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My 97 Coupe will randomly shut down while driving. It runs great/revs up totally fine when running. When it shuts down, it will struggle to restart and backfire/pop until I reset the BCM. There was also an issue where it would start but as soon as I put it in gear, it would quit.
Things I have a code for:
MAF low signal, Cam Pos sensor low signal, trans solenoid switch. MAF is definitely dead (no change while datalogging), CPS is probably dead (errors on tech2 on "high/low"). Tachometer does NOT work. the FSM suggests the tach is based on the crank sensor, but could it possibly be cam sensor driven? I can datalog the RPM on hptuners/tech2, but the dash tacho reads zero.
Somehow, the car will still start without an issue whenever it dies if the BCM is restarted. When I was troubleshooting, I was able to kill the engine by messing with BCM controls on a tech 2 and the car's window switch. I popped open the BCM and can't see any water damage, and nothing looks burnt/smells burnt/cap and ICs look OK. I have NOT been able to consistently been able to get the car to stop.
The MAF WAS working at one point, as was the CPS (I datalogged after I got towed home). Now, with both sensors out, it's running fine and I have not been able to get it to trip off again. I'm wondering now, if the two failing sensors was able to kill the car by confusing the PCM. Now that they both failed, the car is ignoring both sensors and using only the crank position sensor to run, therefore running fine? Ever since noticing the cam position sensor has fully died, I have not had the car quit on me but I have also not driven on the road (just idling).
When the car stopped on the road, it was a very quiet death both times - no hiccups, no coughs like a fuel cut/fuel pump issue. Very computer/ignition feeling. The 2nd time it died, it made a quiet backfire pop, suggesting some confusion on timing. Potentially pointing to the trans switch, both times it was also driving when it quit. Thought maybe it was the CLB so I installed the LMC5 between the first and 2nd time the car quit, so it was obviously not that.
Thinking its wiring, cam, crank, or BCM/PCM issue.... any other thoughts?
P0102 - Mass Air Flow (MAF) Sensor Circuit Low Frequency (its dead - new one on the way)
P0342 - Cam Pos Circuit Voltage Low (H, not C - I think its dead, Tech2 keeps counting up on too low/too high)
P1571 - Traction Control Torque Request Circuit
C1241 - Magna Steer Malfunction
C1277 - Requested Torque Signal Circuit Malfunction
Tacho on dash does not work, or is intermittent. Tacho to DLC works.
First failure, I took a video of the error codes - Only one then was P0705(Trans Range Switch Circuit). I have a switch coming in to swap too.
In the tech2 while stopped, the transmission switch seems to be working fine, but I suppose it could be failling as it's moving. Oddly enough, shifting from anything to [D] has a markedly hard shift, but if I skip D and go to 1, and then back to D, its a nice softer shift.
One more thing - when scanning with the T2, every module communicates fine except radio due to aftermarket radio install. So, doesn't seem like its a module killing the datalink unless its very intermittent. Also have to mention the driver side window did lock up on the "up" position recently and I was able to do the motor screw tap trick to get it working again. It's been problem free since.
You need the crank sensor to start the car…a bad cam sensor will only cause an extended crank…the Tach gets its signal from the crank sensor…what BCM controls on the Tech 2 were you messing with to kill the engine ??…I’d clear all the DTC’s and see what comes back as current…the car shutting off would more than likely be a crank sensor but if the PCM “resets” it can cause the crank sensor to get “confused”…also a bad PCM can cause this so I would check the PCM powers and grounds…this can be a difficult one to diagnose…the video below concerns a C6 but is applicable to a C5…Before changing the MAF sensors I’d check the wiring first…how do you “restart” your BCM ????
With sensor unplugged and with ignition on you should see 12 volts on the pink, 5 volts on the yellow and with a test light connected to B+ the test light should illuminate when probing the black/white ground.
