Can someone help design a 5 volt circuit

[opnspaces]

Yes I realize this is an RV forum but the post is related to wiring on my tow vehicle. I have a diagram below of what I am trying to do. But I will spell it out as best as I can first. My hope is that a EE or very knowledgeable person can look at my two diagrams and tell me if the bottom one will work or why I'm just nuts.

The vehicle is a 1977 ford with c4 transmission that has a 93 fuel injected motor. This is basically all 1977 technology with simple on/off mechanical switches. The existing starter circuit has a neutral safety switch. When I turn the key to start a 12 volt start signal goes to the neutral safety switch. If the vehicle is in park the switch lets the signal through to the starter solenoid. This is currently working fine (top box below)

The vehicle also has an EFI computer that when the key is in the run position it tries to send a 5 volt signal out to the neutral switch. In theory if the transmission is in park the 5 volts comes back to the computer and will allow me to pull engine trouble codes. This is currently not working as the wires are cut.

The system has worked this way for 20 years, but I'm finally wanting to improve it. I need to figure out how to marry a 5 volt key on system with a 12 volt start system without feeding 12 volts into the computer and possibly burning it up.

The second box below is my thoughts on inserting a 5 volt relay into the starter circuit to separate the two signals (5 volt and 12 volt). With the key in the run position the 5 v relay is energized to allow the 12v start signal through.

My initial questions are
Will the computer still see 5 volts returned after going through the wiring in the 5 volt relay? Or does the relay wiring drop the voltage to 0?
Is there and easier way to do what I'm thinking maybe with diodes so that the computer knows when the transmission is in park?

The top box is the existing system. The bottom box is my proposed system. Any thoughts?

[BruceMc]

Keep in mind, the purpose for the circuit is to enable code pull, which may include engine RPM changes, is in Park or Neutral only. The circuit is disabled when in any other gear.

A switch is certainly an option, but one that allows the code pull function to feasibly operate in any gear.

[CA_Traveler]

opnspaces wrote:
This is pre OBD2 so when pulling codes with the engine running it goes through various computer checks including automatically revving the engine up.

Any circuit that allows the computer to rev the engine when the car is running and shifted to neutral from drive seems underdesirable. Since it's not known what the comuter does based on the 2 wires and other inputs there could be other issues. I still favor a switch, maybe a double pole one that would active a light inside when on?

OP Your last diagram will cause the new 5V relay to have 12V+ on the coil due to the much lower resistance of the starter solenoid. Not desirable.

[BruceMc]

BurbMan, here's my understanding:

OP wants to close the 5V source to 5V sense lines to be able to pull codes from the computer only when:
In park/neutral, key on
Else, the connection between those lines is open.

Opnspaces, You asked:

1) I wasn't sure of the blocking diode position so I added two diodes. Were you saying to put in position A or position B or does it not matter and either position would work?

A: The blocking diode can be placed in either position, as you don't want the start line voltage from back-feeding the Run line. The diode is necessary to prevent spurious current spikes on the run line from the starter coil's collapsing current when you release the key from the start position.
As an aside, (in theory) current flows in the direction of the arrow, so it's blocked on the bar side.

2) Would the 5v through the constant duty solenoid cause the starter to constantly crank. I'm not sure if 5v would be enough to energize the starter solenoid but it's a thought.

A: The 5V circuit is completed through the switch when the relay is energized; it is a separate circuit and completely isolated from the 12V circuit. You'll never want to backfeed 12 into your computer!

Here's why the relay works in this setup:
When there's no voltage on the start line (key in Start position), the line acts like a ground when in Park or Neutral. When in any other gear, the neutral safety switch is open, so the circuit is open, so you have a route to ground.

If in any gear other than park or neutral, then the switch is open, but now you have no route to ground.

The reason that it acts like a ground when the key is NOT in the start position and you are in Park/Neutral: There is no 12V on the line from the start circuit, so it is an open circuit at that point. The coil in the starter solenoid is huge and has very low resistance; it takes a lot of current to pull that coil closed.
The coil in the relay is miniscule in comparison, requiring only a few milliamps.
Both coils are in series at this point, and if both were equally sized, it would take 24 volts and the commensurate current to pull them closed. If you put two relays in series, one with a tiny coil and the other a huge coil, then one will activate at 12V and the other simply acts as a wire to ground. There will be a bit of current drop across the starter solenoid, but not enough to be an issue.

If you have a spare starter solenoid and a small relay, such as that used for aux lighting, jig up the setup on your workbench. Let me know if I'm all wet...

There's a chance I'm wrong, but I'm 99% sure this will work.

[BurbMan]

opnspaces, maybe start with a description of what you are trying to accomplish. What is the functional goal here, to allow the engine to start when not in neutral? To pull diagnostic codes while rolling down the road? We are very deep in the tech details without an understanding of what we are solving for.

[opnspaces]

wa8yxm--Thanks for your thoughts, I'm still reading it over and making sure I think that I understand it.

On the reed relay are you basically suggesting a much less restrictive relay to be able to more effectively pass the 5v sense signal through?

