Wiring diagram for 1973 C20 Crew Cab? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**hatzie** · 07-28-2015, 01:55 PM

Quote:

Originally Posted by dwmitchell61

My gal has a 1973 C20 Crew Cab. It originally had the 454ci in it. Someone had swapped that for a 350ci small block engine which is getting tired and the transmission is a goner.

She bought a 1988 Canadian GMC C1500 with a 305ci small block and TH400 heavy duty transmission (maybe a 454 can go back in there in the future).

I am in the process of cleaning out the '88 TBI/engine compartment wiring harness and getting it ready to go into the '73. I am needing to tie into the Park/Neutral switch for the ECM & brake switch for the downshift relay. I also need to tie into the ignition switch "run" lead.

Can anyone supply me with a link to a wiring diagram for the 1973 Chevrolet C20?

Thanks in advance for any help offered...
Dave

Look in my signature...

dwmitchell61 · 07-29-2015, 12:31 AM

Cool!

Mooch-o grassy ass, Sir! Just what the doctor ordered.

I am going to see if I can locate a second junction block like the 1988 had on the firewall for the BATT + distribution.

I will power the second from a relay energized by the '73 IGN RUN lead.

I have to use the Park/Neutral switch to run a DPDT relay to safety the IGN START lead and send a signal to the ECM for P/N status.

I also have to run a relay from the brake light circuit to kill the power to the downshift relay when the brake pedal is depressed.

The 73 has dual tanks, so I need to replace the tank selector solenoid with a 6 port one so the return line has a route back to the proper tank. Have to install a tap in the filler necks to connect the returns to.

The TH400 transmission we are installing is a heavy duty model, has the HD on the bell housing. I am hoping the yoke on her drive-line has the proper number splines.

I will have to remove the reluctor in the transmission for the speed sensor and install the appropriate speedometer gears & sleeve for the tire diameter so the speedometer will work, then get a VSS sensor to go between the gears & speedometer cable to send the VSS signal to the ECM.

I don't think she realized just how much work the project would be!

07-29-2015, 12:31 AM	#2
dwmitchell61 Registered User Join Date: Jul 2015 Location: Benton City, WA Posts: 37	Re: Wiring diagram for 1973 C20 Crew Cab? Cool! Mooch-o grassy ass, Sir! Just what the doctor ordered. I am going to see if I can locate a second junction block like the 1988 had on the firewall for the BATT + distribution. I will power the second from a relay energized by the '73 IGN RUN lead. I have to use the Park/Neutral switch to run a DPDT relay to safety the IGN START lead and send a signal to the ECM for P/N status. I also have to run a relay from the brake light circuit to kill the power to the downshift relay when the brake pedal is depressed. The 73 has dual tanks, so I need to replace the tank selector solenoid with a 6 port one so the return line has a route back to the proper tank. Have to install a tap in the filler necks to connect the returns to. The TH400 transmission we are installing is a heavy duty model, has the HD on the bell housing. I am hoping the yoke on her drive-line has the proper number splines. I will have to remove the reluctor in the transmission for the speed sensor and install the appropriate speedometer gears & sleeve for the tire diameter so the speedometer will work, then get a VSS sensor to go between the gears & speedometer cable to send the VSS signal to the ECM. I don't think she realized just how much work the project would be! Last edited by dwmitchell61; 07-29-2015 at 12:33 AM. Reason: Added final comment.



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