1984 carb to TBI (maybe) question

[TommyD11730]

Guys
Im considering going TBI on my 84 K2500 (350/700r4). aside from stealing the obvious from a donor (intake/dizzy/underhood harness/ ecm and its harness) what else will I have to grab? Yes I will have to come up with a place for an exhaust bung, Yes Ill need a tank and pump/sender from a 87 and plumb new lines. I do have at least 1 question though.
Ill need a speed sensor input. I really dont want to swap clusters if I dont have to. any suggestions?

Thanks

 

[Charlie]

 

[74 Shortbed]

Welcome to the site..

 

[4WDKC]

Guys
Im considering going TBI on my 84 K2500 (350/700r4). aside from stealing the obvious from a donor (intake/dizzy/underhood harness/ ecm and its harness) what else will I have to grab? Yes I will have to come up with a place for an exhaust bung, Yes Ill need a tank and pump/sender from a 87 and plumb new lines. I do have at least 1 question though.
Ill need a speed sensor input. I really dont want to swap clusters if I dont have to. any suggestions?

Thanks

I recommend getting one from a 93-95 truck, they have control options for the 4l60 and 80e transmissions, also a faster connection speed to laptops for data logging. I think they use the vss for input directly where as teh 87-92 trucks use a DRAC module that can be a PITA to locate and make function with correct gears. If you wont tune it the egr,air pump equipment will set a code and may cause undesired running conditions.

You will need a new speedo either way maybe a full cluster. The 87+ trucks use a hall affect sensor on the speedo to send electric signal to the TBI so it knows when to lock up the torque convertor, actuate the egr and air pump. The 91 speedo is electronic and can accept input from the VSS or ecm if it outputs speed.

 

[TommyD11730]

The donor is a 87 C10 auto. I know my 84 K2500 uses a vacuum/electric switch on the firewall to control the 700R4.

I thought as the donor uses a 700R4 I would just have to use a aftermarket VSS inline on the cable. (Howell)
I'll retain the EGR on the factory intake, but AIR could be a problem.

 

[Derrick]

 

[4WDKC]

The donor is a 87 C10 auto. I know my 84 K2500 uses a vacuum/electric switch on the firewall to control the 700R4.

I thought as the donor uses a 700R4 I would just have to use a aftermarket VSS inline on the cable. (Howell)
I'll retain the EGR on the factory intake, but AIR could be a problem.

If you you the full donor then just swap it all, dont forget the tank switch valve also, on the 87 they are rated to 65 psi, not sure about the ones for carbed trucks.

 

[ajd89]

The donor is a 87 C10 auto. I know my 84 K2500 uses a vacuum/electric switch on the firewall to control the 700R4.

I thought as the donor uses a 700R4 I would just have to use a aftermarket VSS inline on the cable. (Howell)
I'll retain the EGR on the factory intake, but AIR could be a problem.

Last time i checked a 700r4 doesnt use vss. It uses a cable connected to the tbi. The 4l60e is computer controlled and would need a speed sensor. Should be a easy swap if you have the whole donor truck. I know a lot of people will disagree but i really liked my 87 with tbi. Fired right up no matter how cold it was and it ran great too!

 

[TommyD11730]

Let me backtrack for a second. Thanks for the welcome! It’s nice to find a square body forum hopefully with a lot of participation. Also if any mods are reading this, feel free to move this thread to the correct forum. I now see there is one devoted to TBI Conversions.


Ok, just a rambling… My truck did have dual tanks. The PS was leaking so I removed it. I have only been running on the DS as I only drive the truck local. The crossover system did work at the time of pulling the tank. The fact that the TBI trucks used a different crossover valve always made sense to me. However I was always assured that both the carb and TBI trucks used the same valve. So looks like you guys have confirmed what I suspected.


Ok, back to the TBI Conversion. Keep in mind the donor is many states away. The seller will hopefully accommodate requests to yank this or that.


What I’m wondering is did the 87 year use a VSS factory on the TBI trucks? I briefed some of the forums and it seemed like the info was conflicting. By that I mean to say the auto trucks did, and the manual ones did not. Perhaps I read that wrong, reading comprehension was never my strong suit. Anyways if the ECM does indeed need to see a speed signal can it be as easy as adding a aftermarket speed sensor (Howell lists them) vs having to go with a later model cluster. (Finally got my cluster nice… don’t want to have to mess with it if I don’t have to.


