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2013 ATS 3.6 Performance
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Discussion Starter #1
Driving along this weekend....gauges flashed on and off and wipers swiped once. turned car off. would not start. got it jumped, and now engine light on reading codes P0685 - ECM/PCM power relay -circuit open
and code P0480 - Cooling Fan Relay 1 Control Circuit

Anyone ever run into these 2 or have had this happen while driving?

I am going to replace the relays in the fuse box first (cheapest).

If that does not solve the issue..........what next?

Replacing the ECU looks to be a last resort (most expensive) and rarely happens.
 

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2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
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2,872 Posts
Start with your battery and battery connectors because a power glitch can set a lot of unrelated codes. How old is your battery? If you car wouldn't start from its own battery but did start with a jump then I would strongly suspect the battery and its connections, it could have a defective internal weld/braze to one of the plate assemblies that was disturbed due to a bump in the road.

If you have a volt meter, measure the voltage at the battery terminals with the engine not running and the headlights off and then on. Compare the resting voltage and voltage drop under headlight load which will give you a clue to the state of the battery charge and health.

You can also use a volt meter for an easy check for bad battery connections, cables, and grounds. Measure across the suspected bad connector or cable when the engine isn't running and the battery is under load. The heaviest load is the starter but in most cases you can just turn on the headlights to provide sufficient load for testing. Set your volt meter to a range that includes 15 volts but not much over and then put one meter probe on each side of the suspect connection and the meter reading should be almost 0 (some meters will display a little because of measurement errors plus the inherent +/- 1 count error in most digital meter circuits). But any significant voltage means a connection or cable issue. For example if you suspect a bad battery connector, put one lead directly on the battery post and the other lead on the connector and check for a voltage reading (aka a voltage differential) across these two connected points. Do the same for the battery cables and the negative post to vehicle ground.

With these high current circuits, only a tiny amount of resistance will cause major issues. I have a specialized four lead meter that reads to a very small fraction of an ohm but for automotive testing purposes you can simulate the same setup with a standard volt meter using the car battery and headlight or starter load to substitute for the other two leads in a four lead meter setup.

Most recent GM vehicles (includes the ATS) use a smart charging system controlled by the ECM to cut alternator output at times to increase fuel economy and if a battery or battery connector glitch occurs during this off time then the sudden drop in the 12 volt bus will trigger errors just as you sometimes see when starting when a battery is on its way out.

And on edit, depending upon where you live this is the time of year when mice start looking for a winter home and it doesn't matter whether you are in an urban, suburban, or rural area these little elements of destruction love vehicles and love to chew on the connectors and wiring harnesses. Take a good look for any sign of mice nesting.

Rodger
 

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Registered
2013 ATS 3.6 Performance
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138 Posts
Discussion Starter #3
Start with your battery and battery connectors because a power glitch can set a lot of unrelated codes. How old is your battery? If you car wouldn't start from its own battery but did start with a jump then I would strongly suspect the battery and its connections, it could have a defective internal weld/braze to one of the plate assemblies that was disturbed due to a bump in the road.

If you have a volt meter, measure the voltage at the battery terminals with the engine not running and the headlights off and then on. Compare the resting voltage and voltage drop under headlight load which will give you a clue to the state of the battery charge and health.

You can also use a volt meter for an easy check for bad battery connections, cables, and grounds. Measure across the suspected bad connector or cable when the engine isn't running and the battery is under load. The heaviest load is the starter but in most cases you can just turn on the headlights to provide sufficient load for testing. Set your volt meter to a range that includes 15 volts but not much over and then put one meter probe on each side of the suspect connection and the meter reading should be almost 0 (some meters will display a little because of measurement errors plus the inherent +/- 1 count error in most digital meter circuits). But any significant voltage means a connection or cable issue. For example if you suspect a bad battery connector, put one lead directly on the battery post and the other lead on the connector and check for a voltage reading (aka a voltage differential) across these two connected points. Do the same for the battery cables and the negative post to vehicle ground.

With these high current circuits, only a tiny amount of resistance will cause major issues. I have a specialized four lead meter that reads to a very small fraction of an ohm but for automotive testing purposes you can simulate the same setup with a standard volt meter using the car battery and headlight or starter load to substitute for the other two leads in a four lead meter setup.

Most recent GM vehicles (includes the ATS) use a smart charging system controlled by the ECM to cut alternator output at times to increase fuel economy and if a battery or battery connector glitch occurs during this off time then the sudden drop in the 12 volt bus will trigger errors just as you sometimes see when starting when a battery is on its way out.

And on edit, depending upon where you live this is the time of year when mice start looking for a winter home and it doesn't matter whether you are in an urban, suburban, or rural area these little elements of destruction love vehicles and love to chew on the connectors and wiring harnesses. Take a good look for any sign of mice nesting.

Rodger

Thanks Rodger!

Battery is less than 1 year old. If the relays don't fix the issue I will take it out and get it tested. I dont own a voltmeter but I will get it tested.
 

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2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
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You are welcome. When you get a relay circuit open code, it generally isn't the relay itself and if the ECM power relay fails then the ECM itself won't let the car run. Basically, the ECM is in low power/sleep mode until the relay is commanded to supply normal bus voltage to the ECM.

I ran into a somewhat related issue when my previous 2006 diesel pickup had a fault preventing me from shutting off the engine with the key. A mouse had urinated in the UBEC (underhood bused electrical center) where the relays and fuses live. The center layer is open copper bus wiring and the urine created corrosion created leakage between the constant on (low power) 12 volt bus and the switched 12 volt bus to the ECM. When the humidity was high enough, the switched bus had sufficient leakage from the constant bus that it kept the ECM in active state so it would not respond to a shutoff command from the ignition switch. The only code set by this fault was loss of communications with the separate transmission control module because it was properly switched off by the key. IF the ECM is not powered by both the constant AND relay power switched buses then the engine cannot run. I suspect that your DTC was set due to a sudden drop in voltage.

