No Start, No OBD Signal
01-26-2020, 09:21 PM
#1
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No Start, No OBD Signal Hi,
I've been trying to get my LM7 swapped Volvo running for quite a while. The PCM does not seem to trigger spark or fuel while cranking. When I connect a bluetooth scan tool it will also not connect to the PCM.
I've tried swapping the PCM and messing with grounds with no luck. Tonight I was messing with it and measured the resistance on the OBD II pin 4 and 5 to ground. I got some confusing results;
OBD pin to fly wire alligator clipped directly to battery negative terminal gave 120 ohms.
OBD pin to alligator clip connected to interior bolt gave 0 ohms.
So it looked like the PCM ground had high resistance back to the battery.
Now the odd part... I connected the battery negative to the interior bolt from which I previously measured using the fly wire. The resistance on OBD pin 4 and 5 back to the bolt jumped from 0 ohms to 20 ohms.
How could this be possible? I have somehow increased the resistance in the ground circuit by paralleling it to the battery negative. Any explanation for this? I expected it would stay at 0 ohms.
I've been trying to get my LM7 swapped Volvo running for quite a while. The PCM does not seem to trigger spark or fuel while cranking. When I connect a bluetooth scan tool it will also not connect to the PCM.
I've tried swapping the PCM and messing with grounds with no luck. Tonight I was messing with it and measured the resistance on the OBD II pin 4 and 5 to ground. I got some confusing results;
OBD pin to fly wire alligator clipped directly to battery negative terminal gave 120 ohms.
OBD pin to alligator clip connected to interior bolt gave 0 ohms.
So it looked like the PCM ground had high resistance back to the battery.
Now the odd part... I connected the battery negative to the interior bolt from which I previously measured using the fly wire. The resistance on OBD pin 4 and 5 back to the bolt jumped from 0 ohms to 20 ohms.
How could this be possible? I have somehow increased the resistance in the ground circuit by paralleling it to the battery negative. Any explanation for this? I expected it would stay at 0 ohms.
01-26-2020, 09:40 PM
#2
TECH Senior Member
You might want to check all grounds again.
01-26-2020, 10:23 PM
#3
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I have battery negative running to the block. Block to frame rail and both heads to firewall. I've swapped the battery negative cable and messed with all 3 ground straps. It does seem like a grounding issue but I cannot figure it out for the life of me.
