1988 GMC C1500 engine/trans into 1973 C20 Crew Cab

billygraves · 08-30-2015

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**1project2many** · 08-30-2015

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 · 08-30-2015

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**garnerm** · 08-30-2015

here is my write up on the installing a drac. http://www.gearhead-efi.com/Fuel-Inj...-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** · 08-31-2015

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** · 09-01-2015

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** · 09-01-2015

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

**buddrow** · 09-02-2015

Originally Posted by 1project2many

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** · 09-02-2015

Originally Posted by 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

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** · 10-14-2015

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.