Yes I am still planning on 2.5 lbs is 40 oz. or 3 and 1/3 134A cans is what I'm going for "ish". 3*12 =36 + 4oz. I'm following the guidelines you put on here and I verified them separately. So I think we are good there.

It stopped taking freon at the ~7psi mark, i am assuming that is the internal can pressure (so then I stopped ;-). From what I read if I can get the compressor on it will boost the high side pressure way up, and lower the low side pressure will go down to take more Freon. There is no cooling yet at all, which make sense since the compressor isn't turning on.

On the gauges both high side and low side show equalized pressures of 7 ish psi. ...so nothing has come on to cause a difference in pressure.

What is interesting is that with the hybrid high voltage driven compressor there is no way to turn it etc. so I think it had everything emptied out of it but couldn't really tell. Best I can tell is it has 2 connectors (1) high voltage +/- and a 12v connector? I know it has a high voltage connection, curious if it needs to be turned on by a module and whether it is always energized waiting for a 12v control signal? If so what signal does it need? i.e continuity?

Interesting idea on the hot water. heh! Looks like from the good truck I need to be somewhere about 50 psi?!?!? I wonder what that low side pressure threshold is...

I have no DTCs set, so I didn't think there was something to clear. Getting out the bootleg GM tech 2 is a pain in the ***, but you think if I do there something there I can do to command it on I guess I should try...

 

sounds like something isn't going right. from my experience there's way more than 7psi in a can. unless it's freezing outside. I don't know off the top of my head for these hybrid, but most low side switch's are around 20-40psi before they will turn on, but for me a can has always put in enough to get the pressure up to that level. I do believe I've seen a place to command the ac compressor on in the tech 2. I've not done it thou, but I've also seen low side and high side psi in the tech 2. worth a check just incase your gauge are incorrect? I'm not sure I'd command it on with only 7psi in there, or at least not for very long.

from my experience, I could get about 2 and a half cans in a 3 can system before I needed to turn the compressor on. these days I have a ac machine at work, so it's been years since I've done it with cans in my garage thou.

I was thinking you had low refrigerant dtc but if not, you probably don't have enough for it to run long enough to set that dtc. there's a whole page of things about that dtc in manuals.
​​last I looked, something like it's under 17psi for 240sec run time or something like that. if you can't get above that, it probably doesn't run 240sec.

you might have something weird with the can adaptor? I've never used a gauge set that needed that?



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​​

 

Ok, you were right! ...I was reading the manifold gauge wrong. Both the high side and low side are reading 70 psi. When I read the low side pressure from the Autel it say 7.6 psi and 4.5v. I got the bootleg Tech 2 out, and it reads 113 Kpa (16.3 psi?) and 4.5 ish volts. So I then used the tech 2 to command the compressor on and the A/C permission went from granted to withheld. Low refrigerant pressure flag is still set. I disconnected the low side pressure switch connector on the accumulator. Now it reads zero volts but says 9.8 psi, and the low refrigerant pressure flag went from yes to no.

Oddly I would think at this point I could compare the high side switch to the low side switch, but I don't see a high side pressure reading on the scanner.

I ordered new low side pressure sensor, its $30 bucks and I guess worth eliminating at that price. The only bummer is I have to pull the inverter to get at that part of the accumulator.

Let me know if you have any other guidance as to what else to try...

It would be nice to have a wiring diagram, on the HVAC module there is a hybrid section live measurements section, so I assume those functions are limited to what controls are modified for the hybrid vs a clutched compressor?

-Tom

 

let me take a look if I can find any useful info.

the high side psi is definitely in there somewhere, I even have hi side on my phone tq app display. I can't find the pid flow low side for it.

there's probably a ohm test to test the sensor. which you're right it's a sensor not a old school 2 wire switch. sorry about that.

I'll check in the morning

 

Thanks I will go look for the high side pressure and swap the low pressure switch with the new one that is inbound. At this point I think both sensors should show the same pressure as the manifold right? I was hoping to bypass the sensors just to get things going, but your confirming my fears that they are no longer just circuit closers. I think all the sensor conditions have to be right to have 300V go to the compressor from all the different sensors (or is it always present? Inside the inverter there is another 300v connector on what appears to be the same terminal block, hmmm?), and then I wonder after HV is present if a command is needed to electronics in the compressor or if it will just run? If it will just run I wonder what the other little connector on the compressor does, it must either be control and sense or both. Thats is what I was wondering if there is a wiring diagram that would show whats what here.

