carb to tbi conversion

[Dan Brown]

I am currently putting a TBI unit on my 1974 Chevy K10 with a 350 and Muncie SM465. I'm using a 1227747 ECM and was told that I won't need a VSS. If anybody out there has done this same conversion, I would be grateful for any help you can give me. Thx a lot. Dan Brown

 

[dyeager535]

TBI will run without it, but there are usually complications. The big one I'm aware of is stalling when decelerating.

Think your easiest option would be to swap in a 1983(?)-1989 cluster as those all have cable drive VSS, but it's part of the cluster. That introduces other issues (different speedometer cable connector being one) but if you are doing the TBI swap, I suspect you will find the other issues are pretty simple to deal with.

 

[Bent77]

I am currently putting a TBI unit on my 1974 Chevy K10 with a 350 and Muncie SM465. I'm using a 1227747 ECM and was told that I won't need a VSS. If anybody out there has done this same conversion, I would be grateful for any help you can give me. Thx a lot. Dan Brown

Done two of these with that exact computer, but had to change the PROM because the computer will look for the VSS.
I had Howell Eng burn me a new PROM each time, each time removing the VSS (it will pattern off the TPS instead) I also had the EGR removed

 

[Dan Brown]

TBI will run without it, but there are usually complications. The big one I'm aware of is stalling when decelerating.

Think your easiest option would be to swap in a 1983(?)-1989 cluster as those all have cable drive VSS, but it's part of the cluster. That introduces other issues (different speedometer cable connector being one) but if you are doing the TBI swap, I suspect you will find the other issues are pretty simple to deal with.

Right now I have a 2PRS speed sensor that I got from Jags That Run. It's driven by the speed-o-meter drive at the rear of the transfer case, and the speed-o-meter cable attaches to it. It has two white wires of which I was told one is a ground. The wiring schematic I'm using for the 1227747 ECM shows pin A10 (brown) wire going to a VSS Buffer. Is that how I should hook it up, ground one white wire and put the brown wire coming for A10 on the other white wire, and, does it make any difference which is which.

 

[dyeager535]

I'd talk with JTR to make sure. I've got slow internet so it's taking too long to load their site and dig for info. A10 is definitely the "input" to the ECM for VSS, as long as the output of that sensor is what the ECM is looking for, should be no problem.

 

[anwat]

That's the setup I have on mine, I think....is yours coming off a 205 transfer case? Does it look like this?

 

[Dan Brown]

That's the setup I have on mine, I think....is yours coming off a 205 transfer case? Does it look like this? View attachment 363509

Yes it does look like that. Who made yours and where did you get it? How many wires does yours have on it, and how did you wire it. I sure appreciate the picture and the help you have been to me. Thx a lot.

 

[Dan Brown]

Yes it does look like that. Who made yours and where did you get it? How many wires does yours have on it, and how did you wire it. I sure appreciate the picture and the help you have been to me. Thx a lot.

Oh, and yes, it's a 205 transfer case.

 

[anwat]

Oh, and yes, it's a 205 transfer case.

I'll go out and check for sure tomorrow morning, but I think the brown wire went to the buffer box first, then to the ECM. I'm running the same ECM, so the wiring should be identical. I just want to check before I steer you the wrong way. My box came from JTR, it is also a 2PRS. I've got it in a 90, but I have a dash and a transfer case from older models, so I still have a cable driven speedo.

 

[Dan Brown]

You've been such a great help to me, and I've been trying to find somebody in my area that has done what I'm trying to do that I could talk to face to face about my project. Don't know where you are, but I'm in Katy, Texas right outside of Houston. If you don't mind, and this forum doesn't mind, I'd really like to talk to you directly on the phone or by Email. My Email address is [email protected]. If you could send me yours I'd really appreciate it. My name is Dan Brown and I'd really like to tell you about my project and hear more about yours. Thx a lot.

 

[anwat]

Ok, mine is going to be a bit different, I think. I don't think you have an instrument panel connector, a flat long connector that has about 21 pins in it...I think that came about with the electronic speedo. My VSS wire goes to that connector, from there it goes to a DRAC module and the buffer. I'm sure someone else will pipe in on which of those two you actually need. My understanding is the DRAC is a way GM came up with to use the older speed sensor with different tire and gear ratios. I didn't have time to dig around and get the wire colors off my module, but I'm pretty sure the wire that goes to the DRAC is the one that just dead ends, since I now have a cable speedo. The other goes to the ECM, but I'm told it doesn't really do much important. I'll have some more time to dig around and see how it's hooked up over the next day or two, if someone else doesn't post up from memory. I'll also get you a message with my email address, I'm happy to help out if I can. There are others that know a lot more than I do, I've only done it once, and mine is with an auto trans, but shouldn't make a difference, since the VSS is coming off the T-case. I'll also see if I can find a wiring diagram for you.....

 

[Dan Brown]

As I said before, you've been an absolute God send with your help. You have used some terms that I'm not familiar with, i. e. DRAC. Too, I built my own dash using Auto Meter gauges so there is nothing in my dash with 21 pins in it. Thx again for your help, and your call sign is anwat, but, what is you name. I guess it could be anwat, so what should I call you?

