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Fuel Pump Issue - Ghost in the Machine?

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Fuel Pump Issue - Ghost in the Machine?

  #11  
Old 04-22-2018, 11:00 AM
ClintJohn
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Originally Posted by HeyYou View Post
May just be a bad relay too...... cheap and easy to swap out.
Thanks again HeyYou, i'd love to be able to swap out the PCM/ECM/TCU and the relay cheap and easily but first i have to locate one as apparently they have been discontinued for years and my mechanic told me that i have to find one with exactly the same serial number... but wouldn't all 95 Dodge Dakotas with a 318 V8 be the same that way? Also, obviously i'm a newbie, so not clear what relay you are referring to there...? you wouldn't happen to know of a video off-hand that explains all that would you?
 
  #12  
Old 04-22-2018, 11:05 AM
ClintJohn
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Originally Posted by HeyYou View Post
PCM controls the fuel pump via a relay in the pdc. (assuming the dakota is similar to the full-sized trucks.) So, not getting power to the pump may be caused by a bad relay, or the pcm not controlling the relay......
please pardon, i'm new and don't know what a pdc is. plus not finding how to swap out either the computer or the relay... i'll continue my research and figure it out. thanks again
 
  #13  
Old 04-22-2018, 11:23 AM
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PDC = Power Distribution Center. Black box right behind the battery. Fuses and relays are in there.

Before replacing anything, need to find out what the actual problem is first. You don't want to be just throwing parts at it, in hopes that you finally find the real problem. Here, I am *assuming* the system works the same way as the full sized trucks. (which I am more familiar with......)

So, the way it is SUPPOSED to work is: When you first turn the key on, PCM will ground the control circuit for the fuel pump relay, and run the pump for three seconds, then it opens the circuit. (pump stops running) After that, the PCM needs to see the engine turning to once again ground that circuit. (and thus, turn the pump back on.) Computer uses the crank sensor (and sometimes the cam sensor) to see if the engine is actually turning. If it gets a signal from either/both of those sensors, it'll turn the pump back on. So, the big question becomes, when is the pump NOT running? If you get the initial prime, but, nothing after that, chances are good, that the problem is NOT with the fuel pump at all, but the PCM not seeing the engine turning. If you do NOT get the initial prime, then you need to look at the fuel pump, and it's control circuits.

Clear as mud?
 
  #14  
Old 04-22-2018, 05:44 PM
ragged89
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Originally Posted by HeyYou View Post
PDC = Power Distribution Center. Black box right behind the battery. Fuses and relays are in there.

Before replacing anything, need to find out what the actual problem is first. You don't want to be just throwing parts at it, in hopes that you finally find the real problem. Here, I am *assuming* the system works the same way as the full sized trucks. (which I am more familiar with......)

So, the way it is SUPPOSED to work is: When you first turn the key on, PCM will ground the control circuit for the fuel pump relay, and run the pump for three seconds, then it opens the circuit. (pump stops running) After that, the PCM needs to see the engine turning to once again ground that circuit. (and thus, turn the pump back on.) Computer uses the crank sensor (and sometimes the cam sensor) to see if the engine is actually turning. If it gets a signal from either/both of those sensors, it'll turn the pump back on. So, the big question becomes, when is the pump NOT running? If you get the initial prime, but, nothing after that, chances are good, that the problem is NOT with the fuel pump at all, but the PCM not seeing the engine turning. If you do NOT get the initial prime, then you need to look at the fuel pump, and it's control circuits.

Clear as mud?
HeyYou, you got it right, the Dakota's PCM/fuel pump/ASD, etc. work the same way.
 
  #15  
Old 04-23-2018, 08:40 AM
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Originally Posted by ClintJohn View Post
please pardon, i'm new and don't know what a pdc is. plus not finding how to swap out either the computer or the relay... i'll continue my research and figure it out. thanks again
A few thoughts...

It's not the computer. The PCM has very little interaction with the fuel pump and no interaction with the gauge. All the PCM does is turn the ASD relay on to supply power to the pump. It's either running or it isn't. The PCM isn't going to tell the pump to stop running at any fuel level. Trust me, I burnt up a pump during my build as I forgot I had the ASD relay in, the tank was empty, and I turned the igniton on (I had removed computer) and thus the relay came on while trying to troubleshoot my lighting circuit.

Your pump is starving. Whether it's crud in the bottom of your tank, or poor fuel mileage. While the gauge might be reading 1/4 full, or your only getting XXX Km out of a tank, your pump isn't getting a solid supply of fuel based on the symptoms you've described.

Fuel gauge reading. Here's a picture of the fuel sender: https://www.rockauto.com/info/26/P74..._ANG__ra_p.jpg All the fuel gauge circuit contains is a wiper resistor pot on the float arm. As the arm moves up, the resistance increases and the gauge reads more. That's it. Nothing else in between. The Empty light is driven by a separate point contact on that same wiper assembly such that when the arm drops low enough it touches this contact and sends a ground signal directly to the empty lamp.

