is responsible for the fuel injection and IAC on the throttle body, uses the compressor clutch state in its computations as it controls the fuel and idle air controls. I think you should now be getting a better idea of how the sensors, modules and logic are all intertwined and co-dependent on each other. This is also why the BCM will signal for the compressor to be disengaged if the low or high temp sensor go way out of value as they are central to the control logic. As the data network is how the ECM and BCM communicate, it is NOT possible to spoof the ECM so this is where unintended consequences begin to unravel things and things get worse if we try to bypass the ECM as well. The ECM is computing the correct fuel injection signals based on the engine LOAD which is of course significantly impacted by the compressor, so if we bypass the ECM control of the compressor clutch, we will run into idle issues as the ECM currently adjusts the fuel injection and IAC to compensate appropriately. In the old days, there were kick up pots controlled either electrically or with vacuum that mechanically forced the throttle open slightly more when the compressor was active. In our Reattas such is done with the IAC and fuel injection which are under the ECM control and the ECM needs to KNOW about external loads. The more I investigate this, the more obvious it becomes that even attempting to bypass the OEM control of the compressor is a bad idea! Even IF we did bypass the ECM/BCM control of the compressor clutch, the problem would remain that the ECM would be getting data from the BCM indicating desired compressor state control and the ECM would in turn adjust the fuel injection and IAC to compensate EVEN THOUGH, the actual compressor state would likely NOT be in agreement. So the idle would be all mess up and might even stall the engine under certain conditions as the compressor clutch could be engaged when the ECM thought otherwise and vise versa, not to mention engaging when other conditions would contraindicate. So I really don't see a viable way to control the compressor outside of the OEM design which brings us back to the living with the OEM design. We WILL solve the low temp sensor issue. I have already discussed why an external clip-on evaporator thermistor is a bad idea (throws off all the compressor control logic in the BCM due to hysteresis). I am not trying to shut down discussion either as such is invaluable, but hard facts are hard facts and we need to understand HOW the GM design functions before we can begin to discuss work arounds. I hope that this helps explain in a bit more detail HOW the A/C system works.
  3. You may want to leave the pulsator OFF as the Bosch is a turbine pump producing a constant pressure rather than a pulsed output like the OEM fuel pump and OEM design aftermarket pumps. The pulsator is just a push-on fit and the ethanol in our fuel attacks the rubber in the pulsator connections causing the rubber to shrink away and then you loose fuel pressure. I fought this exact issue a few years back and at the time did not know about the Bosch turbine pump, so I installed a new aftermarket pump and a genuine GM pulsator. Recently the aftermarket pump I had installed began to whine a LOT and stranded me twice, so I pulled it and installed a Bosch pump with NO pulsator. Even with the pulsator on the aftermarket pump, when I had my fuel pressure gauge on the rail, I had a jitter in the pressure reading (small, but there none the less) and with the Bosch and NO pulsator, absolutely rock solid on the pressure. I also noticed that even after only a bit over 3 years of use, the pulsator I had install new was showing signs of rubber damage due to ethanol. FYI, the Bosch comes with a short piece of fuel line hose and a couple of clamps to bypass where the pulsator would normally be installed
  4. I have the same issue when the trunk area is heat soaked (my reatta is black) in the Tx summer sun. The solenoid will click but the latch does NOT release. I have replaced the gas struts, ensured that the spring across the latch is properly adjusted, but the problem still occurs periodically. I have tried to pull up on the trunk lid while releasing the latch, but the latch itself is simply NOT releasing on mine. I have oiled the latch and can't find anything worn or binding. I guess I should just replace the latch assembly but it only acts up under extreme heat and so I have learned to live with it.
  5. In the past few years I have had to replace the harmonic balancer twice on my reatta and in neither case would my electric impact wrench budge the bolt. What I did both times was, with the car on jack stands, using my long breaker bar angled down such that it hit the ground forward of the engine, with the ICM disconnected, I would bump the starter and bingo, the bolt released. I know it sounds scary and certainly you do NOT want to be near the area just in case the breaker bar snaps, but this trick has been used by quite a few. I suspect that a pneumatic impact wrench might well break loose the HB bolt.
  6. We have such built into the Reatta. Just let the car sit overnight, then go into the ECM and BCM and get the temp readings from all the various temp sensor and compare them. There is NO value in testing the thermistors in our car over a range of temps as they either work or they don't. It is their resistance vs temp that is of value and again, comparing the sensors after sitting overnight gives us that information as we only need ONE measurement point as long as we have other sensors of the same type exposed to similar temps that are reporting.
