I am working on a 2011 Subaru Outback, automatic for a no crank problem. While testing the relay that puts power to the start terminal on the starter I encountered a 9 volt (bias?) coming from the ECM on the Black/ blue wire that grounds the relay. My 45 milliamp draw incandescent test light will pull the voltage down to about 4 volts, and my 97 milliamp incandescent test light will pull it down to about 2 volts. This is either the strongest bias voltage I have encountered or a problem in the ECM. I will update when I know more. Any thoughts?
I was testing at the start relay which is attached to the side of the fuse box in the car on the drivers side. The key was on and it was about 9 volts on the black / blue wire to the pcm with the relay unplugged(at the connector for the relay), and 5ish volts with the relay plugged in (back probed). It would light both test lights, not full bright, but more than I expected.
dhodges wrote: I was testing at the start relay which is attached to the side of the fuse box in the car on the drivers side. The key was on and it was about 9 volts on the black / blue wire to the pcm with the relay unplugged(at the connector for the relay), and 5ish volts with the relay plugged in (back probed). It would light both test lights, not full bright, but more than I expected.
You weren't kidding. That IS a strong bias voltage. :lol: I can't say I've ever seen a bias voltage that strong. Others will hopefully chime in.
Did the PCM set trouble codes related to the starter relay? With a bias like that, I'd expect the PCM to notice.
Not that it's related to the theory of the bias voltage at work, but what did you find wrong with the starting system? Bad starter?
I have just begun to play with this car and was doing some preliminary testing. This came to me form another shop that dropped the motor cradle to put head gaskets in and now it won't crank. I am told there is no comm. with the pcm also (have not checked yet) so I have no idea how the pcm views this lol. The check engine light doesn't not come on and it won't crank so I suspect a power or ground problem to the PCM. I do have 5 volt ref to the ac pressure switch so I think it is partially alive, I have a lot more testing to do before I really know anything. I was just confirming that I had everything I need on the starter relay and that the computer is just not grounding it when I ran across this. I will get back to it on Monday, later in the day but I was wondering if this was a known Subaru thing. I suppose it could be a bad pcm. I will know more later when I finish my homework on this car. This (bias?) was just really interesting to me.
dhodges wrote: I was testing at the start relay which is attached to the side of the fuse box in the car on the drivers side. The key was on and it was about 9 volts on the black / blue wire to the pcm with the relay unplugged(at the connector for the relay), and 5ish volts with the relay plugged in (back probed). It would light both test lights, not full bright, but more than I expected.
That's strange. For the relay to pull that voltage down it'd need a path to ground, which seems unlikely. Does your wiring diagram show the relay coil power feed as hot in run or just hot in start?
Good point!, the wiring diagram shows the other control side of this wire going back to the ignition switch and is only hot in crank.....soooo... interesting. Unless I am incorrect on how I remember my test, But I know I still have the bias on this part of the circuit you can test it right at the fuse(anytime not in crank) since this fuse is only hot in crank also.
dhodges wrote: I have just begun to play with this car and was doing some preliminary testing. This came to me form another shop that dropped the motor cradle to put head gaskets in and now it won't crank. I am told there is no comm. with the pcm also (have not checked yet) so I have no idea how the pcm views this lol.
Gotcha.
I'm gonna throw a WAG out there and say that the behavior you're seeing on the relay control circuit is another symptom of... Whatever is wrong with the PCM. :silly: Smells like a missing/resistive ground.
I was wrong, it makes no difference if the relay is plugged in or not as to the voltage of this (bias?). It is0 volts key off, 8.5 volts key on and battery voltage in crank. Sorry about that. I misremembered my testing. Not helpful I know. I should know more today.
