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I have a 1998 Automatic Coupe and am getting weird electrical issues. The car will crank but not start, it almost started one time but chokes right away and dies. When I turn the key, the DIC indicates "Low Fuel", "Reduced Engine Power", "Service Traction Control System", "Service ABS" and sometimes other various errors. I have the check engine light, ABS light, Traction light, and check gauges light illuminated. Codes pulled:
1518 - Throttle Actuator Control
U 1160 under HVAC - Communication Error
Troubleshooting so far:
1. I checked the battery voltage and cranking amps when fully charged and both were fine. However, It has gone as low as 10 volts during the time the car has been parked for diagnostics and I think something has shorted causing it to drain.
2. I have looked at the grounding points on the chassis under the battery and the two behind the headlights. Also the one near the fuse box inside the passenger footwell. None looked corroded but I did clean them up and sand them with high grit and put them back together.
3. I read about the ignition switch causing problems, removed it and found some burnt contacts. I cleaned and lightly sanded those and put it back together. Still got the lights on my dash when the key was ON.
4. I pulled the BCM out of the box and found a little bit of corrosion down at the bottom of the board, though nothing was currently wet. Cleaned that off with a soft bristle brush and rubbing alcohol and reinstalled it. Still get all the lights on my dash.
5. I believe I have an issue on the serial data bus and unplugged the STAR connector that links the LDCM, RDCM, and SCM. Lights still showed up with the key in the ON position.
I measured resistance between each pin in the other star connector and ground with they key in the OFF position and battery disconnected. Wires like the auto climate (white) and a couple others showed resistance around 150k ohms to ground. The PCM (dark green) showed about 350k but would steadily climb slowly up by 1k the longer the volt meter stayed measuring it. The BCM (light green) showed infinite at this point, but with the battery connected and key to ON I think I got about 80k ohms. The radio wire (orange) never showed any resistance on either key position and this was true for another wire or two as well.
After plugging everything back up as I found it, I measured the voltage from pin #2 in the DLC (goes to the purple wire at the STAR and links DLC to the serial data bus) to ground. I was expecting about 3.5 volts as the data bus oscillates between 7 volts highs and 0 volt lows, however I only measured .05-.2 volt. I think this means I have a short to ground somewhere in the serial data bus. I have tried many various combinations of removing certain systems from the STAR connector by depinning them but did not get anything to stop the dash lights from showing. Unplugging both STAR connectors completely also still results in dash lights.
Is this signs of a short somewhere and how do I go about finding it? If I have missed anything or forgotten to write up a step please let me know. Thanks for the help!
The voltage you saw on Pin 2 with your DVOM is normal !!…do you have a scan tool ??…only with a scope will you see a 0-7 volt square wave…a voltmeter is too slow to show this…I’ve demonstrated this small voltage to members having serial data bus issues…I’ve actually taken my break out box and put 12 volts to pin 2 and you can see what happens to the data bus…fuel gauge to E, water temp maxed out, and all DIC messages scrolling through with a start and stall…won’t damage any module though…good that you tried de pinning some terminals off the data bus…if you remove both bus bars off the star connectors and jump the PCM and BCM the car should start if you don’t have a bad PCM or BCM…you can check each terminal on the star connectors and see if you see 12 volts or near ground…good video below.
I do not have a scan tool unfortunately. I removed both star connectors and put a small jumper wire between the dark green (PCM) and light green (BCM) cables. The car dry cranks and will not start. What is the diagnostic procedure to determine if it is the PCM or BCM from here? Additionally, is there a voltmeter test I can perform to make sure the computers are getting proper ground and power before I spend money on a new module?
You can use a voltmeter on the PCM/BCM power and grounds but it's always best to do "loaded circuit" testing...if you have a back probe you can check these power and ground circuits with the connectors plugged in....system voltage should not drop much on a cranking test and ground should be less than 100Mv's...might be high on initial crank but that is due to all the current flow on the ground side (engine block) going back to the battery. PCM ground BTW is above the starter...battery negative is also connected there !!...BCM looks like it has 5 power feeds and one ground.
Pulled out the PCM and found that someone had drilled a hole in the side of the case to run a wire out where a pin had broken. They did not seal this hole and my PCM had standing water in it and corrosion. I will clean this up and re solder the wire, but if it still won't run I may have a new PCM in my future.
Pulled out the PCM and found that someone had drilled a hole in the side of the case to run a wire out where a pin had broken. They did not seal this hole and my PCM had standing water in it and corrosion. I will clean this up and re solder the wire, but if it still won't run I may have a new PCM in my future.
WOW !!…good find !!…I’d probably go for another PCM especially if there was water in it…can’t remember if the 98 PCM were hard to come by…try calling Flagship One…they may be able to help…if you tried jumping the PCM and BCM together and it still doesn’t start it’s one of those 2 modules.
The 98 PCMs are unfortunately rare along with the 97 ones. I cleaned up the corrosion best I could and re-soldered the wire in place. However, still have all the warning lights and messages on the dash, and gave it a quick crank - no start. Anyway to guarantee it's not the BCM that is the problem before I move forward with a PCM?
I know you found that wire going into the PCM but did you still check all those powers and grounds I had listed for you…if you need I can get the BCM power and grounds if you need them….there is only so much we can do for you on our end…a good scan tool and a scope would definitely help your cause.
Alright I was able to test all the pins you mentioned for the PCM. All the grounds had resistance to battery negative when tested with a multimeter and all the powers had battery voltage (12v) when measured to battery negative. Also, I now own a scope, but haven't done any tests with it yet. Sorry for the delay, I was catching up on school work and haven't had time to test. At this point the car has been out of commission for a month and I feel so lost, I don't even know what's wrong.
"Resistance checking" is not the correct way to check your grounds...loaded circuit (key on or engine running) ground to ground voltage drop testing is....you can also hook up your 12 volt test light to battery POSTIVE and probe those grounds...test light should be bright...light only draws about 500 milliamps.
Okay, I went back out and the voltage with the key in ON and got 0v on the grounds for the PCM. I also pulled out the PCM to double check the wire inside for continuity and noticed a few tiny spots of corrosion on the board still. Put everything back together after cleaning again and noticed the lights on my dash were gone. I cranked the car and it started and ran. I did not make many changes, so either touching up cleaning the board, or reseating the connecters to the PCM might have made the change. I gave it a couple minutes before shutting it off, no lights on the dash and everything seemed normal to me.
I'm still skeptical of it right now so I will update if the problem comes back.
Good idea to remove the PCM Connectors and spray with some contact cleaner...Deoxit is very good….also inspect each terminal for corrosion…afterwards spray terminals with either of these products on it…Stabilant 22 is expensive but the best product to enhance those electrical
connections !!
Electrical Issues - Serial Data Bus
