Where do you go and what would have been your next step?
I'm going to be flat out with you all, my thinking was locked up with this one. But in the end research helped me solve this one and i learned from it. What I'm mostly looking for is to hear from you more experienced diagnosticians/ techs. What methods would you have gone with. Could you see or conclude that there is a fault that is related to all the fault codes/ symptoms?
2010 Chevy Malibu with 3.6L is towed in. Driver said they where cruising down the road and car starts running rough and the "Reduced Power" message is displayed along with CEL. The next morning i hook up the Tech 2 and run codes on it. in the ECM I have 5 faults,
-P0131 H02S circuit low voltage B1S1 Failed/current
-P0171 Fuel trim system lean B1 Failed/current
-P2101 Throttle actuator position performance Passed
- P1516 TAC module throttle actuator performance Passed
-P219A Unknown DTC
I start the vehicle up and its running fine, STFT on both banks are switching 0 to -4 at idle, LTFT are -2 -4, upstream 02s switching in range, downstreams are about 700mv steady. TP actual is the same as desired, both agree. For some reason I'm thinking the fuel trim looks fine the 02 looks to be operating fine. I raise the rpm and the 02s on upstream aren't flat lining lean on either one nor are the downstreams doing anything wild. BTW there was no freeze frame stored for codes. KOEO i graph the TP sensor pids and watch as i do a slow sweep test and I dont see any drops. This is where i break the rule that ive been following and have heard from so many instructors i erase the codes, reset fuel trims and do an idle relearn. I go out on the road and do a hard accel and bam! CEL comes on, reduced power message and very little throttle response. I pull over and look at TAC data my TP is stuck at 33% open my desired is 0. The only codes to come back are my TAC faults the 1516 and 2101. So i cant make it back with it in reduced power mode so i shut the car off and erase faults and drive it back. while driving back to shop I try to get it to act up again. Uh-oh I can't and I just was able to and erase all that fault data.
Now I'm back in the show and I'm doing a visual inspection, looking For TSBs and PIPs and only thing i find is the issue with icing of the TB plate due to water in the intake boot. I notice that the throttle body looks to have been replaced not to long ago and the connector is a new pig tail that is soldered to the harness. I see a new PCM and new black wire running down the wire harness and spliced into the X2 pcm connector. I ask what the history on this car was and if this problem has been going on for a while. The customer tells me yes the same issue has been occurring every few weeks. Its been to another shop where they replace 2 TB, a Accelerator pedal, the Pig tail/ connector for the TB, and a new PCM 2 weeks ago but it happened again. At this point I drive the car 5 times on the highway trying to get the issue to pop up again and i can't. I look at fuel trim data, 02s data, TAC data but i can not find any issues with what data I'm looking at. so i come back to the shop and I'm wiggling connectors and wiring, GENTLY tapping on the TB, doing a TB sweep test again, nothing.
Here is where i lock up, I dont know what else to check. I use my DVOM and check for 5v ref at TB and I have it. I voltage drop the ground and its fine. I looked at the voltages of the TP sensors with my DVOM while opening closing. I realize that I should have done that measurement with a scope and long story short is I own a scope but where I work they provide tools and wont let me bring it in. Next step i do for the heck of it is a resistance check of the TB to PCM wires even though I believe the test isn't full proof and has its place at times. All 6 wires measure .2 to .3 ohms. With both connectors still disconnected i check for a short to ground using ohmmeter, I take one lead of the DVOM to ground and other i probe in each individual circuit and I read OL across the board. Now what? My gut tells me do not let this vehicle go because it will act up again if I do and I will not just through parts at this thing.
Time to step back and regroup. I look in my ATG GM drive ability and code diagnostics manual and read up on the TAC section. I get some knowledge on different reduced power modes. look at code definition and code setting criteria. Then I hop on Identifix and look at what comes up for the TAC codes. Most the direct hit fixes show throttle body cleaning or replacement. 2 other fixes talked about wiring rub through on the trans pan bolt right below the pcm connector. With nothing to lose I look at the wiring at the PCM connector with the wire loom off. Right where the wiring bends by the trans pan I find 2 wires that have been chaffed and copper showing. 1 is a YE wire and the other a PU/W wire. Okay well I know from my notes that the TB has a YE wire for the motor and i know from the PCM connector its pin number 11. I remove the connector cover, find pin number two on the back of the connector and follow the YE wire. Bingo! its YE wire thats chaffed. My mid starts thinking hmmm, whats this PU/W wire? non of my TB wires are PU/W, so i follow the wire up to the back of the PCM connector, its in pin slot #3. Quick look at the connector diagram and pin # 3 is for H02S B1 S1. Now its all making sense to me with the codes i saw when it first came in. I wanna prove this is the problem so i lay those wires up against the bolt the harness was at before i looked and it. I start the vehicle with the Tech 2 hooked up and then gently move the wiring up against the bolt. There it is reduced power mode, TAC codes and 02 sensor code returns.
For the repair I cut out the damaged section of wire and soldered new wire in place. I got lucky on this issue because i really did not know what else to do. I was thinking was there other pids i should have been looking at while the problem was happening? Or did I not look into all the data close enough or was there a test out there that could have lead me to the short? I wanna know what everyone else would have done, what words of advice can you give me so that I can learn more from this? Thanks guys.
Thomas you did very well in your diagnostic path, wiring issues can be very difficult to find and the fact you did your research and did not revert to throwing parts at this vehicle demonstrates your passion to diagnose, well done. Most of the time it is the diagnostic path that takes the time not the fixing of the actual problem. A good technician will often spend 3 to 4 or even more hours
Maurice I have been trying to prove the importance of needing a scope. the Response I get is that with the factory scan tool graphing I don’t need a scope now a days. I just laughed silentl. But your right I am thinking of making a move because I feel that I went from a place where I was excelling in diagnostics to go to a place that pays better and offers benefits but to sacrifice advancing My
I do not know what it is like in your country but here is Australia we lack skilled technicians and many are undervalued because their bosses like yours do not recognize their skills. We all need to be valued and I think job satisfaction is very important. I hope you find the right place of employment were you have the freedom to be a diagnostic technician an get the appreciation you deserve
Thomas, I wouldn't fault anything you did and after all, you did find the problem. That's the kind of fault that can cause a lot of parts changing but you stuck with it until you found the root cause. I've been down that road many times and want to second guess my steps everytime. Intermittents are always tough and there is no "right" way to tackle them. You need to get creative at times…
Bob Ill have to start doing this even though it can be a pain in the butt to find.
I love using scopes and trying to catch elusive problems with them but it can be a little humbling sometimes to spend a couple hours trying to catch an intermittent and then realizing that you could have found it in 10 minutes with a good visual check.
Same here, I work at a place that says "that guy is a good tech" you mean the guy who did a code pull and said the vent solenoid is bad, oh what's that now I spend 3 hours with my testing equipment out that the shop doesn't provide, and found the pinhole in the harness in the fender well, but I have found a visual around problem areas, broken harness clips, sharp bends, road debris splash zones…
Sean I have a Vantage ultra which i love. Really want to get into the picoscope.
Nice work, I think your approach was pretty solid. I have been trying to do a close visual and check wiring diagrams just to have a background on what I'm looking at early on though. Also in addition to measuring resistance between components though, I've been load testing them with a light bulb. A wire with only one strand of copper left will read the same resistance as one with no damage, but
Hans, after last Friday I was thinking dang what if i had disconnected the TB connector and pcm connector, installed a bulb one at a time to each pin of the TB connector and then use a power probe to power and ground the circuits from the pcm connector. I don't know if the result would have been any different but my mind wasn't thinking at that time.