Diagnosing GM Transmissions

In the hot rodding community, the transmission can sometimes become a neglected component in the driveline. It’s common to see a stock or mildly-built transmission sitting behind a mega-horsepower engine without the proper ingredients to handle abuse. Everybody has their own idea of the perfect transmission, and in many cases they may be accurate depending on their application.

I recommend using a high-capacity pan and a transmission cooler for performance-built transmissions. Think of it as cheap insurance. -Scott Miller

As fun as it is to slam gears in a manual car, more often than not it’s an automatic that handles shifting duties in drag cars, and street and strip vehicles. In fact, some of the quickest and most consistent bracket racers rely on a slushbox to achieve their goals. The most common of those, at least with the older GM cars, is the Turbo-Hydramatic (TH) 350, TH400, and 700R4.

All three of these gearboxes have earned their stripes over the years, particularly the TH400 as the unit to rely on in a purpose-built drag car. Even the late-model LS guys have been seen rocking the classic TH400 on occasion.

However, nothing is perfect and everything can be improved upon. To clear the air, we sat down with three of the most respected transmission experts in the business, discussed some of the weak points of all three of these transmissions, and found out what it takes to remedy these issues. Now before you dismiss this as a tech article that doesn’t concern you, it may come as a surprise as to what can be learned.

We’ll be describing the pros and cons of each with Scott Miller of TCI, Larry Speicher of Gearstar Perfomance, and J.C Beattie of ATI Performance adding their input along the way.


The TH350 has been around since the early '60s. While Gearstar's entry Level 2 TH350 (left) will handle 450hp, Gearstar's Level 4 will handle even more and is rated to 700hp.

Since its inception into the 1969 passenger cars, the TH350 transmission is considered to be “Ol’ Reliable,” becoming a mainstay in the production lineup up until 1986. It backed countless General Motors production cars, and is basically considered a three-speed version of the old Powerglide gearbox found in many cars built during the ’50s and ’60s. It continues to be a popular transmission today with drag racers and street rodders, and we feel that there’s always going to be an interest in them, despite the rising popularity of the late-model electronic transmissions.

TCI's Super Streetfighter TH350 (PN-211000) and Streetfighter TH400 (PN-311000) transmissions.

I constantly have customers bring me their cars complaining about a slipping transmission, when in all actuality, it’s suffering from what’s known as a delayed engagement.-Larry Speicher

Scott Miller and Larry Speicher both agreed that if you build them correctly, they can handle over 700hp. The parts to rebuild or repair a TH350 are relatively inexpensive, and since they were offered in everything from family sedans to light-duty pickup trucks during those years, they’re so common that you could still find rebuildable units from your local wrecking yard.

When it came time to find out what the common formalities are with this transmission, both mentioned the “late shift” issue. Occasionally, the TH350 will hesitate on shifts, and this can be caused by a number of things, including; a vacuum line leak, a blown modulator, or the bolts on the hold-down plate not being tightened down securely.

Larry Speicher in particular mentioned how he has seen countless customers complaining about how their gearbox is “slipping” only to find out that it was suffering from what is known as a “delayed engagement.” This issue is simply the result of a low fluid level or a clogged filter/screen. He went on to say that if a TH350 suffers from this “aging complaint” only when cold, it’s most likely that the lip seals have become too brittle. If this is the case, then they are not properly sealing until after a 10-15 minute idle warmup.

Pictured here is the TCI,  36-element intermediate sprag and drum assembly as used in the TH350.

On the other hand, it is possible that a TH350 can suffer from an early shift. The culprit is usually because the original vacuum line running from the intake manifold to the vacuum modulator may have either rusted apart or corroded, and someone replaced it with a generic rubber piece.

Doing this will also cause the clutches to burn out due to a lack of line pressure rise. Obviously, the best thing to do when the metal line needs replacing is to use the correct unit from either the dealer or an auto parts store.

J.C Beattie pointed out that the input shaft is somewhat a weak point in the TH350. It’s fine up to 600 hp, but if your engine is putting out any more than that it will break. The TH400 also suffers from the same problem, but doesn’t break until the 800 hp mark.

Beattie went on to tell us that the sprag for second gear is also less than bulletproof, and will fail above 400 hp in the TH350.

Have you ever heard a knock or a rattle sound coming from under your bellhousing? It could quite possibly be a cracked flexplate. Mark brought it to our attention that the factory flexplate has tendencies of flexing (hence its name), and can be responsible for developing a crack or two overtime.

As a result of the cracked flexplate, eventually this will cause the front seal to leak due to pump bushing damage and converter hub wear. It’s best to get it looked at as soon as you begin to hear that rattle, as it can lead to bigger problems.

