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TBI trouble when converting from Auto to Manual?

TruckNutzDude

TruckNutzDude

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Will I have problems with my computer when I swap from a 700R4 to an SM465 if I don't change the chip in my computer? I am swapping in the steering column from the donor rig so I won't have the park/neutral safety switch there. How will I let the computer know the transmission is in gear? The tranny I'm using is from a '79 Blazer and the t-case is from a '76 3/4 ton. I'm pretty sure the speedo cable will work from my 241 so my VSS will be working. If you can't figure out what I'm asking for, I'll try to explain, I made this post in a hurry /forums/images/graemlins/rolleyes.gif
 
Since this applies to more people than just me, what if you happen to have one of the later model t-cases that use a tone ring for the speedometer? How would you accomodate for that? Swapping gauges and a VSS from a rig that used a cable seems like a simple solution but will it work? /forums/images/graemlins/dunno.gif
 
Like I said in another post, I haven't delved into this much...I AM going to be converting to the tone ring setup in the future though. I already have the Electronic Speedo instrument cluster, the DRAC, and the 205 with tone-ring.

You can't just trick the computer into thinking there is a P/N switch there. You could *probably* rig in a manual switch, but that is beyond ghetto IMO. Even with TPI setups, the correct procedure when going to a manual trans is to change the PROM.

The later electric setups used a DRAC, and the signals for Speedo and ECM split off of it...so there is more wiring involved. Although I have a harness that had the DRAC, I haven't looked at in depth. There are quite a few pins on it, I wouldn't be surprised if it fed cruise, speedo, and ECM from it, which would be a minimum of three wires, plus whatever comes from the VSS at the T-case, probably 1-2 more. Obviously cruise could be left off if you don't need it, but the others are a necessity of course.

I know the '90-91 wiring manuals are around here somewhere, I'm sure someone could email you a copy so you could figure it out.
 
I did the exact same thing you are asking about with no problems what so ever. Every now and then the check engine light comes on at a stop light, but goes out about 20-30 seconds after I start moving again. I think it has something to do with what load the computer is sensing (none with a manual at stop) and the fact that it isn't getting a signal that it is in park or neutral. I have gotten to the point now where I don't even notice it and the thing runs perfectly.
 
That's a good question. Sounds like a lot of work. I know (first hand) here in California, that if that engine light goes on for any reason while you are having your wonderful bi-annual smog test, you fail. It doesn't matter what the reason or what the reading are at the time, you fail.

Also, keep in mind that when that light goes on, you are in "open loop" for the duration that the light is on. Not the ideal situation for a TBI motor.

With the wiring diagram you could set everything up as it should've been for a manual, but I'm not sure how you'd get the correct ECU or PROM. When buying a new ECU, they ussually require the ID number off the old or the VIN, how would you request one for the same configuration you have, EXCEPT with a manual trans? You might have to try the dealer and hope to get a friendly counter man with some free time or maybe find someone with the same year set up as you with the manual trans stock and ask for their ECU part number.

There's gotta be an easier way???

Eric M.
 
if it is as simple as just switching to a manual trans PROM, then i just got a reply from Brian at TBICHIPS.com saying that for $24. he can send me a stock 350 manual trans PROM or he can make a Stage 1 "performance chip" also for $50.

So if it is as simple as switching a chip then thats what i will be doing.

Nate /forums/images/graemlins/k5.gif
 
[ QUOTE ]

The later electric setups used a DRAC, and the signals for Speedo and ECM split off of it...so there is more wiring involved. Although I have a harness that had the DRAC, I haven't looked at in depth. There are quite a few pins on it, I wouldn't be surprised if it fed cruise, speedo, and ECM from it, which would be a minimum of three wires, plus whatever comes from the VSS at the T-case, probably 1-2 more. Obviously cruise could be left off if you don't need it, but the others are a necessity of course.

I know the '90-91 wiring manuals are around here somewhere, I'm sure someone could email you a copy so you could figure it out.

[/ QUOTE ]

i believe only 90+ had DRAC
the DRAC feeds the speedo, cruise, ECM, and abs controller.... to calibrate it for right gear/tire.... click here

oh yeah, i have the '91 GM r/v manual & wiring diagram books
 
I honestly think you guys are giving an OBD I computer more credit than it deserves.

Yes, EFI is smart, but the TBI computer isn't really all that smart. I really don't think the VSS has anywhere near as much to do with fuel trim as Dorian gives it credit for.

