Looking for a sanity check
Regular customer recently purchased this truck and brought it in to have it checked over for the summer. Refrigerant charge was low so we performed a recharge. On checking the operation of the system after the charge I noticed air mix was stuck on heat, compressor engaged fine and evaporator temp was in the low 40s so I knew the A/C was working. Scanned for any codes and found none. Knowing this customer well I knew he would want cold air, so I began pulling service information to start my diagnoses.
Looking at wiring diagrams it looked like a typical 2 wire motor with a 5v ref, signal and return. Not knowing Nissans as well as I'd like, I decided to look at symptom related troubleshooting. Step 1 for no cold air is as follows:
1. Turn ignition switch ON. 2. Rotate temperature control dial to maximum hot. 3. Check voltage between front air control harness connector M180 terminal 17 and terminal 18 while rotating temperature control dial to maximum cold.
The result was No Good. At full cold there was a short pulse of voltage, maybe 9-10 volts for not even half a second. On to step 2 then, which says:
1. Turn ignition switch OFF. 2. Disconnect the front air control harness connector. 3. Check continuity between front air control harness connector M180 terminal 17, 18 and ground.
Sounds good, I guess. Unplugged the connector and check for shorts to ground on pins 17 and 18. Result was a good, no shorts to ground present on said circuits. What does that mean? Well, let's see:
Is the inspection result normal? YES - Replace front air control. NO - Repair or replace harness as necessary.
This didn't sit well with me, but hey I didn't write the service info. Out of curiosity I continued on down the list, and to make an already long story slightly shorter, the only time they ask you to check the motor it's to check continuity across the 2 pins at the HVAC control, 17 and 18. Continuity was good, and that meant the motor was good. Still this result didn't sit well with me. I grabbed my 194 bulb test light just to have a small load and put power and ground to the motor wires. Bright light. Swapped polarity, bright light. Pins 19 and 20 are for the mode door, which works, so I moved over and did the same thing. This time there was no light until the motor reached the end of its travel and then the light lit dimly. Same thing happened when I swapped polarity.
Clearly the mix motor is shorted and the control module is hopefully seeing that and limiting current by cutting voltage, or am I missing something? Why does service info have you replaced the control unit miles before actually checking the motor? Am I losing it?
If you've read this far I appreciate it and am interested in any thoughts.
Edit: Actuator removed, no shorts in harness. Disassembled actuator and hook the motor up to the disconnected battery, it got half a turn, then farted and let the smoke out. Looks like control head was indeed limiting current.
chase around charts.......I follow those to a point but then I need to get serious about the diagnostics. My take on the chase around charts ....... They give me an idea of how the system supposedly works but just replacing parts after doing the chase around doesn't happen. That gets expensive fast. I guess at the dealer level they have these in stock so you can change the part and if that…
Its always best to read into the charts and steps get an understanding of the system and take the most logical or appropriate steps. I would not recommend placing blind faith in the charts because sometimes their approach even seems to lack common sense. Some of the OEM's leave errors in charts and schematics plus leave crucial information about the circuit or operating details out. The…
The parts swapper thing doesn't sit well with me at all. I see it all to often. I agree that we need to make all tests and confirm that what we are about to replace really needs it. Yes we need the chase around charts for a bit to understand a new system or circuits. It sure would be nice though if they would ditch the check bla bla bla if that isn't with in replace part x.
I had to come back to this particular thread just to read all the great posts a second time and up arrow them. When I get an electrical diag job, I joke with the office staff that I'm going to be killing some trees. I print every relevant wiring diagram as well as the OE troubleshooting procedure. I'm one of those guys who has to have it all on paper. And really, having wiring diagrams as a…
I do the same thing. It is easier to take all the papers to the vehicle then drag the computer over and have to work around that too. Plus if I knock over the papers oh well. if i knock over the computer oh shit.......The colored pencils work great too even with colored wiring diagrams.
Your description is how I typically use the charts as well, something to get a general idea of how the circuit works. This particular case was just an especially egregious example, so much so that I thought I was missing something obvious and it just couldn't be as bad as it read. Just no mention of the possibility of the motor being shorted at all, it was a little hard to accept at face value.
OEM charts are very bad most of the time, but I use them as another tool to get an idea of system operation and some value readings. I would recommend you use your own diagnostics strategies along with the diagnostics equipment you have. Remember the dealer has parts laying around and often the people working there get told by tech line to put a known good part in the vehicle a lot of times. One…
Yeah, they've always seemed pretty bad, this one just seemed so much worse than usual that I thought I was missing something obvious. There's no mention anywhere of the possibility of the motor being failed as a short. Granted I guess I may find the wires shorted together somewhere, but on a quick visual with some of the dash removed nothing seems out of place.
It is obviously a flawed testing procedure. They are giving no thoughts to the possibility of a shorted actuator, they are just going with a pass fail test on the control unit and wiring to the actuator. If there is no short to ground on the circuits involved then with no voltage they fail the control unit. It makes me wonder why there were no codes stored in the system. What scan tool did you…
Definately flawed procedure. (Not a tool problem) I don't want to hijack this thread but it was another good example. There was a code for shift motor encoder rationality it remained active. A second code for motor current but did not return. Its a three wire low voltage sensor with the voltage continuously monitored, so I would think it sends a signal on top of a bias voltage. Therefore…
Eric, have the Asian makes finally added DTC's to HVAC systems? I haven't seen that yet out here. GM always, Ford twice. Nothing else. Or, maybe they just don't break here since they never get moved ;-)
I don't remember when they started but they've had them for a while. Of course I'm working on 2015 and new most of the time, except for some older Toyota and Subaru vehicles, a 2012 is old for me. He says as he looks over at a 1947 Cadillac Limo he is rewiring. ;-)
Codes were checked with just a Solus Edge. It was hard enough to convince the boss that the OTC Genesys that he stopped updating in 2009 wasn't going to cut it. I do pay to keep the Solus updated though. I agree on unplugging the actuator, that's what I would have preferred as a next step. I just can't get a hand on it without removing the dash. It's on top of the box up against the firewall. I…