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?

[2112]

You could bring all 4 wires to a DPDT rocker switch under the dash. Reach down and flip the switch while sitting in the drivers seat, turn the key and start counting flashes. Just remember to flip the switch off when done.

I had a '86 Mustang manual tranny, no MIL. The clutch had to be held down during the test while I counted voltmeter needle bumps under the hood. I volunteered my daughter to be the designated clutch crew. I could have rigged up something to make it easier but I only needed to do it a few times in the 15 years I owned it.

Have Fun

[opnspaces]

Yes you're pretty much on the money. I initially didn't want to get to far into the computer details because I didn't want to cloud the waters or dissuade suggestions because it sounded too technical. So I left it as I have a 5v out and 5v return that I need to make work with my one old school NSS.

Donor was a 93 Mustang GT AOD automatic with a C3W computer. It was a complete swap, motor and harnesses but no transmission. It is smog legal in California with a BAR label on the door. I do have the extra wire available but unfortunately I did this swap 20 years ago and don't remember the actual pin numbers. I just have the two bare wires free under the hood next to the self test connector.

To pull codes I twist the wires together. Then I jump the STI connector to the diagnostic connector and read the codes from the blinking check engine light.

I'm assuming when you are talking about tricking the EEC into thinking I in park (that the NSS is controlling pin 30) that you're referring to me twisting the two wires together.

I've run like this for 20+ years with no issues. My hope is to design a system that when it's in park the EEC knows it's in park, and when it's not in park the EEC knows it's not in park. PLUS I need the NSS to safely control the starter solenoid.

I've received a lot of good ideas off this post and I'm going to try a few of them out on the bench and see what I can come up with.

Thanks for the ideas on the double relay by the way I really like that.

Thanks

[bighatnohorse]

This might work: https://www.amazon.com/gp/product/B07TCF26NT/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

[2112]

My understanding is you are trying to pull codes from a 1993 OBD1 EEC-IV controlled engine.

The neutral safety switch on the 1993 donor vehicle controlled pin 30 on the EEC. The EEC verified the neutral switch position on pin 30 prior to entering the self test mode. If the transmission was not in park or neutral it won't initiate the diagnostics sequence. Your 1977 vehicle does not have this feature. As you stated, the 1977 NSS controls the starter solenoid directly. The good news is you can force the EEC to think the NSS is controlling pin 30 by connecting EEC pin 30 to pin 46.

I assume you used the original 60 pin EEC connector during the swap and I hope you left some wire on each pin to splice onto. Not knowing what vehicle your '93 donor engine came out of the following are assumptions.

What you call 5V OUT should be STI on EEC pin 48. It's actually an input being pulled to ground to initiate the self test. Grounding this pin starts the self test sequence

What you call 5V Return should be Test Ground on EEC pin 46

Once you enter self test mode, the codes can be read as pulses on EEC pin 17

Are my assumptions close to accurate?

[MEXICOWANDERER]

Amazon search
TYCO RELAY
They last 10 times longer than cheap Chinese garbage

[opnspaces]

Thanks 2112 I didn't even know that a double pole relay existed. I'll have to think about your suggestion as it takes the neutral safety switch out of the start circuit and relies on the relay alone.

I might have to decide that is good enough. Or I might have to put more thought into an added switch on the linkage.

[2112]

Have you considered having the neutral safety switch control a quality automotive grade DPST relay.

You might also consider placing a manual backup toggle switch across the contacts driving the starter solenoid in case the relay fails.

[Bobbo]

If just adding a switch, I would recommend a momentary contact switch. That way, no one can accidentally flip it on, or accidentally leave it on. You have to physically hold the switch on for it to energize. Release it, and it de-energizes.

[time2roll]

Not sure where the lever and linkage connects but if the tunnel is that tight maybe a switch topside at the other end would work just the same.

Be very cautious with modifying the neutral safety switch. Could leave the vehicle stranded if it fails open or provide a lurching start for yourself or unknown user if something fails connected.

[opnspaces]

I have to say I love the people on this forum. Sure there are differing opinions. But each post might have that one nugget of information that is missing from all the other posts.

As far as the ideas coming in. time2roll, I have definitely considered adding a switch of some kind on the linkage. But haven't had a chance to crawl under and see if there is even room to do so. I know from adding an aftermarket kickdown cable that space is tight in the transmission tunnel.


As far as the adding a relay, I think I'm going to spend a couple of bucks on Amazon to get some relays and mock it up on the bench as suggested. I might not be able to mock the 5v part, but I'll figure something out. my thought is in the end I'm going to have to go back to the bottom picture in my original post and see if it will work. But I plan to try out BruceMc's idea of grounding through the starter solenoid first to see if it will work or if it will cause a constant crank. On the bench of course.

It was suggested or asked why not allow code pulling when driving down the road. I don't have an answer to that except I would be concerned that the sensors are not in a range that the computer is expecting so I'll get a bunch of sensor out of range false positives.

Thanks again for all the thoughts.

[Bobbo]

time2roll wrote:
What is wrong with pulling codes while driving down the road?
Why does it need to be in neutral-park?

Since the computer revs the engine, having that happen while going down the road, in traffic, would be dangerous.

[time2roll]

What is wrong with pulling codes while driving down the road?
Why does it need to be in neutral-park?

I am really inclined not to mess too much with the factory neutral safety circuit.

Consider fabricating a bracket to hold a separate momentary switch that is closed when the lever moves to park.



The two hold down screws for the switch could also hold the bracket. As that lever moves forward the switch closes.
N-S switch is the grey half moon shaped object just to the left of middle picture.

Close up:



Possibly a switch like this: https://www.amazon.com/DIYhz-waterproof-Momentary-Roller-Switches/dp/B07JL3Y36P

[BurbMan]

Thanks guys I think I'm tracking with you now. Sorry I can't help answer the question but at least I can follow the conversation now!

[opnspaces]

Thanks BruceMc, you spelled it out much more succinctly than I could.

BruceMc wrote:
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.