TBI HEADACHE

[Tattoojack]

So I have a 1973 K5 blazer with the 350ci, TH350 trans and np203 t-case. Previous owner put a throttle buddy on it. Injectors lost 12v somehow and were only getting 8v. I ran a wire to the 2 positive wires going to the injectors thru a toggle switch and to the battery direct. I flip the switch, injectors get 12v, blazer starts. While running, I can shut the switch off and stays running fine. Problem is, won’t start unless I flip the switch. Will this harm any of the system they installed? ECU, ECM, other electronics?

 

[Bent77]

Yeah, you could mess it up if it back feeds voltage to the computer

Couple things you want to know about TBI, it is a ground switched system. So before you do anything else, take all the grounds off of everything clean all of them and put them back on.

There is two ways that the fuel system works on a TBI unit. It uses a bypass switch to fuel pump and then it uses the oil pressure switch to keep the drive system voltage going to the pump so that it keeps running. It uses a prime system relay to start the vehicle.

If you go to “the injection section” there are some wiring diagrams that will make more sense of it, presuming it was installed as GM engineered it

 

[Tattoojack]

My fuel pump runs and pumps fuel to the injectors, but the injectors aren’t getting 12v somehow and weren’t spraying. That’s where I ran the 12v wire to the injector hot lines to start it. Once it’s started I turn the hot jumper off. Unfortunately, whoever set this up kinda half assed it and wires everywhere and most the same color.

 

[Bent77]

The partial voltage thing keeps coming back to me, like maybe not grounded. The reason I mentioned the relay, is because we need to see it cycle there on cranking. If not, the system may not know the engine is cranking

When it’s cranking, it will ground for the injectors and see 12v if working

You may need to remedy some of that cousin loving wiring

87-91 TBI wiring Diagrams

TBI wiring diagrams for 1987 to 1991 C/K/V/R trucks. 1 to 5

ck5.com

 

[eodcoduto]

Check the grounds, frame to battery, engine to firewall, and the one on the water temp sensor. Run a ohm them out and try to get under 5ohm between the fayjng surfaces. Then try a new ignition module, it controls the injector pulses.

 

[Tattoojack]

Yeah it needs rewiring and rerouting of what’s there for sure to clean it up. Which relay are you talking about? The fuel pump relay? That works just fine. All grounds are clean and good, that I could find anyways. Lol

 

[eodcoduto]

Check your grounds with a ohm meter, go from the wire to the ground surface to be sure. Its cheap insurance and will save a ton of headaches down the road. Can you find a donor vehicle and pull the complete factory tbi system?

 

[Tattoojack]

Check the grounds, frame to battery, engine to firewall, and the one on the water temp sensor. Run an ohm them out and try to get under 5ohm between the fayjng surfaces. Then try a new ignition module, it controls the injector pulses.

That’s what I was thinking the issue was. The ECM or ICM? under the distributor cap possibly? I’m not throwing any codes tho at the ALDL or obd 1. Whichever it’s called. Seems like a bad ground, but so far haven’t found anything. Cranks over just fine and will start with gas poured in carb throat. Haven’t tried to drive it yet the way I have it rigged, but curious if it would damage anything since it’s only used for starting.

 

[Tattoojack]

Check your grounds with an ohm meter, go from the wire to the ground surface to be sure. Its cheap insurance and will save a ton of headaches down the road. Can you find a donor vehicle and pull the complete factory tbi system?

I’ve called salvage yards around here and can’t find anything with TBI on it. Maybe when I get to Florida with it I’ll find one. Eventually I think I’m gonna go back to it having a carb like original. Carb, intake and distributor and eliminate the whole TBI system and computer completely.

 

[Blue85]

Look at the wiring diagrams for 87-91 to see how the fuel pump relay, oil pressure switch, TBI A and B fuses are set up and follow all that wiring through. I think there used to be a resistor wire for the electric choke or to excite the alternator and it's possible that got re-used for injector wiring. It might just need to be moved. Or you could have a bad connection at the ECM, fuse panel, etc. From your description, this is a relatively simple problem and should be easier to fix than dealing with all the conversion, setup and tuning issues with a carb swap. Don't fall for the "grass is greener" and downgrade the vehicle. You won't eliminate all the problems, you'll just find different ones.

