GM 700-R4 Overdrive Automatic Transmission - Working Under Pressure

The GM 700-R4 is one of the most popular overdrive automatic transmissions used in street rods. And why not, you might ask-they have a 3.06:1 First gear for launching from a standstill, a .70:1 overdrive Fourth for low-rpm highway cruising, and they can be built to withstand a tremendous amount of horsepower. But then if you've had one burn-up or fought with adjusting shift points until you were blue in the face, you just might have plenty to say about the 700-R4-and most likely not much of it would be printable.

When first introduced, the 700-R4 got off to a rather rocky start. GM had not done its homework, and much like the company's first foray into diesel engines in passenger cars, research and development was left up to the consumer. Lots of those early transmissions quit before the payments on the cars they came in were completed and warranty repairs quickly established the transmission's weak points. But, eventually-after a list of updates that were about the size of an L.A phone book when printed out-Generous Motors corrected the 700-R4.

While the 700-R4 ultimately became a pretty bulletproof transmission in production automobiles, the same couldn't always be said when they found their way into a street rod. And while it seems ludicrous to say, it wasn't always the fault of the transmission, but rather the induction system that was in use. Now before you demand that STREET RODDER staffers be subjected to random drug testing, let us explain.

All automatic transmissions use hydraulic pressure to apply friction components; either bands or clutches shift into each gear. The 700-R4 (as well as the 200-4R and the 4L60) uses a cable (called the throttle valve, or TV cable) connected to the throttle linkage to establish that pressure. On these factory induction systems, the linkage arm where the TV cable attaches has very specific geometry that moves the cable a precise distance in relation to the throttle opening to ensure the transmission's hydraulic pressure is appropriate. That's why 700-R4s behind engines with GM TPI don't seem to have nearly as many problems-the TV cable geometry is correct. On the other hand, most carburetors don't have the proper lever geometry; they may work just fine with a Turbo 350, but they won't work with a 700-R4. This condition has become so well recognized that many new aftermarket carburetors now come with a warning that they are not to be used with an overdrive automatic equipped with a TV cable (with computer-controlled transmissions, it doesn't matter).

A 700-4R's shifting can be erratic as a result of improper TV cable lever geometry. In many cases, the shifts are poorly timed and may be early and soft, late and harsh, or a combination of both. Downshifts often fall into their own category of weird, and no amount of fooling with the TV cable adjustment seems to make things right. But then why would it-the cable isn't at fault.

While odd shifting characteristics are annoying, transmission failure is even more so, and it's expensive to repair to boot. And although there are other scenarios that cause damage to a 700-R4, what happens in many cases is basically this: The engine begins producing increased torque before the transmission is ready for it. What's supposed to happen as the throttle opens and the engine produces more power is that the hydraulic pressure in the transmission increases to handle it. However, slippage can occur if that pressure increase lags behind the power increase because the relationship between the throttle and TV cable movement isn't properly coordinated. Sometimes the slippage is noticeable, but often it's not. In any case, it's the kiss of death for an automatic transmission and allowing it to continue ultimately results in total transmission failure.