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post #1 of 5 Old 07-18-2019, 05:06 PM Thread Starter
Jwhite0627
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Crank no start

I have an 06 ram 1500 5.7L that has a crank no start condition. After some diagnosis, I came to the conclusion it's not getting fuel. So I turned my attention back to the fuel pump. I say "back" because I initially looked to it, but when I heard the pump prime at "key on", I went to check spark.
Anyways, so checked for pressure at Schrader valve. Has some but not what I'd like to see.
Long story short, I put a volt meter on the pump connector and I'm getting 12v at key on (when pump primes) and after 3 sec prime it drops to 10.5v, then finally when cranking it slowly drops to under 1 volt.
With a scanner on obd2, no codes, but looked for rpm signal. Confirmed pcm is receiving crank signal both with voltmeter and scanner.

I'm familiar with the 07 and up tipm fuel relay bypass, however it doesnt apply to this style tipm. So 1 question is, does anyone know a way to bypass it on this style tipm.
I would think that if the pump primes, that the relay works just fine? So then the problem should lie inside the pcm? Anyway to verify it is infact the pcm (without swapping a known good one, as it's been hard to find one to borrow)?
Any ideas would be appreciated.

Most diagrams and service manuals do not have documentation of this style tipm so it's been quite the project. I've narrow it down to 3 possible things.
1. Still a faulty crank sensor (though it sends some signal) however not likely
2.pcm receives signal and sends to tipm to send to fuel pump, but relay is bad in tipm so it never gets there. -unlikely imo because the pump primes with 12v
3.pcm receives crank signal and never tells tipm to turn pump on. (Most likely) but want to prove before throwing a part at it.
Maybe pcm controls the relay differently from different ignition switch positions?

Thanks in advance
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post #2 of 5 Old 07-19-2019, 12:22 PM
txwooley
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I'm not that familiar with the modern engine controls and interlocks so this could be way off base. Many years ago I had an Oldsmobile with the same problem. Key on, fuel pump runs for about 3 seconds. When try to crank, nothing. I replaced fuel pump and ECM to no avail. Finally decided to just hot wire the pump to key on ignition. Car ran great for about 2 minutes and then spit a connecting rod out the side of the block. It turns out that there is an oil pressure interlock that kills the fuel pump if there is no oil pressure (the idiot light bulb was missing so no warning until massive valve clatter for about 2 seconds followed by grenading engine).

Could you have a weak or non-functional oil pump and the truck is trying to save itself?

J.T. Wooley


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post #3 of 5 Old 07-19-2019, 12:39 PM Thread Starter
Jwhite0627
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I dont think you're that far off, I do believe that there is something keeping the pcm from telling the pump to turn on. However it's why does it not tell it to, because every signal it needs to do so is there. It just doesnt send the signal during cranking.
I know the circuit itself (all the wires, connectors, etc) is good because for the 3 seconds the pump primes the system functions as it should. It's just upon cranking it doesnt send the signal.

Its definitely either the pcm or the tipm. I just dont know whatelse I can do to prove it one part or the other.

I've eliminated the pump itself, I get the same voltage readings at the pump connector with it plugged in it unplugged.

So the only thing left is why is there a 2volt drop after the prime. Has to either be internally in the tipm or pcm
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post #4 of 5 Old 07-23-2019, 08:23 PM
super99ram
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Hi, recently I had an issue with my 1500 would runrof a bit and then shut off while driving. it would start back up after a 20 minute cool down. Found out the pcm was at fault and not much to do except change the pcm. Flagship one resolders and replaces parts known to fail in addition to resoldering the entire board. I have had this issue on my 99 ram as well as the 2006 charger. Both remedied for about 250 each issue. Hope it helps
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post #5 of 5 Old 07-24-2019, 12:06 AM
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How about:

4. Fuel pump is failing internally.
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