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wiring issues with PCM controlling fuel pump....

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    wiring issues with PCM controlling fuel pump....

    Ok, looking for some help here.

    Long story short, I've been trying to solve hard starting issues on the viper on and off the last couple years. I have changed fuel, fuel filters, and spent $300 getting the injectors fully serviced and balanced.

    It's a common thing for them, which essentially boils down to the fuel pump not running for long enough to fully pressurize the fuel system before you crank. The PCM energizes the pump for 1 second (by connecting ground to the pump relay) before it cuts out due to no engine rotation being detected from the crankshaft position sensor.

    For whatever reason the fuel pressure reg, the screen or the pump just don't quite get it done in time. works fine when its running. annoying as fuck, you have to keep priming it and then it goes. I want it to prime for longer.

    There is a fix, it involves using an ELK-960 delay timer, ( ) a small circuit board, and hooking it up at the back fuse/relay box in the boot to extend the inital prime time by a few seconds. I couldnt get to said relay because the fuckwit that owned the car had a sub box PERMANENTLY INSTALLED in front of it. This resulted in me dismantaling the rear of the car (splash guards, rear bumper) to try and see it it was bolted on. Nope, said fuckwit just drilled the bottom plywood piece of the box into the fiberglass then glued/made the box around it. Had to rip it (with screws) out, so now i CAN get to the rear box. Lots of wasted hours.

    before doing this i fucked around and tried to make the delay timer work at the PCM. I failed, and in doing so I seem to have stopped the PCM from connecting the ground to the fuel pump and energizing it. I confirmed this, if i just connect that wire to ground the fuel pump runs with the start/run circuit and car is fine.

    However, I do want that safety feature such that if it gets crashed and the engine stops, the fuel pump stops running and passengers have a better chance of not being fireballed to death.

    I'm trying to figure out if there is a way to use another 960 or similar to essentially replicate the safety feature, tap into the crankshaft position sensor wire (output ~5V, enough for the 960 module) but I don't think I can do it with just one, or maybe i need a seperate product.

    here's what I think needs to happen:

    - initial start/run power energizes fuel pump, but uses a timer such that if no rotation is detected within.... say 7 seconds (this is configurable on the 960) it cuts it off
    - if rotation is detected (cranking or running) it keeps it on, but either a loss of signal from the crankshaft sensor OR no power from the start/run circuit it stops.

    i THINK its something like example 9 on this sheet for the 960.

    I'm just not a car electrician and am scratching my head a bit. I'll keep noodling. Will draw some pictures and update if anyone is keen or able to help

    i think i figured it out. Two delay timers both set to provide the ground to the fuel pump module.

    first one triggers to start/run and goes for like 6-10 seconds, only on initial power up.

    second one configured to free run with trigger from cam sensor. When engine stops, cam sensor stops, it opens.

    I believe that mimics the PCM safety feature behavior that shuts off fuel pump with no engine rotation (based on cam sensor) even if start/run is energized, and it gives me 7-10 seconds of fuel pump priming time rather than the 1 second fail time.



      no i was wrong

      its the crank position sensor not the cam that it runs off, and that doesn't give a steady output, more of a waveform but something like .2 to 2v when cranking.

      so i need something that can take that and convert it to a steady +12v signal that I can use as a trigger that determines engine rotation. pulsed to steady 12v output. something like that, i dunno

      a better question is why would the PCM stop providing that ground to activate the fuel pump module? When I ground that out and start the car it runs fine, no codes etc. I have the service manuals, and all they tell you is the paragraph about how the PCM achieves fuel pump module control by providing the ground to the relay. No info on how to diagnose or what to do when this stops. wiring diagrams all just point to the PCM, pin 19 on connector C1 or C3.

      Is it a PCM issue? cannot see any other obvious things - all fuses are ok, and everything works as long as I ground out that one pin for the PCM.

      should I take it to dodge, I really don't know what to do, other than to go down the path I am and essentially make a little circuit to replicate the function. that seems do-able and really simple for someone with the know how. or if you're me, a few hours of figuring basic electronics shit out that I haven't thought about for 20 years


        Why does the pressure bleed down and require so much priming? Does the regulator leak? I'd fix that, much simpler.


          Why don't you just put a momentary switch on the relay ground circuit.