Granted, I don't think they put it there for nothing either, but I really don't think that with all the other inputs that the computer has, that the VSS plays a huge part in the fuel delivery.

I was told today, "O2 is KING" and I firmly believe that. Between that and the MAP, that computer is getting some pretty good information. I'm sure the VSS helps, but I would say it's got to be pretty low on the list of importance compared to the others.
 
88 tbi from a suburban with a 700r4 into a 79 pick up with a sm-465...runs great...occasional light that goes away immediatly with no stored codes... wheeled the wee out of it all weekend with no problems. I don't have any diagrams and don't even know what DRAC means, but it WORKS. Stuff it in there and run it.
 
[ QUOTE ]
Yes, EFI is smart, but the TBI computer isn't really all that smart. I really don't think the VSS has anywhere near as much to do with fuel trim as Dorian gives it credit for.

[/ QUOTE ]

What happens with the OBD1 system on the TBI motors is if one sensor is off, the system switches to the nominal open loop mode. It doesn't just read off the other settings and make adjustments with what it has. Even if the neutral / park switch is not functioning, which has nothing to do with any of the engine functions, the computer switches to open loop. I can't remember off hand what perameters open loop uses, I'll need to look at the shop manual for the TBI systems.

You're right, none of the EFI systems are that smart, if they are missing any of the required sensor readings, they can not tell wether it is important or not, it will automatically switch to open loop until the problem is remedied. They aren't able to "think and reason" like people can (well, most people).

Eric M.
 
If it has an electric speedo, yes it's DRAC. That would be '90-91. Sorry if that was unclear. Anything previous uses cable driven speedo and the VSS buffer setup on the speedometer itself.
 
There are many parts to what VSS considers.

First, if you get a check engine light, it isn't running right. There are no ifs ands or buts to that statement. The system throws a code when something isn't right.

I already said it would run. If you don't know what correct is because it was never setup correctly, you can't say it works right.

VSS is important to EGR, as well as IAC. Sure, people have run the systems without VSS, but people throw codes or check engine lights, or have off-throttle issues.

You said yourself, it wasn't put there for nothing. What good is TPS if you don't have a MPH reference to cross with the % of throttle? You are assuming a vehicle powerbraking meets the same criteria as one with the same throttle position on the freeway, (for example) or that one sitting in the driveway or on the freeway turning 2500RPM doesn't need to know the difference.

There is no one "king" component of these setups. You don't build an engine by throwing random parts together, and you certainly don't take random parts off and expect it to work right either, so why would you attempt to on an injection setup? Laziness? Do you think GM's billions of dollars of research is worth second guessing?

Lastly, if we needed anymore proof, witness the fact that the aftermarket wiring harness makers have now started including VSS in their setups, and stating how important it is, where before they just omitted it out and had you ground the P/N switch.

Read Me

Oh, and by the way, if you check out thirdgen's forums on DIY-EFI, there is at least one person that intentionally runs his vehicle open loop. So open/closed loop also isn't as important as you think it is, when you REALLY get into it.
 
the more i read the more confused i get. /forums/images/graemlins/1zhelp.gif alright not to hog the post but i need some specifics here, so here we go. I've got a 1990 K5 with a 700r4/241. I'm swapping in a sm465/208, and i plan on swapping the 208 with my 241 once i get the 32 spline input gear that way i have no problems with VSS or my speedo etc... Now reading the other post in the garage i found that i'll need to swap my PROM with one from a manual trans. (no problem i talked to TBIchips.com and i can get one cheap) But now i'm reading all this talk about the DRAC and i'm stumped /forums/images/graemlins/dunno.gif

But now that you know exactly what i'm using can you be more specific as to what i will need for my set-up to keep everything running smooth once i do this swap /forums/images/graemlins/waytogo.gif thanks

Nate /forums/images/graemlins/k5.gif
 
OK, this is how the system is setup for 90-91, to my understanding. (some of it I know, some I have to speculate)

There is a toner ring and a sensor in the 241. It measures how fast the output shaft is turning. That signal is sent to the DRAC. The DRAC converts that signal (thats my understanding of its function) to usable data for the ECM, which is vehicle speed. (you can see this with a scanner, it will show vehicle speed if VSS is working right)

Basically, the toner ring/sensor is the vehicle speed sensor. It just doesn't feed the ECM directly. Someone can look at the 90-91 wiring diagrams, and see how the DRAC is wired for sure, I'm just basing this off of what makes sense.