It honestly sounds to me that you may have the wrong fuel pump/sender assembly in your tank. Maybe too short. Or could be trash in the tank. This is something you can check yourself...don't sell yourself short. You and a buddy could do it on a Saturday afternoon. I've dropped my tank a dozen times by myself and it's not hard. Two tank straps, some hose clamps, and the electrical plug, and the tank is out. Do it when the tank is almost empty. Then you can pull the pump, and see if it is installed correctly. Reconnect the pump out of the tank to the electrical connector, operate the arm and see if your gauge operates correctly. Inspect your tank for crud. Throw some rocks in there, fill it up 1/4 way with soapy water, and roll it around the yard. Pulling the tank is the only way your're going to get to the bottom of this, IMHO.
 
  #16  
Old 04-23-2018, 09:42 AM
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Honestly, you get what you pay for. I would go with a Bosch or Denso fuel pump. If you would have spent a little more money in the first place you may have saved yourself alot of time and effort.
 
  #17  
Old 04-24-2018, 01:16 AM
ClintJohn
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[QUOTE=HeyYou;3389169]PDC = Power Distribution Center. Black box right behind the battery. Fuses and relays are in there.

Before replacing anything, need to find out what the actual problem is first. You don't want to be just throwing parts at it, in hopes that you finally find the real problem. Here, I am *assuming* the system works the same way as the full sized trucks. (which I am more familiar with......)

So, the way it is SUPPOSED to work is: When you first turn the key on, PCM will ground the control circuit for the fuel pump relay, and run the pump for three seconds, then it opens the circuit. (pump stops running) After that, the PCM needs to see the engine turning to once again ground that circuit. (and thus, turn the pump back on.) Computer uses the crank sensor (and sometimes the cam sensor) to see if the engine is actually turning. If it gets a signal from either/both of those sensors, it'll turn the pump back on. So, the big question becomes, when is the pump NOT running? If you get the initial prime, but, nothing after that, chances are good, that the problem is NOT with the fuel pump at all, but the PCM not seeing the engine turning. If you do NOT get the initial prime, then you need to look at the fuel pump, and it's control circuits.

Clear as mud? [/QUOTE

lol yep, clear as mud! took awhile to piece it together as far as i can tell, the only time the pump is not working/running is when (twice now) the odometer reads at about 3/4 of a tank. at that point, my truck starts bogging down. so, i fill up and we'll see if it's fine a third time. that said, i have it in the shop now and am getting the mechanic to read the computer, it may just need a flash... Chrysler is the only ones who can do it apparently.

i'll keep y'all updated thx again
 
  #18  
Old 04-24-2018, 05:51 AM
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[QUOTE=ClintJohn;3389436]
Originally Posted by HeyYou View Post
PDC = Power Distribution Center. Black box right behind the battery. Fuses and relays are in there.

Before replacing anything, need to find out what the actual problem is first. You don't want to be just throwing parts at it, in hopes that you finally find the real problem. Here, I am *assuming* the system works the same way as the full sized trucks. (which I am more familiar with......)

So, the way it is SUPPOSED to work is: When you first turn the key on, PCM will ground the control circuit for the fuel pump relay, and run the pump for three seconds, then it opens the circuit. (pump stops running) After that, the PCM needs to see the engine turning to once again ground that circuit. (and thus, turn the pump back on.) Computer uses the crank sensor (and sometimes the cam sensor) to see if the engine is actually turning. If it gets a signal from either/both of those sensors, it'll turn the pump back on. So, the big question becomes, when is the pump NOT running? If you get the initial prime, but, nothing after that, chances are good, that the problem is NOT with the fuel pump at all, but the PCM not seeing the engine turning. If you do NOT get the initial prime, then you need to look at the fuel pump, and it's control circuits.

Clear as mud? [/QUOTE

lol yep, clear as mud! took awhile to piece it together as far as i can tell, the only time the pump is not working/running is when (twice now) the odometer reads at about 3/4 of a tank. at that point, my truck starts bogging down. so, i fill up and we'll see if it's fine a third time. that said, i have it in the shop now and am getting the mechanic to read the computer, it may just need a flash... Chrysler is the only ones who can do it apparently.

i'll keep y'all updated thx again
Don't think it's the computer as I posted above...I'm afraid you're wasting your time on thinking it's the pcm.
 
  #19  
Old 04-24-2018, 10:49 PM
Azboyinmi
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As far as the pump itself goes, I've gotta agree with ragged. The pump,pickup, assembly is too long. When your tank is full, the bottom pushes out towards the ground fuel gets picked up no prob, but as the tank emptys it has less weight pushing down on the bottom so it begins to push back up towards the pickup. Then when you've got a quarter tank left it's blocking the pickup. That sounds like a really reasonable explanation. It's one of those little things that you'd never think about that ends up screwing you. As for the guages being screwy.... Are you sure it's not the guages part itself instead of the sender?
 

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