Question on automotive fuel pump relay control, need to convert pulsed signal to steady +12v

bwilliams60

Joined Nov 18, 2012
1,390
Why not get the check valve fixed and repair the PCM as needed? I am fairly certain both of these are attainable. The check valve that prevents fuel from draining from the lines is sure to be found on other models or a reasonable facsimile and there are several places that repair PCMs or do it yourself. This all sounds like a lot of work for such a nice car.
 

Thread Starter

thebvk

Joined Nov 1, 2018
18
Please correct me if I did not understand your question.
You have a pulse the moment the pulse exists you want an output like 12V.
When no pulse the output will go to low after a certain time (say 10 seconds).
This implies that:
When a pulse arrives within the selected time the output is high.

If this is what you want take a counter and count to whatever seconds you want.
Reset counter with the pulse.
That's all.

Picbuster

yes that sounds like exactly what I want. thank you. I'll try and find something off the shelf that can do this.
 

Thread Starter

thebvk

Joined Nov 1, 2018
18
Why not get the check valve fixed and repair the PCM as needed? I am fairly certain both of these are attainable. The check valve that prevents fuel from draining from the lines is sure to be found on other models or a reasonable facsimile and there are several places that repair PCMs or do it yourself. This all sounds like a lot of work for such a nice car.
I'd like to, but as i explained in the original post, there are issues. They dont make the fuel pump module any longer, the bits in it aren't really serviceable or replaceable and they are hard to find 2nd hand, and cost a bomb - at which point it's a reasonable alternative to just re-fuel the car with new components. expensive and messy. Same story for the PCM, nothing servicable, just a $2K replacement, IF you can find one.

functionally what im trying to do is just replace the stock behavior. It will not be noticable at all. So on one hand I have $4K+ in bits and a boatload of work to get it back to stock behavior, or I can spend a few 10s of dollars on basic electronics, some basic wiring, and same result. If I can't make this work then I'm left with no choice but to go the replacement route, but it's not my first choice.
 

bwilliams60

Joined Nov 18, 2012
1,390
The PCM can be repaired. There are reputable companies out there that will repair it and make it good as new. I have salvaged many PCMs by doing board level repairs. I am not sure where you are but I am sure someone close can do this.
As for the check valve which I am assuming is the culprit of your problem, I am sure that if someone made it, someone else can duplicate it. Machine shop etc.
I get what you are saying but it seems like such a nice car to go bypassing systems and probably creating more problems. If it was a common everyday $2000 beater, I would be right there with you. What year and engine is in this Viper?
 

Thread Starter

thebvk

Joined Nov 1, 2018
18
The PCM can be repaired. There are reputable companies out there that will repair it and make it good as new. I have salvaged many PCMs by doing board level repairs. I am not sure where you are but I am sure someone close can do this.
As for the check valve which I am assuming is the culprit of your problem, I am sure that if someone made it, someone else can duplicate it. Machine shop etc.
I get what you are saying but it seems like such a nice car to go bypassing systems and probably creating more problems. If it was a common everyday $2000 beater, I would be right there with you. What year and engine is in this Viper?
fair enough - and yes I would pursue a repair option before replacement, obviously.. it just seems to me that this is the easiest solution to go after right now. Theres a whole story around why this happened (previous owner having sub box made and permanently installed in front of the rear fuse/relay box) and how I took 5 steps backwards by trying to work around it (and breaking PCM) but its not really worth repeating as i ended up taking apart the whole rear of the car anyway.

I would love to get that PCM repaired, I will look into that before i go messing with anything else. I'll do some searching on what company to talk to. I don't care if they're local, i'll send it out, this isn't my daily driver or anything. Right now the car works fine with that pin grounded and the pump tied into ignition, but obviously i need that safety feature and i want the car to operate normally. as you said, its not a beater. 98 GTS with 20K miles. Has history, was a pace car at LeMons events, block is signed by drivers. '98s have the 8.0L V10 fully forged internals and the race block as they messed up with the moulds for the regular block that year, they're the ones people like to mod.

DSC03122.JPG
 

Thread Starter

thebvk

Joined Nov 1, 2018
18
yeah PCM repair seems to be the way to go, i don't know why I didn't think of this first. I usually try to fix my own problems and do my own work, I guess I got ratholed on this one. Any recommendations?

a simple search turned up this mob, $225 repair service...

https://www.g7computers.com/collect...repair-service/products/dodge-computer-repair

thanks again for the comments, help and recommendations. I try top do the same thing on a couple of automotive forums, having people willing to engage in discussions and helping others is a huge part of what makes the internet such an amazing resource.
 

bwilliams60

Joined Nov 18, 2012
1,390
If you could shed some light on your relative location, I may be able to help you better.
On another note, you said that the PCM was providing ground to the pump at one time. What changed?
It may not be the PCM at all. Perhaps something is inhibiting the PCM from providing that ground. With a good scan tool, do you have any codes? Lets start there.
Nice car by the way. A keeper in my books.
 

Thread Starter

thebvk

Joined Nov 1, 2018
18
It's the PCM. I had been through all the troubleshooting steps to get to here, and this thread started as a pretty specific question. I'm in Northern California.

