Author Topic: Mismatched speedo gear/rear axle ratios and 700R4 TV cable adjust  (Read 1168 times)

Offline Snag

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  When I had the trans built I had the guy install the speedo drive gears for the new rear axle ratio , but I haven't changed the gears yet (but soon ...) . I'm driving the truck some now , and it never shifts into OD . Before the rebuild , it usually shifted up between 45-50 MPH , but now I don't think it's up shifting to OD at all - and gas usage reinforces that opinion . I know that indicated speed is the factor , and that I'm going indicated speed plus 25% (2.73 , going to 3.42 and there aren't many places here where it's safe to go over 65 (winding mountain roads) but I haven't felt that last upshift . I'm wondering - since the trans shifts at a slightly higher RPM than it dd before all the rebuilding (motor too , and it ain't stock any more) - if I can loosen the TV cable one or two clicks and maybe lower the shift points a bit . As it is I'm getting dismal mileage - around probably 9-10 MPG - and that's just too lousy . I'm leery of doing this because the trans builder was so vocal about too loose meant pump pressure too low and burning up a trans I just paid a bunch of money to have built .

Offline Mr Diesel

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Re: Mismatched speedo gear/rear axle ratios and 700R4 TV cable adjust
« Reply #1 on: May 10, 2023, 08:09:31 AM »
Do NOT loosen the TV cable to change shift points. That cable and corresponding system was never designed as a way to control shift points. It influences pump pressure which is critical to correct pressure needed inside to shift without damaging interior components. Too much pressure and it damages things. Too little pressure and stuff gets damaged. It has a small window of optimal adjustment (usually by following the GM procedure), anything outside of those parameters WILL damage the transmission.

There are many reasons it will not shift to overdrive. If it started doing this after the rebuild then something was put back together wrong. I actually had this happen on a 700R4 I put back together. I used a small amount of RTV on the pan gasket like I usually do, but the RTV squeezed over a small drain port to the pan which blocked the drain and would not allow it to upshift into OD. Simple fix for me, but there are many other internal reasons for no OD so no promises you have the same issue.
1976 C20 Crewcab, 6.2L/SM465
1982 K30 Crewcab , 427TD/TH400
1983 C30, 6.2L/TH400
1983 K30 Crewcab 454/700R4
1986 K10 350/400. 1989 K30 cab/chassis 454/SM465

Offline JohnnyPopper

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Re: Mismatched speedo gear/rear axle ratios and 700R4 TV cable adjust
« Reply #2 on: May 10, 2023, 12:17:57 PM »
That being said, sounds like a PITA transmission. I was thinking about upgrading my TH350 to one, but better look deeper.

4L65e may be the better route.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline VileZambonie

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Re: Mismatched speedo gear/rear axle ratios and 700R4 TV cable adjust
« Reply #3 on: May 10, 2023, 05:14:21 PM »
Check the cable at WOT with the engine off and make sure it looks correctly set. Ask if he modified the governor or if it is stock.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline MIKE S

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Re: Mismatched speedo gear/rear axle ratios and 700R4 TV cable adjust
« Reply #4 on: May 10, 2023, 09:33:18 PM »
I see in previous posts that you have replaced the oem quadrajet with an edelbrock performer. Make sure that the TV cable linkage is correct. It has to pull the cable in a manner that is identical to the original carburetor. If it doesn’t you will never get the shift points right and possibly damage the transmission.