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Destroyer of Ignorance
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Discussion Starter #1
This one is technical and I'm looking for a very specific answer. Where does the PCM in a 1992 Chevy Suburban with TBI get it's input info to fire the fuel injectors? I'm not talking about what inputs are used to vary the pulse width but rather WHEN to pulse the injectors.

I have copies of all the wiring diagrams for this truck and a Haynes manual (slightly better than toilet paper for my needs). All info shows wiring going to and from the PCM but it does not tell me the main input for the injectors. I think it may be a hall-effect sensor in the distributor but it could come from a number of places like CPS or CS.

I've been looking for this info all over the damned net and can't find it so I figured I'd ask here as you never know who might have the answer.
 

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Cant recall off the top of my head as I got rid of the manuals but it's either off the distributor or the crank at the balancer, which then tells the pcm/ecu the crank position and starts everything out... if you have a cps that is most likely the spot. Remember TBI isnt as critical direct port injection... tbi acts like a carb so it isnt as critical as to when the injector fires so the trigger can be taken at several places but if you have a crank position sensor that is the most accurate place as the distributor will advance with engine speed
 

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The TBI units recieve their basic firing info from the dist.It is then sent to the PCM that then along with other inputs computes when to fire the injectors and at what duration.TBI units are mostly are fancy electronic two bbl carbs with two injectors on top.The injectors fire one side then the other back and forth trying to keep the o2 sensors happy it is not what you would call an accurate way to meter fuel but it is designed to help keep the cat converter alive.
 

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yeah a 92 suburban doesnt have a crank sensor so it must be the dist. i had all kinds of fun with that crap when i put a vortec 350 in my 91 s-10. the computers are supposed to be the same except for the chip between the 4.3 and the 350. the TBI's are the same too just bigger injectors on the v-8s. none of the combos worked right, i ended up going to a carb and then i had a monster. 10 mpg though.
 

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Destroyer of Ignorance
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Discussion Starter #5
Turned out to be the hall effect in the distributor, actually, the shaft bearings were blown and it caused the sensor to mis-read. Bought the whole assembly and installed it. Seems to have solved about 90% of the problem. I'm thinking I may have a dead spot in the TPS or a possible intermittent wiring issue in that circuit. Anyway, the truck runs much better now.

Unfortunately, I didn't read any of this before I made the repair but at least ssonb can now say "Told ya so", LOL. Thanks guys.
 

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The TBI units recieve their basic firing info from the dist.It is then sent to the PCM that then along with other inputs computes when to fire the injectors and at what duration.TBI units are mostly are fancy electronic two bbl carbs with two injectors on top.The injectors fire one side then the other back and forth trying to keep the o2 sensors happy it is not what you would call an accurate way to meter fuel but it is designed to help keep the cat converter alive.
Thanks for the input (no pun intended)... Couldn't remember that system, it's been way too long since I worked on one. Personally the only reason I can see for a tbi unit is retrofitting an older carborated system.
 
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