90 TBI into 85 K10 gauges

[Oldsarge29]

Hey all,
I’m doing my first tbi swap. I have a complete donor 90 c2500 and I have pulled the engine and the wiring for the engine and dash. I have finally got the engine in the 85 and now working on the wiring. I’m not n process of connecting everything. I want to keep my current gauges working but I don’t know how to connect them to the 90 engine harness. I have the engine, dash, fuse box and gauge cluster information for the 90 c2500. I’m not real good with wiring but I do have thick skin so let her rip. Any help is appreciated.
Oldsarge29

 

[Vbb199]

Well, I went thru a similar deal when I had my k5 project. It was originally a 1991 k5 but had an earlier cable driven speedo instrument cluster (someone swapped a th400 in it), and I was using a 1991 engine harness, in tandem with a 1987 suburban interior harness (I made it all work)

The pinouts are mostly the same in terms of the firewall bulkhead that matches up. You may find you'll have to move some pins (I did anyways).

You'll just have to go wire by wire and trace them, or find one of our local pinout/wiring diagrams on this website.

The only thing that really passes thru the firewall at the bulkhead of your k10 is your gauges signal wires..
(temp, oil, gas)
The volt meter I'm not sure of... I know on the 87-91 stuff the actual 12v signal itself that energizes the cluster is what supplies your volt meter..

Fuel pump/gas gauge is powered by 2 wires + ground at the frame rail.

A pink and brown/tan. One is gas gauge resistance, the other is 12v to the pump.

I assume you are using the oem 85 interior/cab harness?

What about the trans from the c2500? You said 1990, so I'm to assume maybe it's a th400? I (recently) learned there are th400's out there that use vss (vehicle speed sensor), in the 1990-1991 range (as well as the 700r4 4 speed automatic)

My point here is = if you're using a VSS driven speedometer transmission, you're gonna run into some minor issues getting a working speedometer.

You will need to source the DRAC module and speedometer from a 1990-1991 , (DRAC decodes the electronic pulses of the reluctant wheel inside the trans and turns it into an analog signal the speedometer can read, and then solder the terminals to match your tire size and gear ratio of your 1985)
Or just keep whatever is in your k10 now. Which is probably a th350, 4 speed, or a 700r4.


I hope this is some helpful information. I can answer your questions about "what this is" when you get to the engine bay harness if you like.

 

[Oldsarge29]

Thanks for the information. I found the wiring diagrams and printed them out. That will help.
Yes, I’m using the 85 interior harness. It’s in good condition so I’m keeping it.
I’m keeping the 700r4. I didn’t want to go through the trouble of converting the th400 to 4wd. The truck has cruise control so there’s a vss signal in there somewhere. I think I need to find it to use for the ecm.
1. I have a purple wire that goes to pin c9 on the ecm and I’m not sure what it connects to. Does that the wire connect to the switch wire that is hot only in start position?
2. Red with white stripe that goes to the fuel pump relay connects to?
3. Gray wire fuel pump input goes to the fuel pump?
4. Orange wire with 2 black stripes goes to?
5. Purple wire goes to?
#4 & 5 I have not found the other end.
6. Red wire from D14 on the ecm goes to?
7. Red wire with 1 white stripe from ecm B1 goes to?
8. Red wire with 2 white stripes from ecm C16 goes to?
9. Purple wire with white stripe from ecm C9 goes to?
The reason I’m asking all of this is because the diagrams that I have doesn’t match the wire colors that are in the harness. This ecm and harness came out of a running 1990 Chevy 2500 with the 1228747 ecm. I’m totally lost.
Thanks for your help.

 

[Oldsarge29]

Ok, I’ve finally got the right wiring diagrams. Everything makes sense now. The wiring is not going to be that bad.
I’ll be searching for the VSS signal in the next few days. Hopefully it won’t be too hard to find.
Thanks again for your help.

 

[Oldsarge29]

I have almost everything connected. I’m waiting on the fuel tank to get here but I did get the engine to fire off by pouring some gas into the throttle body. So, I think I have that part of the wiring done. I pulled the hard gas lines from the donor (long bed) truck and have them in the 85. When I get the tank in I’ll cut the lines to fit.
Now I’m working on the speed input for the ecm. My truck has the optical sensor in the speedometer and I found the speed buffer. I haven’t been able to figure out what wire I need to use for the input to the ecm. The ecm is out of a 90 2500. Any information on this world be greatly appreciated.

 

[Oldsarge29]

Bump.
Anybody have any information??

 

[mcarlo86]

How's your progress? You may want to check out this site: https://harristuning.com/Tbi/

I have a '90 Suburban and got a chip burned from them. They have some good stuff on there to optimize TBI's.

 

[Oldsarge29]

I finally got it figured out. My truck had cruise control and there was an output on the module that I was able to use for the ecm.
Thanks again for your help

 

[bucket]

I'm too late now, but the diagrams for a '87-'89 K5 or Suburban would have helped a bit. They used the same cluster that you desired to keep and were TBI.

 

[Oldsarge29]

The truck is running great. This was my 1st conversion so it was a little challenging. I finally got all of the bugs worked out. The only issue was placement of the ecm. I ended up putting it on the right fender well. I just couldn’t get it to fit behind the glove box. Hopefully it will be good there.