 

[Tattoojack]

Look at the wiring diagrams for 87-91 to see how the fuel pump relay, oil pressure switch, TBI A and B fuses are set up and follow all that wiring through. I think there used to be a resistor wire for the electric choke or to excite the alternator and it's possible that got re-used for injector wiring. It might just need to be moved. Or you could have a bad connection at the ECM, fuse panel, etc. From your description, this is a relatively simple problem and should be easier to fix than dealing with all the conversion, setup and tuning issues with a carb swap. Don't fall for the "grass is greener" and downgrade the vehicle. You won't eliminate all the problems, you'll just find different ones.

Fuse panel is the original. I’ll try to get some pics today of what’s there and the rats nest of wiring.

 

[Bent77]

Fuse panel is the original. I’ll try to get some pics today of what’s there and the rats nest of wiring.

So you know, couple of us are being “extra particular” about the trouble shooting since it’s a conversion, not trying to give you the gears


Since the relay fires, see if the cranking signal wire is hot while cranking. You’ll have to use the diagrams for that

If that wire is hot on cranking, the ICM is a usual suspect

 

[Tattoojack]

So you know, couple of us are being “extra particular” about the trouble shooting since it’s a conversion, not trying to give you the gears


Since the relay fires, see if the cranking signal wire is hot while cranking. You’ll have to use the diagrams for that

If that wire is hot on cranking, the ICM is a usual suspect

By use of a noid light? I bought a set yesterday and without my jumper wire, nothing happens with the noid light. Turn jumper wire on and noid light flashes like it should when cranking. I’m very old school when it comes to old school vehicles. Never really been a fan of computer systems at all in a vehicle. Always been a believer in if it’s not broken, don’t fix it, keep it original, keep it simple stupid and don’t make it more complicated than it has to be. I get the LS swaps people do, if the entire system is upgraded. This blazer seems like the bare minimal was upgraded and kinda rigged together to get it to work half ass. The fuse box is still the original glass fuses for christs sake with a thousand red wires running to it. Seems like most of the wires added were all red and then some trailer wiring thrown in. Nothing labeled either. If I wasn’t already bald headed, idve pulled my hair out by now. Lol

 

[Bent77]

I use a meter, your 8v at the injectors of note may trip the noid or test light, but isn’t the right voltage

 

[Tattoojack]

I use a meter, your 8v at the injectors of note may trip the noid or test light, but isn’t the right voltage

I used a meter first and found they were only getting 8v, which didn’t trip the noid light at all. Once the jumper was on, the noid light functioned and they had 12v to them.

 

[Bent77]

Some of the other circuits in this system are 5VDC

 

[dyeager535]

So I have a 1973 K5 blazer with the 350ci, TH350 trans and np203 t-case. Previous owner put a throttle buddy on it. Injectors lost 12v somehow and were only getting 8v. I ran a wire to the 2 positive wires going to the injectors thru a toggle switch and to the battery direct. I flip the switch, injectors get 12v, blazer starts. While running, I can shut the switch off and stays running fine. Problem is, won’t start unless I flip the switch. Will this harm any of the system they installed? ECU, ECM, other electronics?

How were you measuring voltage at the injectors?

Need to measure from a good ground (like battery) to the injector +, not across the two leads. It was already stated before me, but you may not have caught it.

Or, you did test like this and got 8V while measuring properly.

 

[Tattoojack]

How were you measuring voltage at the injectors?

Need to measure from a good ground (like battery) to the injector +, not across the two leads. It was already stated before me, but you may not have caught it.

Or, you did test like this and got 8V while measuring properly.

First across the leads then to battery negative after trying to the block as a ground.

 

[dyeager535]

Block should ground too, shouldn't matter. So could be an indicator.

As messy as this install sounds like it is, this DID work properly for you at first, correct? So something changed or failed, it's not solely due to a hacked install?

 

[Tattoojack]

Block should ground too, shouldn't matter. So could be an indicator.

As messy as this install sounds like it is, this DID work properly for you at first, correct? So something changed or failed, it's not solely due to a hacked install?

It ran, yes. Not properly tho. The way I have it rigged now, hopefully gets me from Colorado to Florida where I can properly fix it and have the equipment and parts to fix it if needed. I drove it around the block and seems ok, just don’t want to kill something halfway across the country. Lol