1988 GMC C1500 engine/trans into 1973 C20 Crew Cab

[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]

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[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]

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[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!

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!

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]

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[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

[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.