            Originally posted by BigMuz View Post
            Why does the pressure bleed down and require so much priming? Does the regulator leak? I'd fix that, much simpler.
            yeah that's probably what it is, and that would be the thing to do, except on these fucking cars the regulator is contained in the 'fuel pump module' that is buried in the tank. requires cutting upholstery and drilling out the rivets on the cover plate, and then its not a serviceable part of the module, you need a whole new module. They dont make those anymore, and 2nd hand they are like $2K. At that point I might as well just redo the whole fuel system with new pump/reg etc. It's a common thing with these cars and hence someone on the viperclub came up with using the delay trigger module to extend that inital pump prime time, thats all it needs. I had a fuel pressure gauge on it, it works fine everywhere except for the inital 1 second factory prime making it start hard.

            also I just dont know why the PCM doesn't provide the ground anymore, even if I fixed the fuel pressure reg. I think i fucked something, but I just can't tell because that is the ONLY thing not working (that one ground pin from PCM) and new 2nd hand PCMs are also about $2K. the joy of old low production numbers hand made cars i guess.

            Originally posted by Do the safety dance. View Post
            Why don't you just put a momentary switch on the relay ground circuit.
            thats what I've done now, it just triggers off the start/run circuit. I guess its not really a momentary switch, but i dont want to hold down a switch to make the fuel pump work. The issue with my fix now is death by fireball - I want the fuel pump to also stop if engine rotation stops, not just keep pumping fuel because it's energized by start/run but the engine has stopped (when you drive the stupid thing into another object at warp speed) I suppose I could just put a regular on/off switch on it (like I did on my diesel deere tractor) but you'd have to be conscious to hit the kill switch, and I dunno if that is a roadworthy thing or not with the DMV. They're expanding the scope of california smog inspections (have to do one every 2 years)

            I want the trigger for that ground circuit to be twofold, an intial 'priming' trigger that runs for 7-10 seconds only once on initial power up of the start/run circuit. Then it should continue running if the engine is rotating.


              could you use something like an lpg safety switch? does the ~3sec prime and shuts off with no pulse, needs a coil negative pulseor something along those lines. also cheap.
              Originally posted by Skompa
              The throttle linkages jammed on the CDs in my old Triumph 2000 and stuck it at WOT.

              I didn't realise for about 15 mins as it never broke the speed limit.


                i'll look into that, you said the magic word

                I think i've got it sorted, have started a chat on a electronics forum with some real nerds, will update this with results if and when. shit is working currently, and im running around trying to get the farm sorted before the winter rain kicks in


                  yeah i got a bit ratholed on fixing my own problems. I am just going to pull the PCM and send it off to a fix place as that seems to be the issue, and for $200 with a 'if its not broken no charge' condition that will work. Then i can pursue the original fix and stick the delay timer in the boot.

                  however I did get around to figuring out what I need to do if i had to rebuild a circuit to do this:

                  Click image for larger version

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                  The relay turns off about a second after the pulses stop.
                  That time can be varied by changing the value of R1 and/or C1.


                    FWIW, my ECU is toast. Well, it works, but it throws a code on the fuel module control circuit, which can be worked around by grounding that pin (make the fuel pump run with ignition) but I can't get it smogged with it throwing a code and therefore can't register it as I have to smog it every 2 years. I sent the ECU away, its an issue with the actual chip, not something on the board, it can't be repaired, now it's back on the car.

                    In the meantime I've busted into the fuel pump housing/module and will be fixing the regulator etc, might need to get some 3d printing bits made up for that to work, but going to address the hard starting issue there.

                    aaaand im looking around for an ECU, the other option being full blown aftermarket management, not sure I'm up for the challenge on that. Replacement ECU for that car suffers the viper tax, i reckon it will be between 2-3K to get one. No one makes them, 2nd hand refurb etc seems to be the only choice. I can't see aftermarket programmable management being less expensive, and that comes with an awful lot of work to tune and all that jazz. Good fun. Not sure if anyone knows of any other alternatives.

                    In the meantime I'm just driving it while it has rego left, end of this year I'll need to get it smogged though.

                    Otherwise I'm ripping off an idea that I saw from a guy with a Z06. Gonna make a small custom trailer for it to haul tires and tools to the track so as to not wear out my R888s on the road, and I'm gonna get back into autocrossing. SCCA does it regularly at Thunder Hill, the nearest decent track to where I am.


                      fucking cars. but this shit still makes me giggle so I'm gonna stick with it for now


                        I have always wanted a Viper, and way back in the late 90's had some bold stupid plan to get one when I was asked to work in Chicago. I got close enough that having negotiated pay rates and living conditions I was looking at pricing and parking for the winter.

                        Alas, it all fell over when there was a policy change in work visas and it all got too hard.

                        Yours is exactly what I was looking at getting, GTS in red.


                          Surely you'd get a haltech in an tuned for that sort of cost?


                            Emtron KV12. I reckon you might be able to get a deal on one if they are looking to develop a PnP version for the Viper.


                              I'll look into both of those, thanks. I haven't dipped a toe in aftermarket engine management tech for many years, appreciate the suggestions.