fuel system wiring to bypass computer after TBI to carb

[eric 87]

Well I may get shot over this. And frankly you might be doing me a solid by putting me out of my misery and making my wife happy not only for the insurance BUT also to not have to hear about more PARTS.
Without further adieu...I am swapping TBI to carb. Yes I know. And I tried and a shop tried and we/ I replaced every part known to man.
SO now the swap is underway. I was going to go new old style sending units/ electric fuel pump to carb with a regulator even though not really needed.
BUT then I thought....why not just replace the passenger side in tank pump/sender and keep the original fuel system. I have the regulator to cut it down to what I need. Why make more work. But I am dead set on bypassing this computer for everything and need to know how to by pass the wiring through the ECM.
Anyone know how? Is there a way to use the relays and the oil pressure sensor as a safety cutoff as it is now? Is there some way to add a keyed and crank hot to prime and run the pump still through the switch?
At this juncture I can go either way with it. I wanted to do this wiring before install of the HEI / new intake and carb. I have the pressure regulator ready to mount on firewall. Is it possible to wire it so as to eliminate the ECM. And does the AC run through the ECM as well. I want this to be done right and I could then pull the entire TBI system and computer since it is BRAND NEW as well and the new computer distributor and all those new pretty sensor intake and throttle body that was rebuilt to someone wanting the entire system. I think I found one gremlin when taking it apart. The four wire pin to ignition module had lost it's female side of the pin from the module which is what threw the code 42. BUT the idle issues were just leaving me stranded and it became something the wife didn't want to drive and have it die. So it was get it fixed or buy her a car. This swap is MUCH cheaper than a NEW ish car as in 2018 or newer.
Any help would be appreciated.
And the IAC was replaced and tested with ohm and or volt multimeter on the wires. BOTH the new ones I bought as well as all the others checked out fine.
I mention this because I know a lecture is coming. LOL
Thanks in Advance guys. These forums have me reading even about truck issues that would never pertain to me. Love reading the knowledge of others and experiences.

 

[idahovette]

What do you do about the speedometer if you remove the electrics??

 

[WP29P4A]

I would think comparing a pre TBI wiring schematic to what you have should give you an idea of what you need to do to keep the sensors working without the ECM. (If you even still need them) I did some work trouble shooting electrical issues on a 2006 jeep with a Chevy v8 conversion. Bottom line, the client had to retain the factory computer to control the dash gauges and interior lighting, but needed a new computer (ECM) to control the new engine and transmission. I had to cut the air conditioning sensors from both computers and wire them in series to retain the safety switches (hi/low pressure switch)

To completely eliminate the ECM you might need a few more pre 1978, pre TBI parts to replace anything else that needs to communicate with the ECM to function.

 

[Bextreme04]

What do you do about the speedometer if you remove the electrics??

An 87 truck will be a mechanical speedometer.

Well I may get shot over this. And frankly you might be doing me a solid by putting me out of my misery and making my wife happy not only for the insurance BUT also to not have to hear about more PARTS.
Without further adieu...I am swapping TBI to carb. Yes I know. And I tried and a shop tried and we/ I replaced every part known to man.
SO now the swap is underway. I was going to go new old style sending units/ electric fuel pump to carb with a regulator even though not really needed.
BUT then I thought....why not just replace the passenger side in tank pump/sender and keep the original fuel system. I have the regulator to cut it down to what I need. Why make more work. But I am dead set on bypassing this computer for everything and need to know how to by pass the wiring through the ECM.
Anyone know how? Is there a way to use the relays and the oil pressure sensor as a safety cutoff as it is now? Is there some way to add a keyed and crank hot to prime and run the pump still through the switch?
At this juncture I can go either way with it. I wanted to do this wiring before install of the HEI / new intake and carb. I have the pressure regulator ready to mount on firewall. Is it possible to wire it so as to eliminate the ECM. And does the AC run through the ECM as well. I want this to be done right and I could then pull the entire TBI system and computer since it is BRAND NEW as well and the new computer distributor and all those new pretty sensor intake and throttle body that was rebuilt to someone wanting the entire system. I think I found one gremlin when taking it apart. The four wire pin to ignition module had lost it's female side of the pin from the module which is what threw the code 42. BUT the idle issues were just leaving me stranded and it became something the wife didn't want to drive and have it die. So it was get it fixed or buy her a car. This swap is MUCH cheaper than a NEW ish car as in 2018 or newer.
Any help would be appreciated.
And the IAC was replaced and tested with ohm and or volt multimeter on the wires. BOTH the new ones I bought as well as all the others checked out fine.
I mention this because I know a lecture is coming. LOL
Thanks in Advance guys. These forums have me reading even about truck issues that would never pertain to me. Love reading the knowledge of others and experiences.