To go through this story somewhat briefly:

- here is more info on the generic problem and the fix figured out by the viperclub community, the basis for the ELK-960 solution: https://www.viperclub.org/vca/threa...your-viper-fuel-pump-starting-problem.624994/

- I have replaced fuel, fuel filter, plugs, leads, had all the injectors balanced serviced flow tested etc, the problem above is definitely the one im dealing with. Its a PITA, but just turning key on./off/on/off/on then cranking makes it go. It's been annoying me for over a year and i've been sorta trying to fix it via the means listed. its looked after and regularly serviced, i've fully rebuilt cars a few times and look after them, but im not an expert when it comes to the specifics of the electronics. This kind of explains why i got annoyed and started experimenting when i shouldn't have and evidently doing something that broke the PCM.

- the previous owner installed a sub box in front of the rear fuse box. permanently. Long story but I couldn't get at the those wires or the box, so I messed around with the elk 960 up the front at the pcm, did something wrong while trying to work around that solution. this was my error, and all of a sudden the fuel pump stopped working all together.

- i eventually gain access to the rear fuse box, because at this point im trying to diagnose why the pump doesn't work at all now. This involved taking the rear of the car apart (bumper off, rivets out, bodywork stuff, etc) and even then I couldn't get the box out, had to rip out. the idiot had built the box into the trunk with screws on the bottom piece of plywood, then built the sub box around it. I wanted to strangle him. Once I removed it I got to testing. its not the wiring, its not the fuse, its not the relay, its not the pump. Those are all good. Yes I have a scantool. no codes, no check engine light.

- i have the full workshop manuals for the car, so I know what circuits, what pins on what connectors. the manual describes how the pcm controls the fuel pump, by providing the ground to fuel pump relay. On initial power up to the start/run circuit it gives the pump 1 second prime time, then only activates the pump after that when engine rotation is indicated via the crank position sensor. (so extending the initial prime time to the relay by a few seconds with an elk-960, as per the link above, is an easy fix to this common hard starting problem).

- If I just manually ground that connection to the relay (that is connected to pin 19 on connector C3 to the PCM, this is how it provides the ground) that makes the pump work with start/run as that's what the relay is connected to. The crank position sensor is fine. no codes. the only possible cause left is the internal functionality of the PCM, which now not longer provides that ground under any circumstance - not on inital power on, and not when there is engine rotation. The workshop manuals treat the PCM as a black box, they describe functionality but not anything about the internals, essentially its a replacement part for dodge.

does that clarify it? when i started the thread I was already at this last dot point, trying to work around and replace the PCM functionality. it's a bit derp of me that i didn't think to just get the PCM fixed, but that seems like the best next step. If I can get that fixed then I can just install the ELK-960 in the trunk per the original fix and move on with my life.

Like I said, I took 5 steps back, now just working to get back to square zero. pretty typical sometimes :)
 

cork_ie

Joined Oct 8, 2011
428
I know nothing about American GM ECUs but do know that the problem that you have is very common on European GM models too. Our ECUs are a hybrid without discreet components where the circuits are mainly etched on to a ceramic substrate and covered in a thick gel. Connections are made with tiny wires spotwelded onto the circuit board. More often then not they are impossible to repair.
Long time ago back in the late 70's early 90's Bosch K-Jet cars had an electronic relay to power the fuel pump which functioned exactly as you described . It took it's pulse from the ignition coil, points side. You could easily take a pulse from an ignition coil or one of the injectors.
VW K-Jet Pump relay is part Number 321906059F, they are still widely available. There are also several circuit diagrams on the internet if you wish to make your own, but it is rarely worth the effort as they can be purchased online for about $16-00
 

Thread Starter

thebvk

Joined Nov 1, 2018
18
I know nothing about American GM ECUs but do know that the problem that you have is very common on European GM models too. Our ECUs are a hybrid without discreet components where the circuits are mainly etched on to a ceramic substrate and covered in a thick gel. Connections are made with tiny wires spotwelded onto the circuit board. More often then not they are impossible to repair.
Long time ago back in the late 70's early 90's Bosch K-Jet cars had an electronic relay to power the fuel pump which functioned exactly as you described . It took it's pulse from the ignition coil, points side. You could easily take a pulse from an ignition coil or one of the injectors.
VW K-Jet Pump relay is part Number 321906059F, they are still widely available. There are also several circuit diagrams on the internet if you wish to make your own, but it is rarely worth the effort as they can be purchased online for about $16-00
thanks. Yeah it seems from this thread there are certainly a few ways to solve the problem. That seems like a easy solution (along with the ELK for the inital prime) if I can't get the PCM sorted out.

I really really like the idea that the pump turns off when engine rotation stops, so i'd still rather use a crank or cam sensor if thats what it came down to, im not sure what injectors or coils would do in a bingle. These cars were low number to start with (ah the joys of hand made cars... shitty panel gaps and a million spacers) but their numbers have been reduced significantly by attrition. when people crash em they don't do it by halves a lot of the time, not that i plan to but it seems like a good idea to retain what little safety features they do have. No ABS, no traction control, stupid grip and a lot of torque means they do occasionally try to kill you.
 

crutschow

Joined Mar 14, 2008
26,866
Below is the LTspice of a reasonably simple circuit that should do what you want.
The relay (red trace) turns off about a second after the pulses (blue trace) stop.
That time can be varied by changing the value of R1 and/or C1.

upload_2018-11-3_19-30-57.png
 
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