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Limp mode-P0753 and now P1765

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Old Feb 8, 2025 | 07:25 PM
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Originally Posted by HeyYou
OK, that's just bizarre. I am suspecting the 'machine gunning' spot is pin 86? That's fed from the field windings on the alternator.... which is fed from the PCM. Field windings determine whether the alternator is putting out voltage, or not, and I always though it was pulse-width modulated, to keep alternator output in a reasonable range, but, that just wouldn't work for the trans relay.... It needs to be engaged whenever the engine is actually running. The logic there escapes me.....

Have you tried a different PCM??
according to the manual its pin 85 that is doing it. i dont have a spare PCM either to check if thats the problem


 
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Old Feb 8, 2025 | 07:36 PM
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What year truck, I'm surprised they're using a micro relay on the trans, i just worked on a 98 for no overdrive with electrical fault, it had the standard size relays.
 
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Old Feb 8, 2025 | 07:37 PM
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Originally Posted by dodgetruck2
What year truck, I'm surprised they're using a micro relay on the trans, i just worked on a 98 for no overdrive with electrical fault, it had the standard size relays.
Its a 99 with the 5.2
 
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Old Feb 8, 2025 | 07:44 PM
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So I have a 2001 manual in front of me, I hope they're the same. Pin 85 should be on the trans control side to the PCM and be ground only. Issues I would suspect are a failed PCM, corrosion in the PCM connector. The more likely option is im seeing the trans harness is shared in connector C130 with the leak detection pump, and there could be corrosion in this connector causing issues. I might try disconnecting the leak pump and see if results changes, Being as you have an evap code

again hoping 01 is the same as 99
 
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Old Feb 8, 2025 | 08:01 PM
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Originally Posted by dodgetruck2
So I have a 2001 manual in front of me, I hope they're the same. Pin 85 should be on the trans control side to the PCM and be ground only. Issues I would suspect are a failed PCM, corrosion in the PCM connector. The more likely option is im seeing the trans harness is shared in connector C130 with the leak detection pump, and there could be corrosion in this connector causing issues. I might try disconnecting the leak pump and see if results changes, Being as you have an evap code

again hoping 01 is the same as 99
Seems hopeful! What the heck is the leak pump lol never heard of it I’ll have to look it up
 
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Old Feb 8, 2025 | 08:02 PM
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Its for the EVAP purge, i believe its mounted to the charcoal canister down by the fuel tank
 
Last edited by dodgetruck2; Feb 8, 2025 at 08:23 PM.
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Old Feb 8, 2025 | 08:33 PM
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Yeah, Pin 85 should not show any voltage with the relay removed. Looking at the 98 wiring diagram, it's a straight shot from the PDC to the PCM, with no branching.... Which would indicate an internal short in the PCM?
 
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Old Feb 9, 2025 | 11:01 AM
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Originally Posted by HeyYou
Yeah, Pin 85 should not show any voltage with the relay removed. Looking at the 98 wiring diagram, it's a straight shot from the PDC to the PCM, with no branching.... Which would indicate an internal short in the PCM?
just confirmed pin 85 does have low voltage going to it with key on engine off. With engine running it’s getting 12.3 and dropping to 11.8 but the light keeps flashing red stating it is losing power and getting it right back. Tried posting video of it but couldnt
Confirmed key on engine off getting low voltage pin 85
Confirmed key on engine off getting low voltage pin 85
 
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Old Feb 9, 2025 | 11:04 AM
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Originally Posted by HeyYou
Yeah, Pin 85 should not show any voltage with the relay removed. Looking at the 98 wiring diagram, it's a straight shot from the PDC to the PCM, with no branching.... Which would indicate an internal short in the PCM?

https://imgur.com/a/ljtclEu
 
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Old Feb 9, 2025 | 11:06 AM
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It really shouldn't be doing that......
 
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