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

#### Alec_t

Joined Sep 17, 2013
11,991
The ELK can be configured for either positive or negative triggers
I realise that; but which way do you have it configured? This could affect the design of a circuit to give the solution you're looking for.

#### 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.

#### 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.

#### thebvk

Joined Nov 1, 2018
18
I realise that; but which way do you have it configured? This could affect the design of a circuit to give the solution you're looking for.
I don't have it configured either way currently.

#### 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.

#### 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?

#### 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.

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#### bwilliams60

Joined Nov 18, 2012
1,390
What is the part number on the PCM for that car?

#### thebvk

Joined Nov 1, 2018
18
Below is the LTspice of a reasonably simple circuit that should do what you want.
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.

View attachment 163078

thank you very much. I appreciate you doing that, and I will pursue that option if the PCM fix doesn't work out.