There's quite a few ways to go about this. The simplest way would be to just keep everything as original as possible. Only disconnect the plugs on the intake that you have to(distributor, TBI) and swap the intake to a Carb one. Swap the distributor and coil to an HEI unit. Then just run the fuel pressure regulator to the carb. You can connect the tach signal and ignition hot wire from the old system to the HEI and that's all you should need. Leave everything else in place and put the TBI intake and throttle body in a box just in case you sell it. Personally I would keep the TBI, and if you ever want to sell it you cut a huge amount of the value out of the truck if you hack up the original stuff when you convert. If you leave everything reversible, you get the best of both worlds and could sell it to someone that prefers carb OR to someone that wants the EFI

 

[eric 87]

An 87 truck will be a mechanical speedometer.

There's quite a few ways to go about this. The simplest way would be to just keep everything as original as possible. Only disconnect the plugs on the intake that you have to(distributor, TBI) and swap the intake to a Carb one. Swap the distributor and coil to an HEI unit. Then just run the fuel pressure regulator to the carb. You can connect the tach signal and ignition hot wire from the old system to the HEI and that's all you should need. Leave everything else in place and put the TBI intake and throttle body in a box just in case you sell it. Personally I would keep the TBI, and if you ever want to sell it you cut a huge amount of the value out of the truck if you hack up the original stuff when you convert. If you leave everything reversible, you get the best of both worlds and could sell it to someone that prefers carb OR to someone that wants the EFI

Appreciate the help. I have all the parts you mentioned. HEI dist. Intake for carb for TBI heads and carb and fuel regulator. I am initially going to do as you said using the in tank pumps etc. But if the truck runs good with the carb which I have no doubt it will since the pre computer 305s came from the factory with carbs...I will eventually get a wiring harness for a pre EVERYTHING electric. I was told NOT to use an 86 even though carbed it had I think an ESC or something. A mechanic told me get one for 78 or something. They had AC and just get a wiring diagram if needed for that year truck. If you look at the picture and see the truck and is that nice otherwise....I have so little invested currently that even if I dropped a 6k engine in it the value is still making me money. BUT if I do all this I am not selling anytime soon. And I want it to be a professional level job in that I don't have a nest of spaghetti wires in the truck.
As far as keeping it original.....Not worried about it to that extent. There are thousands of these trucks and I am not the keeper of antiques. Mine is a truck to be used. I want it to look as good as new and show level nice. But most important I want it to work for me and don't care about admiration of others. I would rather have a great running rust bucket with tin floors covering holes than a beauty queen that runs like crap. OR doesn't get driven.
If it is sold I will tell folks what is done to it. They buy or not. They pay me or not. But if you look at the forums there are many people converting this year of truck. The TBI on this was horrible. I spent over 600 in parts including new computer, distributor, rebuild TBI, 2 IAC 2 Temp sensors, o2 sensor TPS sensor Map Sensor two fuel relays replaced the brand new ignition module. Idle was still at times over 2500. Other times so low it died. It died going down road.
I mention all this because I nor my wife want to ever walk out in our garage and feel like it is a roll of the dice as to whether it gets us there and back. Again if the carb doesn't work I cut my bait and take what I can get out of it. I will be upfront to the persons looking at it. After that it is on them. I intend on also swapping out the fuel pick ups for either in tank low pressure pumps or regular sending units and electric fuel pump.
That will happen if it runs good. That will be determined in the next week. I was just wondering if there was a way to wire LONG term the current pumps in tanks since I have the regulator.
Honestly after I am done you will have a truck that is like a 70's in most ways except body style. I am keeping all records and part numbers etc as I go. If I croak and wife sells at some point then she can pass that along to others as well. I had thought about going aftermarket TBI with its own controller but that cost is high in comparison to doing this. Since I am doing all the work I saved Labor costs.
I am curious at every post I see on the swap issues why people that love TBI are so adamant about it. I look at it the same as changing heads or cams. Which I thought about as well. The engine is solid though. It is only a 305 but I don't need to race from red light to red light. It is a truck not a race car.
If I rewire etc I will sell the complete TBI system and computer and all for cheap including sensors. That and the hideous wheels in the picture.
Thanks for the answer to the questions on the basic swapping. And Don't mean to sound like an Arse. Just when you get to the point with it that you almost want to use it for bon fire material and walk away because of the TBI....well. I have had them before and they were good and dependable especially in winter etc. But when it gets gremlins in wiring. Which this one is. I don't want issues down the road using old stuff with new. I also did the work myself so I can record what is being done and not wonder what a mechanic did later.

 

[Bextreme04]

Well, if you want dead reliable and with the starting base of it already being EFI... your best bet probably would have been to go with an LS swap. You can get the engine and a standalone ECM/wiring harness for maybe $1500-2500 depending on where you are.

