question for 700r4/fuel inj. users or really smart guys

Hey guys, like most of you, i suffer from horribly bad gas mileage. well, no longer will i be tyranised by my gas tank!

the cause:
i was looking through the wiring diagram for the speed sensor, and it appears that the computer uses a speed sensor signal from the trans. Now, if the computer does not have this signal than it would make sense that it would tend to run rich all the time cuz its not thinking that it is moving at all.

the solution:
hook the speed sensor wires up to the computer

the problem:
the speed sensor wire goes through a buffer chip located in the origional combonation meter (speedometer) in order to convert the analog signal into a digital one. this would be nearly impossible to hook up corectly.

the question:
has anyone who runs a late 80s 700r4/fuel injection setup corrected this problem? is there a way to bypass the chip? or reprogram the computer? if so, feel free to tell how. i truely believe this will drastically improve fuel economy. thank you, and sorry for the long post


-Brian M

Have you looked into any aftermarket places? Painless wiring off the top of my head may be a place to start or any company that specializes in swapping fuel infection into older vehicles.

Chris

I think it was painless that I saw a module where you put the speedo cable into it, and it sends the speed signal you need to the comp. Also, this module comes with a box that allows it to be adjustable. Then, the other side of it, you put your stock speedo cable, and you are good to go.

Of course, that assumes you have a cable speedo off the tailshaft of the 700R4.

If it's a speed sensor/wire pickup, there are a plethora of places to pick up the module you need. Mile marker's TruSpeed comes to mind. Although, I am not sure it will do exactly what you describe.

Go to this website www.jagsthatrun.com they make the speed sensor you are looking for. They also sell a great book that helps unravels the mysteries of fuel injection motor changes and tells you which speed sensor , a 4 pulse or 2 pulse sensor you actually need. You'll probably need a ninety degree adapter for the speedo also. The book has really helped me on the fuel iinjected motor swaps that I have done.

Brian: early 700r4's do not feature such irritants. I think you might be talking about the electronic 700r4, the 4L60E. Either way, I have one here, out of an 87 van, if you need any small parts. Free, you come down or I can come up-- assuming you are in Ogden lol.. of course I'd ship to NH. For the record, mine was stand alone. I think it was after they changed the name that you have the electronic lockup etc etc. Same tranny but new name. If memory serves the case on the 700r4 is smoother than the revamped 4L60E.

anyway.

Quote:

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Originally Posted by tacoma

Brian: early 700r4's do not feature such irritants. I think you might be talking about the electronic 700r4, the 4L60E. Either way, I have one here, out of an 87 van, if you need any small parts. Free, you come down or I can come up-- assuming you are in Ogden lol.. of course I'd ship to NH. For the record, mine was stand alone. I think it was after they changed the name that you have the electronic lockup etc etc. Same tranny but new name. If memory serves the case on the 700r4 is smoother than the revamped 4L60E.

anyway.

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On most GM TBI injection setups, the VSS only affects a few things:

1. TCC lockup (yes, even on a "early" 700R4) on auto trannys or "Shift" light for manuals. That can be eliminated by using a aftermarket "lockup" harness that uses a vacuum switch or having the tranny rebuilt to eliminate lockup or be locked up all the time in OD.
2. EGR operation -- disabled until certain speed/load conditions exist.
3. CANP (canister purge or evaporative emissions) -- disabled until certain speed/load conditions exist.
4. Idle Speed -- adds a "dashpot" feature to prevent engine stalls
5. On SOME applications "Lean Cruise" where the computer will continously alternate between a stoich (14.7:1) a/f mix and a leaner mix during steady-state low-load driving at speeds usually above 50mph.

Adding the VSS is easy:

1. Get a VSS from most any 80's Jeep with factory cruise (Grand Wagoneer or XJs). It is inline with the speedo cable (you will also need the "intermediate" cable between the VSS and speedo.

2. Get a "VSSB" (Vehicle Speed Sensor Buffer) from certain GM cars (usually front-drive mid-80's models with an electronic speedo -- Buick Park Aves/LeSabre/Olds 88/98 etc.). It is a small plastic box with 2 plugs, usually Yellow or "Natural" (clear) in color and usually near the steering column. A "DRC" (Drive Ratio Converter) from an '88-up C/K truck may also work, but I have not tested it (the DRC is functionally similar to a VSSB, except it has provisions to recalibrate the speedometer. It is used on GMs with electronic speedos. NOTE: the VSSB type that clips to the back of the speedometer will NOT work (this style is found in S trucks and S blazers) and pre-88 C/K pickups/blazers.

3. Email me and I will dig up the schematic to hook them up.

I came up with this several years back when I put 4.3 TBI injection on a 3.4L 60deg crate motor that replaced the carbed 2.8L in a '84 XJ. Worked beautifully.

the transmission i have uses an electronic speed sensor and does not have a speedometer cable. it is from a two wheel drive firebird, so it already has the output shaft with electronic vss. i have the factory wiring diagram, and it says there are two wires coming from the vss, and one going into the computer. the speed buffer is built into the speedometer/guages. so, i would need a complete substitute for the speed buffer becuase i really dont want to go tearing apart the combination meter to get to the speed buffer chip.

anyway, if what you say about what the vss actually controls is true, should i even bother hooking it up? i dont have a carbon canister, my egr is not hooked up, and the trans. lockup is on a manual switch. would going through all the trouble even be worth it for better fuel economy?

thanks for all the great info.

-Brian M

The speed sensor is an input but may not be that important. You will need to test the computer for closed loop operation. Make sure the MAP sensor, O2 and throttle position sensors etc. are working and readings are correct. You can test the fuel mixture by watching the O2 readings to see if is running rich. If the O2 voltage is really in the high range above .5 or .6 volts to .9 or so all the time it is running rich and poor mileage. There are a bunch of factors involved for this to happen. Hopefully you used the OE computer and wiring. A missing sensor might turn on the check engine light, a missing check engine light may not let the computer work. How did you install all that stuff?? If it was done correctly it was a lot of work and it should work ok, if it wasn't done correctly the system may run in a default or limp in mode thus the poor mileage??
Good luck, Gillus

gillus, you said that a missing check engine light may cause the computer to not work, well i didnt bother putting a light in becuase it would pretty much be on all the time. do you think i should consider putting one in? i have an old gm scantool that i use to check out the engine control systems, the o2 is operating at around .45 and is oscilating properly. i installed everything the best i could, knowing that i couldnt really do anything about some of the missing sensors and such. if it was operating in limp mode do you think i would notice? it runs perfect and has plenty of power, just really really bad mileage. my tank just ran out of gas and i got about 150 miles out of it, which = about 6mpg. mostly city miles. that aint normal, is it?
thanks guys for the input.

-Brian M

I got exactly that out of a dying 460 in a 725. I think something is wrong. I'm going to look over the specs again on the deals I've got sitting in the garage, maybe there is something being overlooked. I know my van ran fine w/nothing hooked up from the tranny.

are you here in UT yet? I wasn't kidding about the cookies hahahah

same as a dieing 460 out of a 725? that makes me feel a lot better. haha.

yeah, i'm back in Ogden, finishing up my last semester of college for an associates degree for now. maybe i could stop by or somthing one of these days and check out your ride and get a second opinion on my truck. oh yeah... and the cookies of course.

-BrianM