1988 GMC C1500 engine/trans into 1973 C20 Crew Cab

[O-B-1]

Donor truck is a 1988 GMC C1500 made by General Motors of Canada, LTD manufactured 01/87

Receiving truck is a 1973 C20 Crew Cab, long bed.

1. What do I do with the TBI fuel regulator return line? Can I just tee that back into the line from the tank before it gets to the pump? Or will that possibly cause vapor lock problems? The truck has dual tanks.

2. Locating the EMC. Would it be safe to install the EMC in the engine compartment atop the fender? On the firewall? Would there be heat problems for the EMC? The EMC was located behind the dashboard in the donor truck. If it cannot be located in the engine compartment I will have to extend the harness and locate it under the passenger side of the front seat.

Thank you in advance for the help and advice,
O-B-1

[jim_in_dorris]

#2 first. That pcm will not live in the engine compartment. it was designed to go behind the glove compartment in the cab. under the front seat would work fine.
#1 if you T the line into the feed, you will have pressure on both sides of the pressure regulator and it won't work. The return line needs to run all the way back to the tank. OOPs, i just re-read your post. I assume you have an inline pump for your fuel line. i am not sure if it would cause a problem there or not.

[PJG1173]

for the return line you can T into your filler neck. take a piece of 1 1/4 pipe drill a hole and slot it. then take a piece of 5/16 tube and weld/braze it at a 45* angle to the 1 1/4 pipe. as the return fuel comes back to the tank it will shoot right down the filler neck tube into the tank.

[O-B-1]

The truck has TWO tanks with a valve to switch between the two.

We are installing a E2182 in-line fuel pump. I was going to put an inlet (Purolator F21111) filter BEFORE the pump, then the pump, then fuel injection filter (FRAM G3727) AFTER the pump to catch any fines before they enter the injector.

I thought putting a tee between the line from the tanks BEFORE the inlet filter would allow the return from the regulator to return back before the pump.

I was thinking that if the return from the regulator just went back to the tanks there should NOT be any pressure to speak of unless there is some other mechanism at the tank to impede it's flow.

The inlet at the pump would normally create a vacuum...

I have one other question... The ECM has a signal that would come from the shift selector active when the selector is in PARK and NEUTRAL... how do I rig something like that?

Thank you for your help and ideas...

O-B-1

[fastacton]

You can get a dual tank selector from a truck similar to the donor (it switches the feed and return lines), but reliability might be an issue, mine never seemed to want to work right. I wouldn't tee the return into the feed line, it would might work, but I could see it causing problems that could drive you crazy as well.

Once you have the wire harness torn down, you might see that you can get enough length out of it without extending it. But if you can't, a van harness has tons of length built into it.

[O-B-1]

It looks like getting a 6-port tank selector valve and two of the Moroso filler neck adapters with returns is in order.

I still have to figure out how to get the park & neutral switch set up. Maybe magnetic reed switches in the linkage somewhere, or give her a switch on the dashboard.

[PJG1173]

switch on dash = bad Idea. magnetic reed switch now that's a new one. I'll have to remember that one it will probably work. I have a aftermarket floor shifter with the park/neutral switch built in so I wired into that one.

[garnerm]

How hard would it be to just mount the tank from the donor truck? ECM needs to be in the cab because it is not water tight. you could use a 1227727 ECM and it is made to be under the hood.

[billygraves]

.......

[hammertime371]

When running duel tanks the return would have to auto switch to the tank your using otherwise if both tanks are full you will over fill 1 when using the other

[billygraves]

.......

[1project2many]

I have one other question... The ECM has a signal that would come from the shift selector active when the selector is in PARK and NEUTRAL... how do I rig something like that?

Doesn't the '73 use a neutral safety switch to prevent starter operation? For some 1973 technology you could use a two relay circuit with a couple of relays to send a signal. Please excuse MS Paint artwork...

[billygraves]

.......

[O-B-1]

A closer look at the tanks revealed vent hoses from top of tanks to the top of the filler necks on both tanks. So I bought a 6-port tank selector solenoid valve that switches the return as well as feed. I then put tees down low in the vent hoses and teed the returns from the tank selector solenoid into them.

@1projecto2many - Thank you for the relay suggestion. That had actually crossed my mind! I saved the drawing to be printed out.

My last obstacle, it seems is the 1988 VSS signal.

I have the TH400 HD transmission. It has the electromagnetic speed sensor in the transmission. I was hoping we could replace the setup with standard drive gear and appropriate driven gear for the rear end ratio and tire diameter within the tail shaft, then get an external sensor to go between the transmission & speedometer cable. No joy! :yikes:

My friend that is a mechanic by profession came out today, pulled the tail housing, we took a look at the setup. The reluctor ring appears to be pressed onto the tail shaft, not pinned! :mad1:

That being said, the 1988 donor had the electronic speedometer, and the VSS signal to the ECM was conditioned WITHIN THE ELECTRONIC SPEEDOMETER CIRCUIT BOARD and then sent to the 1227747 ECM. There is no external DRAC/buffer module. I understand the signal from the transmission sensor is a sine wave and the ECM is looking for 2000 square wave pulses per mile.

