Air Conditioning Chaos
Got another Case Study everyone. This is on a 40ft. New Flyer Low Floor Transit Bus that has intermittent operation of Air Conditioning. This Bus has been back multiple times. When each prior Technician looked at it they could fine no issues but they would have codes 15, 11, and 10 every time. The last Technician opted to replace the wiring harness on the AC Compressor and it still came back with the same symptoms. So I then hooked up the Thermo King Intelligaire 2 Software and took a look and this is what I have to work with.
I have a code 15 first for Compressor Clutch Output Fault, a code 11 for Low Pressure Shutdown, and a code 10 for High Pressure Shutdown. Ok so before I continue I will go over how this circuit works and I will then go into detail on how I approach this issue. In order for this Circuit to work properly Thermo King must first see a complete Closed Circuit through the High Pressure Switch, Low Pressure Switch, and the Clutch Coil. Each Pressure Switch has a Resistor Built Inside of it that is Parallel to the Current Carrying Contacts so when one of the switches go bad, the Computer knows which one is bad by looking at the Current flow through the Circuit. The HPCO Switch has a 8,060 Ohms Resistor and the LPCO Switch has a 11,300 Ohms Resistor. So when one of these Switches opens or their is a wiring problem on a particular end of the circuit it will log a code for either switch. If you get a Clutch Output Fault Code with pressure switch codes it means more than likely you have a switch issue or wire issue for one switch or both switches. Now if you get just the Clutch Output Fault Code that can mean either a short to ground or open circuit. So next question is how does the computer know if you have a open circuit or a short to ground? The TK Computer sends out a Bias Voltage to test for opens and shorts. Thermo King sends out a Bias Voltage of 4 Volts on this system to test the circuit.
Alright so the problem is intermittent, how do I find the problem? Time to hook up the Pico Scope. Here is also a pic of my scope setup. I decide to break out all available channels with both of my scopes. I hooked up to both sides of each pressure switch and then hooked up to the Clutch Supply and Ground. I then used 2 Current Clamps to look at the Clutch Current. Originally I wanted to use a trigger for both scopes but I changed my mind. I decided to just let the scope run on both scopes because I was right there able to monitor the laptop. I made my switch connections at the inlet and outlet of each switch.
The First picture is the High Pressure Cutout Switch and the Second one is Low Pressure Cutout Switch. I set my Pico Screens up to 5 seconds a division and set my triggers to repeat and moved the triggers away from having any possibility from being contacted by the channels so the traces can draw continuously across the screen. So basically it's like a roll mode. I like using the Pico like this to help find intermittent problems as long as I am around the scope and can see it while I wiggle wires, connections, etc. So I turned on the Ac and started wiggling wires and as you can see by the scope captures when I wiggled the High Pressure Cutout Switch wires I lost Voltage from there and then the loss of Voltage replicated on the rest of the connections. So what this means is the High Pressure Cutout switch is faulty and therefore took away the Voltage from the Low Pressure Cutout Switch and the Clutch Coil. And if you look at the end of the capture the voltage at the inlet of the Hpco Switch goes to the 4 Volt Bias I talked about. Also if you look at my Clutch Current it is low. Normally on these Clutch Coils I will see 2 amps. So I also inspected the Clutch Coil and measured the resistance for a quick check of it. This is what I found on the Coil.
On a good coil I will see 11 ohms, so yes not much difference but the old coil was melted, that's why I had the lower current because as it heated up the current went down. This is a good reason why you want to check circuits live and not just use ohm readings alone. Here is a after capture of the new Clutch Current showing the live resistance with a Pico Math Channel and here is the old capture showing how hi the resistance was with the Hi Resistance Pressure Switch and the Hi Resistance Coil. After everything was done I also replaced the Low Pressure Cutout Switch for good measure because it looked real old. These switches have a knack for going bad right at the wire connection to the switch. So experience came into play here but if I didn't have the scope it would have taken me longer to find. Anyway hope everyone found this enjoyable. I plan to show more AC Case Studies as time progresses plus I have some other interesting case studies on the way as well. Have a good day.
One more thing to add everyone. I did check for codes again and I had none. Sorry forgot to post the picture.
Hi Michael: Nice rig. I like where your head was at in diagnosing this. I've got a few questions if you don't mind. 1: Do you prefer the "Pomona" style piercing probes over back-probing with "T" pins? (I was one of the beta testers. The original probe pin was too brittle and would shatter.) 2: How are you healing the wire when finished. I used to use nail polish. We now know that's not the
When I hook up to test it depends on accessibility and if I'm trying to find a intermittent fault or not. I prefer piercing though because I know I'm making a good connection and I dont take the chance of spreading a pin. Bit if I have a breakout lead I go that route. I try to use those as much as possible. Saves time repairing wires. I also like the Pomona Piercers. I like them the most because
Hi Michael: The reason that I asked about the "T" pin (hat pin) vs. the Pomona is because a "T" pin will usually slip through the connector grommet and between the wire insulation and strain relief. At that point, you should have a good connection with no insulation damage. Of course, you need to be mindful of where the "T" pin is at or else life gets "interesting". HTH, Guido
The wire broke off right at the switch, I did not see any piercing holes. Ssometimes the wires will break at the switch due to heat and vibration.
Michael, I'm impressed. That's a helluva set-up. Makes perfect sense where the fault is when you have multiple channels going, but could get confusing to keep in your head if just taking multiple voltage measurements at various points while trying to replicate. I'm mentally filing this one under "8 channel diagnostics". Keep the case studies coming. I'm learning more with every one.
Thanks Chris, hopefully Pico will make software to link multiple scopes together soon, then I can actually get 8 channels when triggered lol. Hopefully Pico will do this. It would make everything expandable and you could buy as you go.🙂🤓
That's a million dollar idea there, especially with that case you have set up. A Pico is on my list, but I keep getting ahold of old vantage meters and modis and verus scanners with scopes for under $100, with leads that have never been used. I have been known to have two to three on at a time.
Great case study Michael, thank you for sharing your thought process and techniques with us.
Using Load Pro leads and piercing probes in the same diagnostic process, Dan Sullivan would have a conniption!
Yes probably,lol. I am not a fan of piercing but sometimes unfortunately it's the best choice, just depends on logistics. :)
Sometimes you gotta do what you gotta do. I have a load pro too and it has never steered me wrong.
Hi Bob, sorry for the late reply, been busy. Yes you are correct Load Pro has it's place and it works. I call it a fancy test lamp,lol. Don't zip tie the button down though and leave it on for an extended amount of time, the plastic on the tool will swell, lol. Opps. I should of used a test lamp for that test. Lol.