Overall, the Turbo 350 is a dependable transmission that is basic, easy to repair, and not to mention, affordable for just about anyone. It’s common problems aside, it’s become a staple in the performance transmission market and will continue to be in the years to come.


A Gearstar-prepped TH400 (left) and an ATI unit (right). Despite the current technology of today's transmissions, the Turbo-400 remains to be a favorite amongst enthusiasts.

The TH400 was essentially a heavy duty version of the TH350. Introduced in Cadillacs for 1964, it slowly made its way into the “big cars” of the General Motors divisions, before eventually ending up in many GM muscle cars and big block-equipped pickup trucks.

If you didn’t want the Muncie 4-speed manual, it was the automatic transmission you got in your Chevelle SS or GTO. It was so good in fact, that Ferrari, Jaguar, Bentley, and even Rolls-Royce used it for a time in their vehicles. The TH400 also sat behind the engines of SUV’s from various makes, including Toyota, Nissan and Jeep.

Gearstar Performance relies on high-end internals for all of their custom, built-to-order transmissions. In this case, Gearstar uses a line of Reybestos Blue Plate Specials (left) for the TH400 forward and direct clutches. Also evident during the Gearstar custom build process are the use of specialized Low/Reverse band (right).

Over the years, the TH400 would see improvements and even a few name changes. It still lives on today in GM’s full-size trucks and SUV’s as the 4L85E, and like the TH350, continues to be a favorite amongst drag racers and street rodders alike the world over, being swapped into both classic and late-model vehicles.

When we spoke to these three gentlemen, they all mentioned that the TH400 was similar to its TH350 sibling in terms of its problems. It must run in the family. An example of this would be the the late shift, which in the case of the TH400, can be caused by the electrical kick-down switch, which may be stuck on disconnect.

This is the 34-element intermediate sprag found in the Turbo-400, and is usually replaced during a rebuild.

Normally at high throttle, the TH400 can suffer from early shifts as well. It’s usually caused by the kick down (KD) switch not functioning. The KD switch is necessary for the passing gear, and also for maximum rpm upshift “hold out”  to operate properly. In terms of the transmission upshifting early, it’s more than likely caused by someone replacing the factory-issue vacuum hard line that runs from the intake manifold to the modulator with a rubber unit.

The problem with installing a rubber line as opposed to a hard line is that the vacuum and heat from the powertrain will cause the rubber line to collapse, trapping the vacuum at the modulator. This will fool the gearbox into recognizing low throttle, and it will run low line pressure. As a result, it will slip and burn clutches.

Other common problems our experts experience in both the TH350 and TH400 is the amount of automatic transmission fluid (ATF) in the vacuum modulator. Our trio of professionals suggested to check the modulator, pull the vacuum line off and check for signs of ATF in the hose or the the nipple of the modulator. There shouldn’t be any.


The 700R4 (left) is pictured here next to its later, 4L60E version (right). It was the gearbox GM called upon to install in its popular muscle car offerings from the '80s up until just a few years ago. TCI offers the 700R4 in Super Streetfighter spec (PN-371000).

The 700R4 was released in 1982 as the first 4-speed automatic transmission with overdrive available for GM vehicles. Initially only available in the Corvette, Blazer/Jimmy, and Chevy Caprice, the 700R4 would eventually end up in the rest of the front-engined, rear drive GM lineup, and would continue to evolve into what is known today as the 4L60E/65E transmissions.

The early examples were considered weak by many, thanks in large part to the aluminum internals fitted inside the gearbox housing. Reason being the technology for fuel-efficient V8’s just wasn’t there at the time of the 700R4’s birth, and the engineers had to do what they could to alleviate the problem.

During the ’90s and the early part of the 21st century, however, it would see various changes; enough to be able to back true high-performance applications like the GMC Syclone/Typhoon, and multiple LS-powered vehicles including the reborn GTO and C6 Corvette.

The representatives from Gearstar, ATI and TCI all agreed that many of the same issues, like those found in the TH350 and TH400 transmissions, can affect the performance of the 700R4. However, the 4-speed automatic uses a throttle-valve (TV) cable as opposed to using a modulator to sense engine load like that found in the TH350 and TH400. It’s also been noted that the kick-down cable and switch for the passing gear downshift is absent in the 700R4 as well.

It's important to mention, Gearstar's technicians follow each custom build from start to finish. Each Gearstar transmission is tailored to each individual application. This means no off-the-shelf parts and hands-on attention to each build. Here, a qualified Gearstar technician installs the 700R4 2-4 band (left), and the input clutch (right).