The DRAC is what is used to calibrate the speedometer for larger or smaller tires and different gear ratios, so obviously the ECM nor speedometer is fed from the toner ring sensor directly, but instead from the DRAC...the "converter" for the toner ring signal.

I would speculate that from the DRAC, you have wires going to the speedometer, ECM, and cruise if present.

I hope that clears up the DRAC issue. DRAC has nothing really to do with the PROM, it is another vehicle monitoring sensor, like O2 sensor, TPS, etc.

The PROM contains the vehicle specific "calibration" that runs the vehicle. This is what contains the spark advance tables, the fuel tables, etc. Since these are all different based on vehicle specs, (engine, weight, aerodynamics, gearing, transmission, etc.) there are many, many, many factory PROM's out there, it is not just a manual trans vs. auto trans issue. This is why you will see knowledgable people harp on burning your own PROMs, and expecting it to take 5-100 trys to get it perfect, especially after engine mods, etc.

But anyways, the burning PROM issue is not really a necessity in your case. If the vehicle runs good as an auto, and no hardware but the transmission is changed, keeping all the rest of the PROM calibration the same, and changing anything relatd to auto to manual (like the person you mentioned will do, I'm sure) you should be fine.

Obviously there were trucks out there made like you are doing (465/241 TBI) so a PROM out of one of those trucks would work perfectly fine, and thats probably what the guy you will buy a chip from, will do...copy a stock TBI manual trans PROM.
 
We have had this discussion numerous times. There is no question in my mind that the fuel requirements while moving under load are much different than when idling and the VSS plays an important part in this.

However, I'm not so sure that the VSS input isn't just telling the computer that you're moving or not moving and not much else. The MAP sensor is there to measure engine load and the O2 to adjust the fuel mixture accordingly.

I really don't know enough about this entire system to say one way or the other. I know when I do my TBI I will include a VSS, but if I didn't run one right away and had to add one later, I don't have any doubts that it would run and probably run better than it ever has with a carb even missing that one input.

You do make a good point that removing one input from the system probably isn't a good idea. I agree with you completely, but I do also feel that this particular input is much less important than some others.

GM wouldn't have put it there if it didn't need to be there for optimum performance.
 
[ QUOTE ]
However, I'm not so sure that the VSS input isn't just telling the computer that you're moving or not moving and not much else. The MAP sensor is there to measure engine load and the O2 to adjust the fuel mixture accordingly.

[/ QUOTE ]

If that were true, it would be a simple switch, moving/not moving, just like the EGR temp switch which tells the ECM if the EGR valve is closed or open.

I'm using that as an example (off the 1988 TPI system) but you get the idea. A switch would be a HELL of a lot cheaper to incorporate on every vehicle than something that actually knows the speed. Obviously with the advent of electronic cruise the VSS is still important, but even before electronic cruise VSS was used.

Obviously on the later vehicles this is even MORE important because of the Speedometer feed, but VSS (that actually measures vehicle speed) has been used on GM cars since they were CCC (first year 1980 on some Cadillacs IIRC, 1981 for all US destined GM gas cars for sure) which should tell you something. If they could have used something simpler, it would have happened in 24 years.

If the MAP sensor could do all the work, then VSS would have been obsolete in 1980 or 1981. There has to be more going on with VSS if it was still present.

O2 sensor is irrelevant in some cases, such as WOT and cold engine temps, but thats neither here nor there...you could just as easily calibrate your setup to run without an O2 sensor if you wanted to, (and it is done) but adapting to different conditions is more prolematic in that case, obviously.

I'm not arguing, just throwing a few things out so people get an idea of why I think it is so important.
 
cool, thanks for explaining man. I see the light /forums/images/graemlins/yikes.gif /forums/images/graemlins/yikes.gif /forums/images/graemlins/yikes.gif

Nate /forums/images/graemlins/k5.gif
 
dyeager is right.... the VSS sends an AC signal (40 pulses per revolution of the tailshaft) to the DRAC (in 90-91) or the VSSB (in 92-95)..... the DRAC/VSSB in 90-95 has the same function, and look almost identical. they convert the 40 pulse/revolution AC signal to a calibrated DC signal which feeds the ECM with 2,000 digital pulses per mile, 4,000 digital pulses per mile to the cruise module, and 128,000 digital pulses per mile to the ABS controller. the 92-95 VSSB unit also sends info to controll the new electronically controlled transmissions for '92 and up.
 
Couldn't the VSS be used for something that's as cheesey as a maximum speed limiter (a speed governor)?

I used to have a Beretta GTZ that felt like it "shut down" @ about 115mph...
 
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