Also did the early TBI trucks use air injection? I know my carbed 84 had it, but my 91 TBI Blazer does not.

 

[4WDKC]

Let me backtrack for a second. Thanks for the welcome! It’s nice to find a square body forum hopefully with a lot of participation. Also if any mods are reading this, feel free to move this thread to the correct forum. I now see there is one devoted to TBI Conversions.


Ok, just a rambling… My truck did have dual tanks. The PS was leaking so I removed it. I have only been running on the DS as I only drive the truck local. The crossover system did work at the time of pulling the tank. The fact that the TBI trucks used a different crossover valve always made sense to me. However I was always assured that both the carb and TBI trucks used the same valve. So looks like you guys have confirmed what I suspected.


Ok, back to the TBI Conversion. Keep in mind the donor is many states away. The seller will hopefully accommodate requests to yank this or that.


What I’m wondering is did the 87 year use a VSS factory on the TBI trucks? I briefed some of the forums and it seemed like the info was conflicting. By that I mean to say the auto trucks did, and the manual ones did not. Perhaps I read that wrong, reading comprehension was never my strong suit. Anyways if the ECM does indeed need to see a speed signal can it be as easy as adding a aftermarket speed sensor (Howell lists them) vs having to go with a later model cluster. (Finally got my cluster nice… don’t want to have to mess with it if I don’t have to.


Also did the early TBI trucks use air injection? I know my carbed 84 had it, but my 91 TBI Blazer does not.

My 87 TBI 700r4 4x4 has a cable that turns the speedo, the speedo has a hall affect sensor on it that converts to an electric signal. This is then sent to the DRAC module that is calibrated for gears and input into ecm.

 

[1987 GMC Jimmy]

84+ have the VSS buffer, but what’s this DRAC module you speak of on a pre-electronic speedo truck? I thought the output went straight from the buffer to its destination. The buffer on the OP’s cluster should work fine for the 7747 ECM without needing anything. I’m not sure where the wiring modification would take place on the ‘84 since it’s used only for cruise control, but it’s the same little green box that was used until they ditched the speedo cable. Plus, if the OP is going with a totally different harness, it should be plug and play.

 

[CSFJ]

The '87 model trucks did have air injection also.

 

[4WDKC]

84+ have the VSS buffer, but what’s this DRAC module you speak of on a pre-electronic speedo truck? I thought the output went straight from the buffer to its destination. The buffer on the OP’s cluster should work fine for the 7747 ECM without needing anything. I’m not sure where the wiring modification would take place on the ‘84 since it’s used only for cruise control, but it’s the same little green box that was used until they ditched the speedo cable. Plus, if the OP is going with a totally different harness, it should be plug and play.

My 87 definitely has a speedo cable in the t case and in the speedo. Looks like the DRAC was on older trucks based on this link.
https://ck5.com/forums/threads/end-all-post-for-drac-vss-yeah-right.124012/

 

[1987 GMC Jimmy]

My 87 definitely has a speedo cable in the t case and in the speedo. Looks like the DRAC was on older trucks based on this link.
https://ck5.com/forums/threads/end-all-post-for-drac-vss-yeah-right.124012/

I took this as saying the DRAC was not in there, and it was telling you what you’d have to do for an electric speedo conversion. It says in the first post that the signal was sent from the buffer to the cruise control and the ECM.

 

[TommyD11730]

My 87 TBI 700r4 4x4 has a cable that turns the speedo, the speedo has a hall affect sensor on it that converts to an electric signal. This is then sent to the DRAC module that is calibrated for gears and input into ecm.

Ok more educating... good stuff. I had to google the DRAC module to see just what the heck it was. Learned a bit about them, what they do, and how to move jumpers for different sized tires. However.. where are they mounted on TBI equipped squares??

Also, the actual in cab harness for the FI. After it exits the ECU is it standalone and exit the firewall via a grommet? Or is it part of the bulk connector that ... well exits the firewall.

Thanks guys