Mark from Gearstar went on, “The TV cable is the only communication between the engine and transmission, and perfect alignment of the TV cable is critical for proper operation.” If the cable becomes loose or out of adjustment, the car will experience relatively early shifts across the range of throttle position and no kick-down passing gear.

Sidebar title

Things to remember when maintaining a performance transmission:

  • Keep your transmission fluid level full and clean
  • Run a high-capacity pan
  • Install a transmission cooler
  • Replace your filter/screen with an OEM unit
  • Never substitute rubber hoses for metal vacuum lines
When you reinstall or adjust a TV cable, you must always make sure it isn’t too far out of adjustment, especially on the loose side. Otherwise, it will trigger a safety feature inside the transmission that will spike line pressure as to slip the clutch, which could ultimately damage the clutch packs due to the insufficient line pressure rise. This condition normally occurs if the TV cable becomes disconnected from the carburetor or throttle body.

Unfortunately, the TV cable is usually attached to the throttle body or carburetor with a clip-on plastic fastener, and overtime, the underhood heat from the engine will degrade the fastener, causing it to break. However, this doesn’t apply to everyone since they are sometimes fitted with a metal unit. In the event that the TV cable should ever fail, you can still pick up a quality replacement from the dealership or an aftermarket parts source.

If your 700R4 refuses to upshift, then it’s most likely due in part to a failed governor. Made from a combination of plastic and nylon, the gear-driven governor can eventually break off from wear overtime, resulting in a loss of upshifting. A replacement unit can be found at your local transmission shop.

Other issues with the 700R4 include the loss of the second, fourth (overdrive), and reverse gears. While many of us would write this off as a burnt transmission, it’s more than likely just a destroyed 3-4 clutch pack. This can be caused from a misadjusted throttle-pressure cable. It turns out that the 3-4 clutch pack in the factory configuration is the weakest link in the 700R4.

A common issue we see are failed band servos in the 700R4. This is usually stems from a low or dirty transmission fluid level. -J.C. Beattie

Mark, Scott, and J.C all pointed out that inadequate line pressure due to an under-adjusted TV cable will ruin both the third- and fourth-gears quickly. Walk continued, that whether or not that the transmission in question had already received a fresh overhaul or has over 100k miles under its belt, proper adjustment of this cable is absolutely crucial.

The final fault with the 700R4 is a non-functioning overdrive gear. If yours refuses to shift into overdrive, but operates perfectly from 1-3, it’s highly recommended that you get it home or to your local transmission shop quickly, but carefully.

When this occurs, the input spring is more than likely destroyed, and the only component engaging forward is the overrun clutch, which is totally inadequate to hold engine torque for acceleration.

The 700R4's 3-4 clutch pack is the weakest link in the whole transmission. The 10-vane billet pump rotor, and the 5-pinion front & rear planetaries. TCI's Super-Pro kit for the electronic version of the 700R4, the 4L60E (PN-378900) offers performance enthusiasts firmer shifts and improved internals.

So after we had these guys tell us more than we ever knew about this trio of slushboxes, their recommendations were simple; keep an eye on your transmission fluid level, use only AC Delco replacement filters/screens and run a transmission cooler with a high-capacity oil pan in performance applications.

Not only will this information help extend the life of your transmission, but it should help your car perform better as well. We like to think of this as cheap insurance, and since these guys were among the best in the business, we’ll take their word for it.

Article Sources

About the author

Rick Seitz

Being into cars at a very early age, Rick has always preferred GM performance cars, and today's LS series engines just sealed the deal. When he's not busy running errands around town in his CTS-V, you can find him in the garage wrenching on his WS6 Trans Am, or at the local cruise spots in his Grand National.
Read My Articles

Late Model LS Power in your inbox.

Build your own custom newsletter with the content you love from LSX Magazine, directly to your inbox, absolutely FREE!

Free WordPress Themes

We will safeguard your e-mail and only send content you request.

LSX Magazine - The Late Model GM Magazine for Camaro

We'll send you the most interesting LSX Magazine articles, news, car features, and videos every week.

LSX Magazine - The Late Model GM Magazine for Camaro


We will safeguard your e-mail and only send content you request.

LSX Magazine - The Late Model GM Magazine for Camaro

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

Corvette Enthusiasts

Performance Driving

Engine Tech

LSX Magazine - The Late Model GM Magazine for Camaro

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

  • Corvette Enthusiasts
  • Performance Driving
  • Engine Tech

LSX Magazine - The Late Model GM Magazine for Camaro

Thank you for your subscription.

Thank you for your subscription.

LSX Magazine - The Late Model GM Magazine for Camaro

Thank you for your subscription.

Thank you for your subscription.