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Old 08-27-2015, 07:09 PM   #14
Katrina/10
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Join Date: Oct 2010
Location: Ocean Springs, MS
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Re: 1988 Canadian GMC C1500 305 TBI w/TH400HD going into 1973 C20 Custom Crew Cab

Quote:
Originally Posted by dwmitchell61 View Post
Thanks, Gary... That's good info to know.

I have an article from elsewhere in the Internet that says the VSS signal is optional also. But elsewhere I found that not using it may cause rough deceleration and stumbling at stoplights. I have the wire in the harness, sio I can connect it later if there are problems.

If there are issues then I will have to do the following:
1. Open the tail shaft on the transmission
2. Remove the electronic speed sensor reluctor and pickup.
3. Install the right speedometer gears
4. Then put an external speed sensor between the speedometer cable and the transmission

That being said, I wonder if the speedo gears from the old TH350 will work in the TH400?

Dave
I put an 87 engine and an 89 700 trans in my truck. I did just what you said, pulled the tail housing off and knocked off the reluctor wheel, and installed a set of standard mechanical speedo gears. Then bought an external VSS from Jags that run, and everything was happy. I did some testing with no VSS, and I would say you would want to use one. Without it, the ECM thinks the vehicle is never moving, causing rapid deceleration when you lift off the gas, and a rough transition when you reapply throttle. It will also go dead if you come to anything more than a slow stop. And a code 24 you will not be able to get rid of.
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Gary

1971 Chevrolet C/10
1951 GMC 100
1977 GMC C15
1955 Chevrolet 3100
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