 

sorry that took so long, I wasn't near a computer for a bit.
​​​​​​so yeah, looks like both are sensors with 3 wires.








low pressure runs into the hvac module, so that's why you found the data for it in there with command on the compressor, rpm and stuff like that. the high side sensor looks to by the radiator. another 3 wire sensor but this one runs to the pcm. so you'll found it's data under engine data. also explains why I can easily find the high psi Pid for torque app but not the low side. it's in a different module.

but yes, I believe the sensor reading in a scanner should match your gauge set. so I think when you get a good sensor reading, it will work. if the new sensor still reads bad, i can find the pin number and wire color at their module for you to check.

here's what I found, might not be very useful but hopefully you get it running.

 

Excellent info. So from the diagram I take it that the HVAC control module "hybrid" section is the controls for the 300V to the compressor, I guess that fits with our theorizing, the sensors must be sensing pressure to control the logic as opposed to be just open/close to enable the compressor. So from what I can tell is there is no other way to "trick" the compressor on through a relay or something. My 3 pin connectors are a little different in that they are staggered in a T configuration but i am guessing they are the same wiring wise. It appears that the system between the non-hybrid and hybrid trucks is essential the same with the added 300v capability on top to drive the compressor. You were right on the high side pressure was in the engine data, and showed 78 psi!!! Ahha. Also, you are correct the high side pressure sensor runs from the top line of the condenser down the left side of the condenser looking in to the engine bay, the input and output lines then meet and travel as a pair back into the engine bay. So now I know that the low side sensor is definitely not reading correctly, but I also notice that the low side and high side pressure sensors are different diameters, which I guess makes sense. Unfortunately Amazon (or whoever the seller is on their site) sent me a high side switch as the low side, which wouldn't help me with the problem. Anyway knowing now that I have to get there, I pull the 300v disable, pull apart all the inverter covers, disconnect the high voltage wires to the inverter, remove the two clamp bolts on the left side, tip the inverter up onto its connector face (always seems sketchy, be careful here) and now I have access to the accumulator. What a pain!

...and the most appropriate diagnosis you made was "something is wrong", yup, you're right, I found the culprit!! Me. As soon as I touch a wrench to the low side sensor it turns. I guess after I screwed it on finger tight and then got caught up transferring all the insulation and such to the accumulator I never went back and turned the extra few turns to get it to open the shrader like valve! DOH!! ...all that work for a dumb should've double checked my connections maneuver! ...I am guessing it was reading atmospheric pressure. I need to go back and check that, I from school though I think thats like 5 or 6 PSI gauge if I recall correctly.

Reassemble everything, re-enable HV, and start the car with AC off. BAM! 80 psi on the low side! ...hold my breath and turn on the AC! ...scanner reports RPM on the compressor! Yipppieee... a few seconds later I can tell its definitely cold! manifold pressures match high side and low side pressures in the data!! Low side drops to like 40 PSI and high side climbs to 140 ish. ...things are going right on track. I then start adding the remainder of the 134a from can 3 and it goes right in (maybe a 1/3 can) and it goes right in. i hook up Can 4 and it won't take it. hmmm.... Low side pressure is back up toward the can pressure. I get the car and its cold in there! ...I guess everything turned off. I shut off the car and let things warm up for an hour. Restarted it and it ...slowly... took the last 1/3 of the can. So I get to roughly 40 oz in and the guages are reading 55 ish PSI on the low side and 140 PSI on the high side. I notice as the compressor runs the pressure goes up to just shy of about 150 PSI and then shuts down dropping. Cycles seem to be a few minutes. It was about 75 degrees here yesterday for the high, so it seems to match the general guidance I saw on AC forums. I am guessing this all sounds like what one would expect with a full charge? Interesting that I really noticed no change in pressures profile after adding the additional 134a over what the system would take on can pressure. In any case! I am happy! Decide that's the ticket, pulled everything, reassembled all the trim, shut the hood and went for a long drive. Far as I can tell, everything works as it should.