 

[anwat]

Here's the GM wiring diagrams for your ECM. looks like the two wires coming from the speed sensor (light green and black, and purple and white) do go to the DRAC. From the DRAC the brown wire carries the signal to the ECM. I think I tried to cut the DRAC out of my setup, and I either had the 4x4 indicator light stayed on all the time, or not go on at all. Not a big deal, but I figured that if it did that, it probably did other things and I should leave it in. The DRAC is specific to gear ratio, so you'll have to figure out which one works with the final trans and rear end gear ratio you have.

According to JTR, though, you can wire the 2PRS directly to the ECM. I'm assuming if you did do that, it would just go to the same brown wire as the DRAC signal goes to. I wouldn't ground the other wire, I'd just leave it unhooked, I think the unit is grounded by the attachment to the T-case. Unless, of course, JTR said it should be grounded. Both of the wires from mine are plugged into the DRAC module.

Hopefully someone will chime in if I'm wrong on this. The beauty of this way of getting info is that it is usually verified by at least a few people who read the thread and correct the info you are getting. Since it sounds like you're new to the message board game, another spot I've found helpful is 3rd gen.org. Just veryify everything before you use it! Mine is wired through a DRAC, because it originally had one installed and I never took it out. I'd try wiring the 2PRS directly to the ECM, (smaller connector, A row, third spot up from the bottom. If you have both sides of the connector is should be a solid brown wire) and if that doesn't work, then go to the junkyard and find a DRAC suitable for your ratio and try it that way.

 

[dyeager535]

DRAC was used '90-91 on the R/V trucks. It is used with the VSS sensor mounted on the transfer case (trans if 2WD). The output from that VSS is AC, the DRAC converts that 40 pulse/AC signal into the various outputs needed for the electronic speedometer, ABS, and the ECM/PCM and cruise control.

1983-1989 VSS (2000 pulse per mile IIRC) was done off the back of the speedometer, and used an optical eye. The VSS buffer (green or yellow box under dash) was what provided the signal for the ECM and cruise control.

The good thing is that the input to the ECM/PCM these vintages always stayed 2000PPM...ECM doesn't care what the source signal is, it just wants it, in the correct 2000PPM.

 

[anwat]

Do you know if that VSS from JTR is already in that format? They claim you can hook it right to the ECM, so I'm assuming he can bypass the DRAC, correct?

Any idea why when I cut the DRAC out of my setup the 4x4 light came on and wouldn't go off? I'm running a cable speedo out of an older truck, but the ECM and DRAC are original to the truck, which is a 90.

 

[dyeager535]

The problem I remember with JTR in the past was they focused on cars more than trucks. Some of their statements regarding VSS were wrong when talking trucks. page 2 on the article you linked to is utterly wrong on their timeline with anything to do with trucks. Probably a decade ago I wrote them for the correction, but I see they didn't listen.

2000PPM is the right signal for 1983-1989 "stock" truck applications. I don't know what JTR shows as the output for that device. Just know you need the 2000PPM input to the 7747 ECM.

As to the 4WD indicator, all I can think of is that somehow they share a common wire for power or ground. I don't see it in the diagrams you posted but I'm working off a phone and it's hard to read them. ECM doesn't care about 2 or 4WD, and since the output shaft is what drives the VSS, even low/high 4WD is irrelevant to the ECM.

 

[Dan Brown]

Dyeager535 you seem to know quite a bit about JTR. The last time I talked to somebody there about the 2prs vss, the guy told me, I think I'm remembering it right, that his brother, who owned it had died an that he didn't know much about the 2prs. He's the one that told me that one of the white wires on it was a ground and the other was the signal, and to be careful about not putting a signal on the wrong wire or you'd ruin the sensor. Do you know if this is true, and if so, why both wires are white and not marked as to where they go?

If you have a better suggestion for a vss for my application, I'd like to know what it is you'd suggest. My speed-o-meter is an Auto Meter using a cable. I used to have a 1987 Chevy Astro that had a cable driven speed-o-meter with the vss on the back of the speed-o-meter and the cable mated to the vss. Does anybody make a vss that works this way?

Too, anwat, where did that schematic come from as I'd like to have a paper copy of it.

Thx to you guys for any help.

 

[anwat]

I have the whole wiring manual, I'm trying to find the link. If you look in Dyeager's line at the bottom of his posts, there's link to a ton of good manuals. Just be very careful downloading them, I know the 1990 wiring and service manuals both have viruses. It's possible that I even got the manual I have from there prior to it being infected. I'll look for the link, if I can't find it, I'll send you a dropbox file or something with the whole PDF manual. Your ECM was used in multiple years, so it should generally cover your wiring for the ECM, but your chassis wiring should come from the older year manual.
Also, fwiw, I have the two white wires coming off my 2PRS....neither one goes to ground in my setup...they both go to the DRAC, different ports.

Found It!:
http://www.73-87chevytrucks.com/techinfo/7387CKMans//Wiring/X_9130_1991_GMC_RV_P_Wiring_Manual.pdf

 

[Dan Brown]

Got it Anwat, thx a lot. I'm going to try to find a DRAC next week at my favorite 'pull you own parts' junk yard.

Too, I'd like to put a picture of my project on the site like the one you have of the yellow what appears to be a Blazer with the cover off, which I assume is your project, on it and can't figure out how. Can you please walk me thru it.

 

[Dan Brown]

Thx Anwat, I just found that manual on Ebay and bought it.