Fuel Pressure

[Cameron]

Hi again.

so, lots of time has elapsed, much of it non driving winter time for the RV.  the last I posted here, I thought I had solved my problem - as a recap, I ran a new ground wire to the back of the engine and improved the connectors on the ground going to the ebl.  I do really think this helped.  however..

the current symptoms are kinda similar to the previous symptoms; driving along and the engine just dies.  not *nearly* as frequently as before, but still occasionally happens.  I had a theory that when the fuel line was coming to the T where the main tank and alternate tank merge that the fuel was heading back to the tank not in use.  my theory was that the back pressure from the efi was sufficient enough that the path of least resistance for the fuel was back to the tank instead of getting into the engine.

so, I bought a couple check valves that operate at very low pressures to prevent this back flow.  I also tried to get a fuel pressure sensor working.  a task that I am still failing at!

so, with the background out of the way, I have 2 questions please.

1. have any of you tried the check valve idea to prevent the back flow?  is this a 'dumb' idea?

2. I do not understand why I cant get a fuel pressure sensor working.  my first attempt was just plain ignorance, tried to wire a 12v power to a 5 volt sensor.  my second attempt included a 5v step down.  this never recorded any fuel pressure, so I assumed I wired it poorly.  so I tried again with the same result. I can test in the coach where the step down is, and at the sensor and it is getting the correct 5v.  however, on the signal wire I only get .7ma when the vehicle is running.  I cant imagine what I am doing wrong, but am I doing something wrong??  

thanks for any advice or direction!

Cameron

[Bill Van Vlack]

There definitely needs to be check valves to prevent fuel from one pump going backward through the other. My pumps have internal check valves; if yours don't then you need external check valves.

We'd have to know a little more about your pressure sensor; manufacturer and model number. My sensor ties into the EBL/ECM using one of the analog ports; AN2. DId you measure the signal voltage referenced to sensor ground?

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[Jon Roche]

Would like to see some photos of what you have and how it is wired. 

On Mar 30, 2024, at 7:58 PM, Bill Van Vlack <bill.va...@gmail.com> wrote:



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[Cameron]

Yes, my testing has been to test at both the sensor and in the cabin where I was trying to locate the step down.  The signal wire just doesn’t seem to have anything.

I will say, that when I first put it together, I saw fuel pressure.  Was about 8lbs.  Gradually decreased, then nothing again.  But the wiring is so simple I’m not sure I could screw it up..

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[BruceHislop]

Hi Cameron,

I need to impress on you the importance of running your main battery negative wire directly from the battery negative terminal to the engine block.  You then ground the frame to the engine block.  The body is then grounded to the frame.  There is a smaller ground wire from the battery negative to the aluminum plate under the passenger side hood and from there on to the dash.  This is because those items are basically mounted to the fiberglass (SMC) body and are isolated form the main body ground.

Also, do not stack ground wires on the same bolt. You can have a good connection to one wire and a poor connection to another even when stacked on the same bolt!

I know many people think the more ground wires the better, but this is not so and can have unintended consequences.

The battery negative is wired directly to the block because the engine block is ground for the high current devices in the chassis.  Specifically the starter and the alternator. 
 - Consider this with the way you currently have your ground to the frame then to the engine. If the engine block were to lose ground (or the ground connection to the block became poor).  When you activated the starter, the current to the starter would find another electrical path back to other grounds you have provided, in your case the frame.  This ground path may be through smaller wires and even bearings causing damage to them.  You may not even realize this is happening if the alternate path can provide enough current to the starter and continue to do this until the alternate path fails.

 - If you lose your engine to battery ground, you want no other path back to the battery negative for the high current starter.

For more information on electrical grounds, read this document by GMCer Ken Burton (an Electrical Engineer)
https://www.gmcmotorhome.org/resources/electrical-grounding-in-the-gmc-motorhome.54/

Bruce Hislop

On 30/03/2024 7:50 p.m., Cameron wrote:

Thanks for the input, will try to combine responses.

Both the 5v step down and the fps are new.  I also assumed I killed the one with 12v power.

My grounds are: battery to frame, engine to same spot, then a ground from that spot to the back of the cylinder head.  I am grounding from that spot on the cylinder head to the ebl.  

I typo’d the mA it should have been mV, I was just also trying to see if maybe the voltage was correct but not the amperage.

The step down is described as “12V to 5V DC Converter Car Power Voltage, Waterproof DC 6.3-22V 12V Step Down to DC 5V 3A 15W Voltage Regulator Buck Converter Power Supply Step-Down Module Compatible with Vehicle Car Truck Volt” (sorry, on a phone, so editing is challenging).

That description is what made me try to check the amperage.  Wondering if I was getting the right amount.  (I am obviously not an electrical engineer.). For grins I ran power directly from the battery to the step down to the sensor, no difference..