9 months and 15ish days, it literally $75 in parts and 4 cans of freon and everything is resolved!!! Never mind the cost of a battery rebuild (needed anyway), several different scanners, tools, etc. Oh and a twin hybrid truck for the price of what GM wanted for a battery (which I will say after a few fixes and upgrades - The dreaded oil pressure sensor issues (3 sensors), is a pretty darn fine and reliable ride). Wow, what a journey over a high impedance short on the compressor.

I will post some pictures of the measurements on the old compressor, after getting as much oil out of it as I could per Kieths thoughts none of the electrical measurements have changed. So its seems the oil isn't the only thing providing isolation, admittedly though I am not sure one can get all the oil out of the thing because you can't turn the compressor. I guess I could find some high voltage to connect to it and spin it, hmmm.... I'll see if I can disassemble it before I throw it in the trash!

So the saga is over!! ??? I commend you for all your help and perseverance of always being there to offer thoughts and sound boarding! Thank you! Truly a gentleman and a scholar when it comes to these things, and with a hybrid or electric car, wiring, voltage and other measurements, and computers are a must in the process! I know all the data you packaged and sent me will help so many others as there are likely several culprits which can put the car into a lack of isolation fault. ...I do recommend if you face one, first and foremost get the isolation tester and test the battery and everything north, that splits the problem in half as to where to work next. If you have on and find a lack of isolation on a drive motor, I am guessing its off to the junk yard, as i can't see how it would be economical to put a new tranny in. ...but hey what do I know, being the proud owner of two of these vehicles! ...BTW, both are now getting a respectable 23 miles to the gallon, and even higher when I stay between 40-60 Mph which on so cal freeways is pretty darn often unfortunately.

Thanks all!

-Tom

 

that's amazing work.. good job. I believe you now know more about these trucks than 98% of the gm dealer techs out there. so that's great.

for draining the compressor oil, you might find just leaving it draining for a while will help, with time it might drip out. that said, I knows it's a 300v dc input, but it does leave me wondering if you hooked it to a 12v battery if it would move at all. I'm sure it would be very slow but might be enough to pump out and test it for isolation.

as for a tranny, there's one tranny guy in Las Vegas that has showing some interest in rebuilding them, he says he can still get parts but can't find a cheap core to work on. he makes youtube videos tearing down and upgrading the 6 speed ones from the non hybrid trucks. without a few fixes, those things seem to blow up every 100k and cost about $6k to rebuild with labor. so honestly if you can find a used one, or someone that can actually rebuild them. I feel like from a money standpoint it's not any worse than a standard tranny. it's the limited quantity that makes it hard. there's a local one to me on Facebook for 400$. but it's 2wd and I have a 4wd. can't find any info on if it's swappable, he says it's got 150k on it and it did work, but no way to test that. so I've held off since mine seems to he in good shape.

good thing is, while there's a few threads around about trannys going bad. with just a standard filter and fluid change, they seem to last a long time. I sent a fluid sample off to have tested from my 150k mile one and it came back looking good. no signs of abnormal wear. I have since upgraded from acdelco dexton 6 to amsoil tranny fluid, as it seems to win out in every test I can find for wear and abuse. figured the cost of the fluid was worth it, since I think my first change at 135k to the acdelco stuff was the first it every had. no sign the previous owner touched anything. so I considered the first change just a 15k mile flush haha.

on a side note, it might be interesting if you run a load test on your rebuild battery as a base line and then a year or so from now if you've put some miles on it.

I know in theory they won't do as well as the oem batteries, but could be nice to know by how much. I'm trying to do that with my lithium ones. the longer these are on the road, I feel like the more creative we're going to need to get with replacement batteries. I know forsure the battery module could be cracked and adjusted how it handles batteries, but it's above my head and I doubt anyone else will do it to such a small discontinued market

 

How did you go about unhooking the AC system to fix your no start condition? I am having the same no start situation you are and I wonder if this may be my issue as when I bought the Tahoe there was a thermometer in one of the AC vents, an AC sensor was disconnected, and the guy mentioned he did AC work for a living before he retired. I wonder if he serviced the AC and caused a similar situation to what you had.

Thanks
Matt