You need the crank sensor to start the car…a bad cam sensor will only cause an extended crank…the Tach gets its signal from the crank sensor…what BCM controls on the Tech 2 were you messing with to kill the engine ??…I’d clear all the DTC’s and see what comes back as current…the car shutting off would more than likely be a crank sensor but if the PCM “resets” it can cause the crank sensor to get “confused”…also a bad PCM can cause this so I would check the PCM powers and grounds…this can be a difficult one to diagnose…the video below concerns a C6 but is applicable to a C5…Before changing the MAF sensors I’d check the wiring first…how do you “restart” your BCM ????
I don't remember what I was messing with on the tech2 - I think I just tried to read the codes or connect to a module and the car shut down on me. I was also able to get the shutdown to occur when I was messing with my window controls on the left door controls. I thought that pointed at some issue with the serial bus, but ever since both the cam and MAF sensor are reporting as failed on the tech2, the car has not shut down by itself.
The yellow wire on the MAF is giving back a ~1.45V, not 5V to ground. The car still runs so then I guess it kind of rules out a crank sensor? I am getting the cam sensor code as well as the trans switch code so ill swap both out and retest at the MAF - do you suspect one of those is the cause of the lower voltage? PCM issue?
BCM Restart consists of me pulling fuse 25 in the passenger footwell.
The last few times i've reset BCM and restarted, the only errors that populated were MAF fail and CAM fail on low voltage. First time the issue happened, those two were "fine" and only the trans switch P0705 was current.
I wish I had an o-scope. I may end up picking one up soon.
Only 1.45 volts…WITH IT UNPLUGGED ??…so you used battery negative as your ground ??…you should check some other sensors with a 5 volt reference…unplug the ECT sensor and you should see 5 volts on the yellow wire….also 5 volts on the gray wire on the AC Pressure Sensor…if the 5 volt reference goes down so will the engine…the 5 volt reference is like the “heart” of the PCM…most shops these days don’t use a scope which is essential to diagnosing cars…I’ve been using one for almost 15 years…BTW there is a 5 volt reference on on EBCM Requested Torque Input circuit…that may explain your C1277.
Only 1.45 volts…WITH IT UNPLUGGED ??…so you used battery negative as your ground ??…you should check some other sensors with a 5 volt reference…unplug the ECT sensor and you should see 5 volts on the yellow wire….also 5 volts on the gray wire on the AC Pressure Sensor…if the 5 volt reference goes down so will the engine…the 5 volt reference is like the “heart” of the PCM…most shops these days don’t use a scope which is essential to diagnosing cars…I’ve been using one for almost 15 years…BTW there is a 5 volt reference on on EBCM Requested Torque Input circuit…that may explain your C1277.
The AC sensor and the ECT both return a 5V as it should... triple checked the MAF and its still 1.45V. I felt around the harness and don't see any kinks or any other issues. Car still turns on and runs OK with zero MAF input. Assuming it is going off of crank position and MAP? I can almost reach the cam pos sensor, will get to it tomorrow and retest the MAF to see if its still at 1.45.
Does the sensor 5V get shared across all sensors or are there groups of sensors that share them?
Tomorrow, I'm planning on pulling the battery, checking pins on the PCM for corrosion, and also pulling the intake mani to get at the CPS. If I get thru all that, perhaps the CKP or Trans Switch also.
Well verifying 1.45 volts I would look for high resistance or a short to ground on the signal wire…that terminal is pin 48 on the C1 connector and will have to be disconnected to check for a possible short to ground with an ohm meter…now on my C6 there are 2 separate shared 5 volt circuits but I haven’t seen that on the C5…the MAF’s backup is the MAP sensor…I do know however that the 12 volt Ignition feed CKP and CPS circuits are shared…if you have a piercing probe you can check an inch or so upstream on the MAF connector…maybe just use a straight pin if you don’t have one.
pulled IM off, with key on cam pos sensor gets 12v, 0v, .02v . maf now gets 12, 2.5v.
cam pos sensor visually isnt damaged. taking it out of the sensor circuit doesnt increase voltage. now leaning towards maybe crank, trans switch, or pcm. going to pull any sensors i can off the trans to see if voltage goes up.
for the maf and cps, is the wires supposed to show 12, 5, and ground? i checked continuity to see if i had a ground fault on the maf and i got 130 ohm, no continuity, no continuity.
for the cam, i got 130, 56k, and no continuity.