If you are eventually going to go with an entire new harness and pull the TBI wiring... I would look for an '85 truck that had a 350 originally. All 305's had ESC as far as I know. I would not recommend swapping the whole harness though, you only need ignition power for the HEI(which should already be there for the TBI coil) and maybe an electric choke wire. You can run that off the fuel pump oil pressure switch and then through a 10ohm resister to the electric choke. Everything else that you are going to be unplugging can just get rolled up and taped up into the harness.

 

[yevgenievich]

Leaving tbi ecu and the fuel pump relay switched through the oil pressure switch, just add regulator. Tbi ecu will prime the pump, and oil pressure switch will engage the fuel pump relay once cranking/running

 

[eric 87]

Thanks for the help to all of you. I think this project became more than I initially had envisioned. But as you get started you see things that you think "while I have this apart" and it snowballs. I can do the conversion as everyone else has and leave the computer and fuel system alone. Ride around with the SES light on with all those sensors unplugged or take the cluster out and remove the new bulb I put in. But then I thought lets do this as if I have an 87 that was wired and functions like a 75. Make it streamlined. PS PB and AC with nice Stereo is all we want along with going anywhere in the country we want. No GPS and all that crap. Nope just back to basics. One in tank fuel pump isn't working or is it the selector valve? So with this swap I intend on fixing everything. Get it running first with the current wiring. Then fuel system is next or may happen now. I think I can just remove the pumps in tanks by dropping them. Then use a 3 way valve and electric pump in line. But I have to get it up and running first before going into this further. One step at a time IT has the must haves above but need to wire the AC if it needs to be changed. That was one of my questions. Does the AC get any control from the computer? If not then I don't have to mess with that part of the wiring and can focus on Cruise and fuel.

 

[eric 87]

I would think comparing a pre TBI wiring schematic to what you have should give you an idea of what you need to do to keep the sensors working without the ECM. (If you even still need them) I did some work trouble shooting electrical issues on a 2006 jeep with a Chevy v8 conversion. Bottom line, the client had to retain the factory computer to control the dash gauges and interior lighting, but needed a new computer (ECM) to control the new engine and transmission. I had to cut the air conditioning sensors from both computers and wire them in series to retain the safety switches (hi/low pressure switch)

To completely eliminate the ECM you might need a few more pre 1978, pre TBI parts to replace anything else that needs to communicate with the ECM to function.

thanks. That is what I think I will have to do. Trying to determine how far back to go. All either of us need is AC and a good stereo for town driving and the cruise control for longer drives on interstate. I can live without the cruise if I have to. We do not need all the modern electronics that seem to occupy peoples attention instead of driving the car.
The KEEP IT SIMPLE STUPID thinking. It is a truck after all. And if it gets sold at some point I want to be sure the next owner gets something done correctly in what is done to it. However if I put all this work into it then I will keep it most likely long term.
Thanks again. I downloaded the wiring manual for the truck and will do the same with an older model. Maybe I can inspect this wiring and only make a few mods to the harness to do what I want. Would be nice to find a local shop that does GOOD wiring work. But there is something satisfying about doing the work yourself to know how it works and the sense of accomplishment.

 

[eric 87]

Well, if you want dead reliable and with the starting base of it already being EFI... your best bet probably would have been to go with an LS swap. You can get the engine and a standalone ECM/wiring harness for maybe $1500-2500 depending on where you are.

If you are eventually going to go with an entire new harness and pull the TBI wiring... I would look for an '85 truck that had a 350 originally. All 305's had ESC as far as I know. I would not recommend swapping the whole harness though, you only need ignition power for the HEI(which should already be there for the TBI coil) and maybe an electric choke wire. You can run that off the fuel pump oil pressure switch and then through a 10ohm resister to the electric choke. Everything else that you are going to be unplugging can just get rolled up and taped up into the harness.

Since my wife wants something that is like a new truck but in an old style body....and was fine with me putting in a turn key fully dressed engine for 6k that had the accessories and all...
What my idea is though is to get away entirely from the electronics in my vehicle. One thing I learned from this first edition computer/TBI system is that when it ages there is going to be demons. And I have had other cars with all the climate control and extras that are extras when it comes to cost of repair.
Here are the marching orders from wife and I agree with. PS/PB/AC . Looks nice and you can get into it start it up and go ANYWHERE in the country you wish to go. Put that into this 87 Truck and we are happy campers. Interior and body are good. Only getting a couple bubbles starting and I intend on addressing this myself. Catch it early.
But LS would be more sensors etc. I will agree it is more efficient and cheap horsepower. But I am willing to put a 290 hp 350 in it and be happy as a pig in a mud puddle. I intend on keeping the rubber on my tires and not in stripes on the road. I feel I can do this with a carbed engine.
But I want to not have a bunch of extra wires and remnants.
For now I will live with it but I will convert each item over time. Getting it running reliably first. Then the wiring and fuel systems will be changed most likely. Fuel being first. Actually that may happen as soon as I get the parts.
Thanks. There is another website group for earlier years and a member on that site uploaded all sorts of manual to download. I think the persons name is Hatzie or something. Man did he do a lot of work. He has MANY years of manuals and diagrams including our years.