I’m getting the 12v from the power to the fuel pump.  The same 12v is powering the WB sensor and the speed sensor which work fine..

I will check with AFI on Monday to see if the fuel pump has a check valve, didn’t think of that..  glad my idea wasn’t dumb, just likely unnecessary..

Thanks again for any assistance!

Cameron

On Sat, Mar 30, 2024 at 3:52 PM BruceHislop <bhis...@gmail.com> wrote:

On the second item, the fuel pressure sensor.  I suspect you may have destroyed the 5 volt sensor when you powered it with 12VDC.  I fed my fuel pressure sensor with 5 VDC from the EBL itself (same 5 VDC line as powers the throttle position sensor).  When you say you only get 0.7mA on the output of the fuel pressure sensor wire, you should be measuring voltage output, not current.  The voltage should vary from 0 (zero) volts to near 5 volts depending on the fuel pressure and the sensors pressure measurement range.

With respect to the ground wires, you should have a single #2 or #4 ground wire from the battery negative to the engine block.  Then all the grounds for the EBL system should go directly to the bolt on the back of the engine head (or any solid bolting point on the engine).    There should also be a ground from the battery negative connector up to the aluminum plate behind the passenger side hood. 

Resist all temptation to run additional ground wires from the battery negative to the frame etc.

There are then a couple of ground straps from the engine to the frame and from the frame to the body. The engine block is the "Mother of all Grounds" in the GMC motorhome.

Bruce Hislop

[BruceHislop]

A single good ground is just as good as two grounds... but how would you know if one ground went bad?   You wouldn't.. until the second ground went bad as well, so why complicate things.

Some people argue the cast iron engine block is not a good conductor.  Well cast iron may not be as good as copper when compared as equal cross-sectional area conductors. BUT the cast iron engine block has a huge cross-sectional area compared to a #2 or #4 wire. Therefore the electrical resistance of the engine approaches 0 (Zero) ohms between any two points on the engine.

Just using the threaded stud hole at the back of the passenger side head will work just fine as a grounding point for the EFI system.  I would clean the casting around the hole and use some anti-oxidant paste to keep the connection clean.

The drawings at dynamicefi.com show a +5 V supply on pin C14 which supplies power to the MAP and TPS sensors.  This is what I used so I know that 5V is present only when the EBL requires operation of external sensors.  When you applied 12V to the fuel pressure sensor (FPS), the output wire of the sensor may have also supplied 12VDC back into the EBL's analog to digital convertor (ADC) chip.  I do not know if the EBL protects the ADC from high voltages or not, so that analog input may be fried. The ADC may be protected against high voltages and this circuitry may be shorting that ADC input to ground.  Try disconnecting the FPS  output wire from the ADC input and measure the FPS output then to see if the sensor is working.  If it does, then you can try connecting the FPS to a different ADC.  Check the Ebl manual because I believe there are only a couple ADC inputs that show up on the WUD screen.

Bruce Hislop

On 31/03/2024 1:25 p.m., Cameron wrote:

thanks for the grounding doc.  I will read it.  I am not using the method you described at this time.  when I was following the randy instructions for the install, the guidance was to ground to the back of the cylinder head with an extension from that central location by the battery.  would you suggest against that?  just ground the ebl to the back of the cyIinder head without pulling a ground from over by the battery? will try to get it reorganized this coming weekend.

regarding the pressure.  I was letting the engine run when testing with the voltage tester, so there should have been something there.  pump had to be running.  if I didnt know better (which I dont) would think that some how I killed the fps.  just not sure how I could have done that.  here are the parts I bought for the fps and the step down;

https://www.amazon.com/dp/B07Y2V1F8V?psc=1&ref=ppx_yo2ov_dt_b_product_details

https://www.amazon.com/dp/B07XZ4BML8?psc=1&ref=ppx_yo2ov_dt_b_product_details

thanks!

On Sun, Mar 31, 2024 at 7:42 AM BruceHislop <bhis...@gmail.com> wrote:

Cameron,

You said you saw fuel pressure of 8PSI at first, but then it gradually decrease to nothing. 
  - Just to clarify... when you turn the ignition key to ON,  the EBL powers the fuel pump for about 2 seconds, then stops.  Was this the reason you saw fuel pressure which then dropped to zero PSI?  The fuel pressure drops off quickly once the fuel pump stops.
     - The EBL does not power the fuel pump again until you crank the starter, or the engine is running.
    - You may need to bypass the fuel pump relay so you can keep the pump running while you troubleshoot your fuel pressure sensor issues.
 

Bruce Hislop

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[Bill Van Vlack]

I've lost part of the thread here, I think. Was the fuel pressure sensor connected to the EBL, or to a separate stand-alone gauge/display?

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