Cam sensor unplugged you should see 12v on the ignition feed, less than 100 mv’s on the low reference and 0 volts on the signal…the signal gets pulled up to 12 volts as it passes the camshaft reluctor since it is a Hall Effect Sensor…the MAF is 12, 5 volts on signal and ground…to check for short to ground on MAF signal wire MAF connector and PCM C1 connector must be unplugged…with ohm meter on highest resistance scale one lead of ohm meter on the MAF connector yellow terminal and other lead of ohm meter on a clean ground…ohm meter must show “OL”…or whatever your ohm meter shows when the leads aren’t touching…some meters say “1”…if anything other than OL you have a short to ground…I have no idea of what kind of “ground fault” continuity testing you are doing…are both ends of the circuit unplugged doing this ??…so have you checked for 5 volts on the MAF signal wire anywhere between the MAF connector and PCM ??…I normally don’t do resistance checking on circuits…the only resistance check I’ll do is for 60 ohms on a C6 or later CAN bus circuit.
Confirming all wires seem to NOT be shorted to ground anywhere on the PCM loom. Only "weird" ones are the fan relays (80-100 ohms with key in ignition), and oil level sensor at 76.2 ohm to ground.
All switched pcm pinouts appear to be working fine too.
Really leaning towards broken PCM? However, after checking for any grounding issues, I put the PCM back in and rechecked the MAF 5v wire and its at 5V. Wondering if my messing around with the wire loom/PCM pins somehow fixed a bad contact? Or, is the PCM just "cold" so its working fine until a component overheats and fails?
I also did the dumb thing and pulled the PCM out of the housing to check for water marks - looks totally OK inside but where the loom attaches has some aluminum corrosion. Perhaps some aluminum oxide powder bridged a few pins and my reseating it fixed the issue? I don't see any obviously burnt looking ICs, traces, or caps either. I DO notice one IC looks slightly like it may have an issue (185-1625045) - slightly less oily/shiny in the middle, leading me to think its overheated at some point. A lot of the joints do look kind of cold-joint-ish...
Actually, looking through the first error code I found, the trans fluid pressure switch, I have C1-22 having roughly 12.6 ohm to ground (Trans Range B input)... If im reading the bekow schematic correct, I should not be having it grounding out in park?
Im confirming the trans position switch is correct, with pin 26 and 32 grounding (A and P pins on the switch)... Could the pressure switch reading wrong confuse the PCM enough to cause it to shut down the engine?
I would say it was a connection issue vs a heat one…if you have a heat gun you can heat up the PCM a bit and see what happens…I use the 2 products below and they are the best…Stabilant 22 is expensive but is the best contact “enhancer”…before condemning a PCM I would check all powers and grounds…I use an old headlight bulb to check the powers and grounds…don’t think
the trans switch will cause a shut down.
I do have bad news though, the 2.8~ish volt MAF voltage came back. I did do a PCM relocate to the cabin interior - hooked everything back up, checked MAF and its below 5V. I am thinking it might be a board issue- cold solder joint, when installed in the housing it is torqued just enough for it to fail. When I remove it from the housing, the flexing is gone, and the solder joint has enough contact to give me 5v? tomorrow, ill try remove it from the housing again and retest.
Trans switch looked totally fine too - cam also looked fine. Car starts, runs totally OK without the MAF and I still have not been able to get the car to stop on me over the last 5-6 hours of testing this damn thing!
I might pick up a PCM from the bay and see if it fixes my issues. If not, I guess ill have an extra PCM?