 

[75gmck25]

Word to the wise - do not use the 350/290 hp engine I’d your replace your TBI. Either use the base 260 HP Goodwrench 350 engine, or consider an L31 Vortec crate. The 290 hp engine still has crappy old smog heads and about 8.1 compression; not a good match to the 222/222 @ .050 old-school cam they used. Use the price difference to buy a good intake, carb and headers.

 

[eric 87]

Word to the wise - do not use the 350/290 hp engine I’d your replace your TBI. Either use the base 260 HP Goodwrench 350 engine, or consider an L31 Vortec crate. The 290 hp engine still has crappy old smog heads and about 8.1 compression; not a good match to the 222/222 @ .050 old-school cam they used. Use the price difference to buy a good intake, carb and headers.

the engine that was the 350/290 hp was a GM performance crate engine. That is the base now. And even on it I am going carb. But now that the carb swap has been done minus the new project of redoing fuel system and trying to figure out if I need to do anything to the AC wiring to completely delete the ECM from the truck....I can now buy any engine long block and use these parts I have put on this 305. The HEI and 600 cfm Holley street warrior. And with a little work on the center holes I can use the intake as well.
Nope this truck will never see TBI again in it's life while I own it. And if I put all this work into it most likely until it hits the shredder being recycled.
I am taking out the 6 way valve for the tanks. Going to a standard 3 way selector electric fuel pump (my block has mounting but no hole through the block). I will wire the fuel pump through the oil pressure sender and the stock relay cutting out the ECM. This process is taking forever but I have spent as much time online looking at how to's as I have been under the hood and under the truck.
I intend on making a HOW To list with complete item list and part numbers for anyone choosing this route in the future. Glad GM improved this first year bunch of mess. But for those that think go aftermarket I would have swapped the TBI out for aftermarket if the prices weren't higher than these parts.
THose self learning systems are not cheap.

 

[eric 87]

What do you do about the speedometer if you remove the electrics??

speedo is manual cable on mine. It is a th400 from factory. 87 it seems had a lot of 700r4's but wonder if it wasn't just from plant to plant.

 

[eric 87]

Okay Ladies and Gentlemen.....He is an idiot idea of mine. I was going to order a new wiring harness for my 87 GMC. this one I am doing the TBI to carb swap. I want it to look professional after I am done. But was thinking instead of doing all this rewiring how can I use 95% of the existing harness. With that in mind the computer it is coming out. All the leads to all the sensors will come out. I am going to wire the new electric pump through the oil sensor switch and fuel relay. Anyway I was looking at the ECM B fuse and for that matter the ECM fuse as well. IS either a switched power circuit off the ignition or are they both constant hot even with KEY Off.
I was thinking take the ECM B circuit if a switched power circuit to the oil sensor switch and then fuel relay before going to pump. That would supply fused power. But didn't know if it is constant on or switched. I wanted to make use of that circuit if I can. THen just disconnect the ecm from it. I am not sure if A1 pin is constant on either. I thought about using that for the relay as well. I think you have to have one switched source and one constant hot on the relay if I have it right in my brain. Alternative is start from scratch with my own circuits as there are plenty diagrams to show what I want to accomplish but the factory wires are in place and are run back to the tanks.
I will be taking off the hots to the in tank pumps so they don't kick on. Was thinking of using that hot wire to the current tanks for my electric pump. Since it is already in place.
And is the current hot to the tank also feeding the gauge wire back to gauge?

Just thought I would run it by all of you first. I may sit here tonight with a pad and pen and draw out my own diagram so I know what I am seeing on paper as I go. I have all the mechanicals in place and it is down to wiring the Fuel system and AC circuits not sure there is much on AC to do.

Thanks in Advance. I intend on doing a how to and what parts to buy list. It has probably been done multiple times.....I just didn't see them. I have a few parts already going back to Summit and a couple local stores. Some lists aren't exactly right I have found.

Hey if I don't speak to any of you before.... MERRY CHRISTMAS AND HAPPY NEW YEAR. OR as I often say and get side eyes from people....HAPPY HOLIDAYS! This allows me to say those two and can include those Jewish folks I know that celebrate Hanukkah. That is why I say happy holidays....not to take Christ out of Christmas.....I left that to all the stores to do. Every watch Charlie Brown Christmas? Even in 1965 when it was made they worried about too much commercialization of Christmas. I thought it funny that a time we thought Christmas was still celebrated correctly many already worried.

Take care all.

 

[Dave Arends]

Did you get this figured out? I’m doing the same thing and was wondering on some wiring issues.