How do I go about processing the signal from the speed sensor to be used by the ECM?

[fastacton]

You can still wire in a DRAC. There are schematics around with the info. You just need a ground, a keyed hot, the two wires from the VSS and one going to the ECM. You'll also have a speedo out if you want to run an electronic speedo.

On your gas tank vent lines. If you use them as the return lines, make sure there isn't a restriction or check valve in it. You'll never get the fuel pressure right if there is.

[billygraves]

.....

[1project2many]

I believe that TH400 reluctor is removable. You have to warm it some and often you need to get creative with prybars or other tools to remove it while the trans is still assembled. I'd look for a replacement part number as a guide to whether or not it was expected that it might be removed. The shaft would need to have a hole in it for the plastic gear retaining clip otherwise the gear will spin instead of the speedometer cable.

As billygraves said, the electronic speedo programming is done on the back of the cluster. The same chart used for the DRAC works with the dash type. If you look at this page: http://www.gearhead-efi.com/gearhead-efi/doc/DRAC2.pdf you'll see the chart on page six that shows the jumper settings for each divide ratio for the DRAC. This is actually a chart for the early speedometer with the built in converter. That funny looking "key" at the top would be ordered from GM then "programmed" by removing the specific legs to match the chart. Although some dealers list limited availability for this part, I make my own jumpers by soldering together lengths of paperclip.

[billygraves]

.............

[garnerm]

here is my write up on the installing a drac. http://www.gearhead-efi.com/Fuel-Injection/showthread.php?4713-How-to-Installing-a-DRAC-on-a-7730-ECM
747 is the same one just waterproof. please know that your drac wont be water proof so it needs to be inside the cab even though you ECM doesnt have to be.

[EDZIP]

FYI only..just installed a LT1 drive-line in a 37 Chevy. I drilled and tapped a hole in the sending unit for the fuel return line.

[O-B-1]

What years were the DRACs external to the speedometer? I will need 2,000 pulses per mile for the ECM & firmware version. Can anyone recommend one?

I have the 1227747 ECM from 1988 GMC C1500 and the TH400 heavy duty transmission.

Thank you,
Dave

[1project2many]

The dash will provide that signal if you choose to use it. 90 or 91 was the beginning of DRAC IIRC.

[buddrow]

The dash will provide that signal if you choose to use it. 90 or 91 was the beginning of DRAC IIRC.

My 90 is in the cluster so it's gotta be after that. Which DRAC you need is a loaded question. It is all dependent on your tire size and rear gear ratio. There are links on this site which show you how to build a configurable DRAC. And if not google is useful for that sort of info.

Buddrow

[fastacton]

What years were the DRACs external to the speedometer? I will need 2,000 pulses per mile for the ECM & firmware version. Can anyone recommend one?

I have the 1227747 ECM from 1988 GMC C1500 and the TH400 heavy duty transmission.

Thank you,
Dave

In full size trucks, you didn't see the separate DRAC until '92, but my '90 S-10 had one, so it was a bit earlier for them. You can try to find the appropriate one in the junkyard, mod your own or buy one that's already modded with DIP switches.

[O-B-1]

I bought a programmable DRAC/buffer from Warr Performance of Fresno, CA. I mounted it piggyback on the ECM, both are now under the passenger side of the front seat.
I did not connect the P/N signal, but I have the wire in the harness should I do so in the future.
I also did not wire the brake switch into the battery + lead to the kick down solenoid. I am presuming the switch opens, disabling the kick down when the brakes are applied. I can wire that in later should it be necessary to do so.
We fired it up and the engine runs beautifully! No trace of oil smoke, no steam or sweet smell of coolant being burned. Exhaust does darken just perceptibly, when one "blips" the throttle.
I need to purchase a TBI mounting gasket and the gaskets under the air cleaner. The distributor was 180 out and we re-stabbed it, but missed by one tooth. One we remove the TBI for the gasket change we will re-stab the distributor whilst the TBI is off.
The driveline needed to be shortened as the 1988 heavy duty TH400 seems to be 3" longer than the 1973 TH350 that was in the truck. It is in the shop being shortened as I type this.
I just have to reconnect her wiring harness we cut to get the exhaust pipes out, secure the fuel lines & wiring harnesses with some Adel clamps. Should be a driver before this week is out.
the 305 may be a bit sluggish in a truck this size, especially when towing, but my gal never gets in a hurry when towing anyway. She always takes the back highways rather than the super slabs.