Also, dumb question - will a bad BCM be able to kill an already running engine? Just trying to hedge my bets and maybe order a BCM at the same time?
(I can always get the engine to start - sometimes it takes a little longer cranking to go, but i have been able to get it running always. it just dies randomly and quietly like something quit).
Final thought:
My tacho has been spotty for a while. I'm reading that the PCM signal is the one used to move the tacho, I can pull RPM info from a OBD connected device (over serial, maybe?). The tacho had been out for almost a year, then it all of a sudden started working right before the failures. Thinking bad contacts/bad IC on the PCM is acting up now.
Also, dumb question - will a bad BCM be able to kill an already running engine? Just trying to hedge my bets and maybe order a BCM at the same time?
(I can always get the engine to start - sometimes it takes a little longer cranking to go, but i have been able to get it running always. it just dies randomly and quietly like something quit).
If you buy a PCM on ebay, don't buy from Flagship1. They've been giving me the runaround for the replacement. I'll probably end up buying from somewhere like speed engineering and just have a full standalone PCM or something.
Anyway, ive been starting the car up on and off and it runs totally fine except for the weird sensor issues. I cleaned and checked all the grounds in the engine bay and no improvements anywhere. about to start heat gunning the PCM to see if I can improve some solder joints and get the sensors running fine.
After the last post, I got irritated at waiting for FS1 so I took my PCM apart to check it out. I found out how to fully separate the board from the case and was able to open it up and inspect both sides of the sandwiched boards. I found a LOT of cold solder joints so I started reheating and flowing a few of them. Then, I noticed the grounding pins on the board had broken joints. There are 3 pins on each board - four of the six pins looked nice and shiny but upon closer inspection had actual cracks running through the joint. These were really hard to reflow since they go directly to the PCM inner frame and pull a lot of heat. The solder back in the early 2000s/late 90s were really weird too. I've had other circuit boards where they do not like mixing with my solder. Fully removed all the solder from the broken joints, reflowed new solder - When I reconnected the PCM, my MAF voltage came back, my idle changed (no longer in limp/no MAF mode), my trans no longer SLAMS into gear from Neutral/Park, and my in dash tachometer also works properly!
If anyone experiences intermittent tach/weird temp sensor/MAF sensor dropping/random engine cuts/slamming gear shifts keep those grounding pins on the PCM in mind!
I'll update if the engine cuts out again, but based on a few of my issues being resolved I have high hopes the PCM is fixed.
C5 (stealth edit - sorry used to posting C6) Diag, thanks for all the help - you were right, it was a grounding issue - just internal to the PCM.
After the last post, I got irritated at waiting for FS1 so I took my PCM apart to check it out. I found out how to fully separate the board from the case and was able to open it up and inspect both sides of the sandwiched boards. I found a LOT of cold solder joints so I started reheating and flowing a few of them. Then, I noticed the grounding pins on the board had broken joints. There are 3 pins on each board - four of the six pins looked nice and shiny but upon closer inspection had actual cracks running through the joint. These were really hard to reflow since they go directly to the PCM inner frame and pull a lot of heat. The solder back in the early 2000s/late 90s were really weird too. I've had other circuit boards where they do not like mixing with my solder. Fully removed all the solder from the broken joints, reflowed new solder - When I reconnected the PCM, my MAF voltage came back, my idle changed (no longer in limp/no MAF mode), my trans no longer SLAMS into gear from Neutral/Park, and my in dash tachometer also works properly!
If anyone experiences intermittent tach/weird temp sensor/MAF sensor dropping/random engine cuts/slamming gear shifts keep those grounding pins on the PCM in mind!
I'll update if the engine cuts out again, but based on a few of my issues being resolved I have high hopes the PCM is fixed.
C6 Diag, thanks for all the help - you were right, it was a grounding issue - just internal to the PCM.
Weird BCM? PCM? Issue