Take a look at the relay and fuse for that circuit to make sure that they are well seated and that corrosion isn't present on their connector blades. If the plug in contacts are dirty, WD40 is a very useful cleaner for this type of electrical malady. Spray a SMALL amount on the male and female ends of the connector/component and then insert and remove the relay and fuse a few times to clear the contacts. CAUTION: WD-40 is largely comprised of light distillates of petroleum so don't spray it around a running engine or allow a large quantity to collect and with any work like this keep a fire extinguisher handy. At one point, WD-40 was used as a somewhat safer alternative to ether when trying to start cold IDI diesel engines; it isn't as flammable as ether but like any light petroleum product it will ignite easily. It does evaporate quickly so it doesn't take long if you do use an excessive amount to safely clear the area.

Rodger
 

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Registered
2013 ATS 3.6 Performance
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138 Posts
Discussion Starter #5
You are welcome. When you get a relay circuit open code, it generally isn't the relay itself and if the ECM power relay fails then the ECM itself won't let the car run. Basically, the ECM is in low power/sleep mode until the relay is commanded to supply normal bus voltage to the ECM.

I ran into a somewhat related issue when my previous 2006 diesel pickup had a fault preventing me from shutting off the engine with the key. A mouse had urinated in the UBEC (underhood bused electrical center) where the relays and fuses live. The center layer is open copper bus wiring and the urine created corrosion created leakage between the constant on (low power) 12 volt bus and the switched 12 volt bus to the ECM. When the humidity was high enough, the switched bus had sufficient leakage from the constant bus that it kept the ECM in active state so it would not respond to a shutoff command from the ignition switch. The only code set by this fault was loss of communications with the separate transmission control module because it was properly switched off by the key. IF the ECM is not powered by both the constant AND relay power switched buses then the engine cannot run. I suspect that your DTC was set due to a sudden drop in voltage.

Take a look at the relay and fuse for that circuit to make sure that they are well seated and that corrosion isn't present on their connector blades. If the plug in contacts are dirty, WD40 is a very useful cleaner for this type of electrical malady. Spray a SMALL amount on the male and female ends of the connector/component and then insert and remove the relay and fuse a few times to clear the contacts. CAUTION: WD-40 is largely comprised of light distillates of petroleum so don't spray it around a running engine or allow a large quantity to collect and with any work like this keep a fire extinguisher handy. At one point, WD-40 was used as a somewhat safer alternative to ether when trying to start cold IDI diesel engines; it isn't as flammable as ether but like any light petroleum product it will ignite easily. It does evaporate quickly so it doesn't take long if you do use an excessive amount to safely clear the area.

Rodger

I checked the wiring both in the trunk and in the front. I do not see any visible corrosion or wear. Unless there is a bad link somewhere between the rear battery terminals and the front engine harness. Not sure how I would be able to track that without a lift.

I will replace the relays since they are only $9 a piece. Then check the battery. If that doesnt fix it, idk if I will be able to avoid a mechanic shop. I know if I go to Cadillac they will go straight to replacing the ECU even though that is most likely not the culprit.

Thanks for the advice Rodger, I will check the housing connections for the relays when I replace the new ones tonight.
 

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Registered
2013 ATS 3.6 Performance
Joined
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138 Posts
Discussion Starter #6
You are welcome. When you get a relay circuit open code, it generally isn't the relay itself and if the ECM power relay fails then the ECM itself won't let the car run. Basically, the ECM is in low power/sleep mode until the relay is commanded to supply normal bus voltage to the ECM.

I ran into a somewhat related issue when my previous 2006 diesel pickup had a fault preventing me from shutting off the engine with the key. A mouse had urinated in the UBEC (underhood bused electrical center) where the relays and fuses live. The center layer is open copper bus wiring and the urine created corrosion created leakage between the constant on (low power) 12 volt bus and the switched 12 volt bus to the ECM. When the humidity was high enough, the switched bus had sufficient leakage from the constant bus that it kept the ECM in active state so it would not respond to a shutoff command from the ignition switch. The only code set by this fault was loss of communications with the separate transmission control module because it was properly switched off by the key. IF the ECM is not powered by both the constant AND relay power switched buses then the engine cannot run. I suspect that your DTC was set due to a sudden drop in voltage.

Take a look at the relay and fuse for that circuit to make sure that they are well seated and that corrosion isn't present on their connector blades. If the plug in contacts are dirty, WD40 is a very useful cleaner for this type of electrical malady. Spray a SMALL amount on the male and female ends of the connector/component and then insert and remove the relay and fuse a few times to clear the contacts. CAUTION: WD-40 is largely comprised of light distillates of petroleum so don't spray it around a running engine or allow a large quantity to collect and with any work like this keep a fire extinguisher handy. At one point, WD-40 was used as a somewhat safer alternative to ether when trying to start cold IDI diesel engines; it isn't as flammable as ether but like any light petroleum product it will ignite easily. It does evaporate quickly so it doesn't take long if you do use an excessive amount to safely clear the area.

Rodger

Just to follow up Rodger. It turned out the Battery Fuse Box Distributor in the back blew the main fuse. I don know if this was from the relay being bad...or if there is a loose ground wire somewhere in the circuit loop. however, i checked around the battery and up front around the fuse box and +connection and there is no ground loose or corroded. i am replacing the Fuse Distributor, and I already replaced the corresponding relays. if it happens again (cross fingers) i will take it to a certified electrical engineer auto shop that I know of around my area. But i think what happened is i drove with a bad relay...which then caused the fuse box fuse to blow.

sound about right?

thanks for all of your input.

Vic
 
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