Also I do know that the Neutral park switch does not go to ground. It's a simple through switch so I unfortunately don't have the option to use it for a ground.

[opnspaces]

Thanks BruceMC I think I followed what you were suggesting but I'm not sure if it will work. I tried to draw out what you said. Can you see if I got it right in the picture below. If I have it right I have two questions.

1) I wasn't sure of the blocking diode position so I added two diodes. Were you saying to put in position A or position B or does it not matter and either position would work?

2) Would the 5v through the constant duty solenoid cause the starter to constantly crank. I'm not sure if 5v would be enough to energize the starter solenoid but it's a thought.

Thanks for your thoughts on this

[BruceMc]

That lead from the ignition switch to neutral/park switch is energized ONLY when the key is at Start. This won't help for your use-case.

You'll need to pick off the Run lead, not the Start lead.

Put a small continuous duty relay on the Run lead, and wire it as follows:
Coil:
Positive: Run
Negative: Ground

Switch:
Common: 5V source
NO*: 5V sense

*NO - Normally Open (when the relay is not energized)

Now the 5V circuit is closed when in Run mode.

If you want to open the circuit when the transmission is not in park/neutral, then wire the negative coil wire to the start line between the key and the neutral safety switch, AND add a blocking diode between the coil and Run line.

Now, when the key is in Start, 12V+ will be on the coil negative line, as will be on the coil positive side from the Run line. The diode prevents 12V+ from back-feeding the run line.

Wire the diode as follows: Run -->|-- Start (in my best ascii graphics).

When in Run mode, and in parK/neutral, then the coil will pick up the ground through the neutral safety switch, and the starter solenoid.

Good luck!

[wa8yxm]

I disagree with JK Wilson the line you mark 5 volt return is in fact Sense. not return... Now. I do not know the specs on that computer but. have the following suggestions.

The relay.. use a very low current pilot relay (like a reed relay) and if needed a second relay .. the 12 volt from this relay does not go to the starter but the starter solenoid.
The "low" end of the relay goes to ground. not to the 5 volt return
The 5 volt return goes to the 5 volt side of the relay.

The reason for this is it is not a 'return' it's a 'sense'

IF this does not work you are 2 wires from the plan you purpose so its' an easy switch.

you may fnd the N/P switch goes to ground already.. If so simply move it in the circuit to the ground side of the relay

[wra]

What I would fear is that the five volts out of the computer may have a delay time before the five volts are available. That would cause the safety switch not to de-energize when it needs to keep the starter from turning.

Another question is, where and when does the computer get its power. The five volts may not be available when it is needed.

Just having a box diagram for the computer does not tell what the computer is doing, and how it does it. It is not possible for anyone to answer your question with the information given.

[opnspaces]

Thanks for the replies, I really do appreciate the thoughts and I am listening. Here's what I do know.

As far the computer 5v out and in wires...
This is pre OBD2 so when pulling codes with the engine running it goes through various computer checks including automatically revving the engine up. If I forget to twist the two computer wires together the computer does not see a neutral signal and it will just sit at idle. It will not rev the engine, and will not spit out codes. When I twist the wires together it goes through the checks, revs the engine and spits out the codes. So my relay idea is to just make a fancier switch to twist the two wires together. But I was unsure of the effects of the wires inside the relay.

I do agree with jkwilson and could run a switch that just puts the two wires together whenever I want to pull codes. In fact that's exactly what I almost did last week. But then my brain started churning to see if I could make it better. I am probably going to take that safer route and just run a switch, but wanted to get some thoughts and ideas first.

For the record this is the relay I was thinking about using. This extra relay only has to power the starter solenoid through the existing 16g wire. Link

What I am hearing from all of you though is that I will probably get a voltage drop and the 5 volts might not be strong enough to trigger the relay. This is really what I wanted to hear or maybe not want to hear but what I needed to hear. I definitely need to do some more thinking on this.

I welcome any additional feedback positive or negative that you might have.

Thanks

[time2roll]

How does the 5v signal to the EFI computer improve the function?

[BB_TX]

I will simply echo the above. Without knowing the capabilities of the 5 v out and 5 v return and the intended design function of those there is a definite risk to your computer in connecting anything to those points. I would be surprised if it could provide enough current to energize a relay of any size.

[CA_Traveler]

My advice is to do nothing until you can get full analysis on what/how the computer circuit is suppose to work, perhaps someone knows. And you certainly don't want to damage the computer.

FYI There will be a voltage drop across any relay coil in order to engage it.

X2 jkwilson

[jkwilson]

A return is generally the negative terminal of the power source, not where power goes into something, so I’m not sure what you are trying to accomplish.

If you need to feed the 5V out to what you have labelled 5V return ( which I would not do unless you are certain that’s how it works) just put a switch between them and turn it on when you need it.

For the circuit you have drawn to work, the 5V return would need to be the negative output of the 5V supply, but your circuit won’t do anything. Since you have no info about the rated power output of the computer or the coil resistance of the relay, it might burn up the computer