  7. Bob, Trying to get around the low temp sensor on the Reatta's is about like saying you want to get around the fuel injection. Sure, it is possible, but the system is what it is. The thermistor itself is NOT a big issue, even thought its impact can certainly be considered an issue! GM stopped selling replacements and the 3rd party market has yet to fill the vacuum. Thermistors are extremely reliable, especially in lower temp environments like the A/C system (both low & high). GM used a disc thermistor with two small solder joints and as I have stated, solder becomes brittle after countless thermal cycles and as it becomes brittle, it becomes resistant to the flow of current until it essentially becomes an open. The problem with an evaporator clip-on thermistor is one of hysteresis (where the state of the system is dependent on its history). The thermal mass of the evaporator is such that it severely lags in temperature response compared to the actual refrigerant. Yes, GM could have engineered a system based on a something as low tech as the old capillary thermostat that was used in aftermarket installed A/C systems years ago but GM was working to maintain as close to suction throttle behavior as they could as that was the reference standard for GM and had been for literally 3 decades AND the system on the Reatta WAS the one designed by and for Cadillac. What 2Seater and I have done is establish the curve for the thermistor by exploiting the fact that the software in the BCM was written around the ideal curve for the part GM incorporated, so it is a fixed reference. Now that we know the curve, we can PREDICT, with ACCURACY, what the thermistor should read at ANY temperature. That allows us to either use the other temp sensors in the car as a group normal to compare against on a car that has sat overnight, OR we can measure the temp of the suction line adjacent to the thermistor with an infrared thermometer to determine the ACTUAL sensor temp. Then we can measure the resistance of the thermistor and see how much it has drifted which allows us to correct it with a parallel resistor, assuming that the drift IS in the thermistor NOT the solder joints. Since we have limited experience with the longer term success of trimming to correct drift, trimming is likely to turn out to be a short term fix at best. However, trimming COULD be useful to correct a fully functional thermistor that is just a tad off optimal, but NOT drifting per say. I am not advocating that this is as desirable as being able to run to your local FLAPS or even a GM dealership and buying a new low temp sensor, but it is certainly NOT difficult. With Ronnie discovering that his failed low temp sensor was actually no longer mechanically secure between the support leads, that was the smoking gun for me. I had a hunch regarding the solder joints, but no evidence outside of years of dealing with damned solder joints in electronics. Ronnie has mailed me his failed low temp sensor and we shall see what I find. If my hunch is correct and I can reflow the solder successfully, then THE solution to this issue is to fix the sensor, NOT try and go around the way the system was designed. ANY external thermistor that you might find will NOT work correctly in the Reatta system as again, the system was designed to directly monitor the refrigerant temp immediately after the orifice with ZERO hysteresis. The first problem you are going to find is thermistor with a value of 2600 ohms @ 25C simply does not seem to be available (I have done a pretty extensive search and found nothing). The second problem is that mounting it externally is going to swing the evaporator between freezing up and too warm due to hysteresis of the evaporator compared to the refrigerant itself. There is NO WAY to simply change the way the BCM code is written. The code in the BCM is ALL binary, the source code and the development environment is LONG gone and attempting to find memory locations, let alone interpret what is at those locations while not totally outside of technical feasibility is a matter that is obtuse and VERY prone to error. After 30 years, we still don't have a complete understanding of the even the ECM and nothing on the BCM. But remember, we are NOT talking about a failure of the BCM, but of a simple sensor.
  8. The problem with using PC duster for evaluating a thermistor is that we need to know the temperature of the exposed thermistor and PC Duster is a pretty uncontrollable environment coming out the can! However, it certainly would allow you to watch the resistance of a thermistor rise as the temp fell.
  9. And back to the beginning of this thread, we want to "manage" the expansion amount such that it does NOT overexpand, dropping the temp at the low temp sensor, below the -2C trip point, which we do by ever so slightly overcharging the system to create a tad higher suction side pressure as this is a closed system which can only hold so much refrigerant (again, more refrigerant=higher pressure=higher density=higher energy density and LESS room to expand). In the good old days of GM's suction throttling system, this was done dynamically and the compressor ran all time with the suction throttling valve controlling the suction side pressure precisely at the point to achieve 0C in the evaporator at all times (there is a direct relationship between suction side pressure and evaporator temp).
  10. It is a tad more complex than just a simple venturi. It has to do with expansion of the refrigerant liquid as it exits the orifice. No expansion=no lowering of the energy density=no ability to absorb energy in the evaporator. The expansion is due to both a large area into which the high pressure liquid exiting the orifice can expand, but in addition, the compressor is creating a dramatic pressure differential aft of the orifice. If you take ANY compressed gas and allow it to rapidly expand, what we perceive is the it gets REALLY cold, but in reality all that has happened is that the energy density has been vastly reduced and less energy density=lower pressure=lower temp. Now expanded and much less energy dense, the gas can travel thru the evaporator absorbing energy (heat) from the interior of the car which then after passing thru the suction side of the compressor which compresses the lower density gas into a much higher density gas then is passed thru the condenser where energy is removed turning the high density gas into a high density liquid and back it goes to the orifice.
  11. I have read on the forum that if you are quick on the swap, you can pull the old low temp sensor and screw on a new one without loosing all the freon, although you WILL loose some but at least you don't have to pull a vacuum if you are quick on the swap. R-12 just got to be such a legal nightmare that shops stopped dealing with it. As for an APB on salvaged low/high sensors, I am going to see what I discover with Ronnie's failed sensor first, but if you run across ANY of them (GM used them on almost all their high end cars of that period), then I think it is good idea to pull them, regardless of condition, now that we have a good way of testing them and potentially a way to repair them.
  12. I think we are good with the readings at this point. I am not finding a thermistor in the proper resistance range, but as I mentioned a 2.2Kohm@25C is common these days which we could use with a series installed 400ohm resistor (the curve for the NTP thermistor itself would be ok just offset which the 400ohm resistor would correct). As for the fact that your BD028 seems to indicate a lower temp than the BD027 which is confirmed by your infrared thermometer, you ARE giving up a couple of degrees of cooling. As the resistance of the thermistor rises as the temp falls, the curve predicts that your low temp sensor should read about 3.6Kohms. Did you think to get a direct resistance measurement on the sensor at the same time? Assuming that it does read around 3.6K@21C, then we need to trim it back to around 3.28K to restore accuracy, which would be a 37Kohm resistor in PARALLEL across the thermistor. Before you do anything however, I am following a hunch on what is happening with the drifting thermistors and Ronnie may have just confirmed it. My hypothesis is that the thermistor, which don't typically have a drift failure mode except when repeated exposed to HIGH temps well above anything in the A/C system), is FINE! The problem may well be the solder joints on either side are becoming brittle as solder DOES tend to do this when constantly temp cycled. Ronnie located his old failed one and found that the thermistor could be MOVED between the two support/connecting legs of the housing. He is shipping it to me for further investigation. If this turns out to JUST be a solder issue, the solution is simple, reflow the solder on both sides. If this is the case, we have a simple fix for at least some of the failing sensors. This is not to say that ALL low temp sensor failure modes will be solder joint related, but my hunch is the majority WILL fall into that category.
  13. It is pretty apparent that the thermistor used by GM in our Reatta's have a value of around 2.6Kohms@26C based on the data 2Seater has gleaned by spoofing the BCM with a trim pot to allow a range of resistances vs CRT BD028 readings to be logged. This would confirm what I read on my installed sensor (2.624K ohms@25.56C). I have been searching for a suitable replacement thermistor that we might be able to use to "repair" an failed low temp sensor, by soldering in a new thermistor. While at the time that GM designed the Reatta's A/C system the thermistor chosen might have been commonplace, but similar value thermistors are not on the radar presently. I can find all manner of 2.2Kohm@25C and that would work by simply adding a 400 ohm resistor in SERIES with the thermistor (i.e. placed in the lead connecting to the low temp sensor housing). Note that this is NOT the same as putting a resistor in parallel with the sensor in order to trim its readings DOWNWARD slightly.
  14. Ronnie, In order to correct a drifting sensor, even if short lived, you have to know its resistance at a specific temp. Getting a good thermistor curve allows us to predict the proper resistance at ANY temp and therefore determine how much is out of range. This is much quicker and more accurate than hit or miss on the trimming resistor value. That said, the comparison against the other temp registering sensors on the Reatta after sitting overnight is an easy way to see IF the sensor is out in the weeds but again, we need the data 2seater provided to create the thermistor curve that the BCM expects in order to accurately trim the sensor OR find a suitable replacement. Of course none of this would be important IF we could buy new sensors!
  15. Thermistors typically don't exhibit a drift issue except under high temperature conditions (well outside the range they are subjected to in our systems). What I actually believe is happening is NOT thermistor drift but the weld/solder joints between the thermistor leads and the support leads are becoming brittle and introducing added resistance (i.e. making the temp seem lower than actual). I have not had one of these out where I could investigate if the leads were spot welded or soldered. If soldered, a simple reheating with flux followed by a good cleaning with denatured alcohol likely could cure the problem. If they are spot welded, I would still attempt to solder the joints as that might well restore the proper conductivity. Cleaning thoroughly after soldering is an absolute must! We are going to need to figure out a good solution regardless as even though other GM cars of the same vintage used these sensors, this is another one of those items that is approaching unobtainium status!