Common 350 Turbo Transmission Problems: What To Note?

The 350 Turbo transmission system has been around as early as the 1900s – and, despite the market’s intense competition – still remains one of the most critically-acclaimed automatic transmissions to this day.

However, like any other same-sector transmission, there are several 350 Turbo transmission problems you should be aware of.

My guide will dig into some of the most common issues and extra tips to ensure smooth performance; keep scrolling.

How to Identify A Turbo 350 Transmission?

350 turbo transmission problems

What Is It?

The 350 Turbo Hydramatic (or TH350) used to receive rave feedback among numerous automobile enthusiasts as the best 3-speed auto transmission ever created.

Its popularity experienced impressive peaks in RWD vehicles (trucks and cars alike), V8, and V6 engines.

Unfortunately, the sweeping entrance of 700R4 transmissions gradually phased out 350 Turbo’s productions a few years later.

But you can still find 350 Turbo in many classic jeeps and vehicle models nowadays due to its great power output.

Turbo 350 set itself apart from other siblings (say, Turbo 400) with its transparent absence of central support. That might sound like a downside at first.

But in fact, such designs allow the transmission to fit into Corvair models perfectly and deliver two identical power ends. Transmissions with central supports clearly cannot achieve the same feat.

What Are The Identification Features?

The 350 Turbo, measuring about 21 3/4 inches in length, arrives with integrated bell housings and 100% aluminum alloy casts. It also feels fairly light – no more than 120 pounds.

There is a custom-made oil pan to fit the transmission, aided by a rear passenger chamfer. Look at the case’s back to spot its modulator.

Regarding 350-C Turbo (Turbo 350’s offshoot version, produced in limited numbers in 1970-1974), most drivers will be delighted by the inclusion of locking torque converters.

They employ electronic converting programs to increase the fuel efficiency of both your car and your transmission when traveling at high speeds.

Furthermore, a plug installed on the 350-C’s left side clearly distinguishes it from the 350 version. But if you still want to double-check the part codes to make sure, then:

  • Turbo 350: M33, M39, M38
  • Turbo 350C: MX2, MX3, MX5, MV4

A few other Turbo 350 derivatives exist, but these two are the most popular versions so far. Equipping them with extra aftermarket conversions can increase their performance by 10%.

What Are The Common 350 Turbo Transmission Problems? 

th350 troubleshooting chart

350 Turbo transmissions have regular glitches in their shifts (often ignoring the 1st or 3rd gear), gear engagement (both hot and cold), engine brakings, and transmission noises.

This section will break down each case in further detail: 

Vehicle Slipping in Park

Customers report the Turbo 350 transmission struggles to keep their cars stationary whenever the gear selectors are pulled to “Park.” 

The vehicle will move or even roll forward slightly while parking, causing tons of inconvenience during emergencies or urgent situations.

Some possible reasons behind the slipping:

  • The parking pawl guides are incorrectly installed
  • The parking pawl guides are severely damaged (worn smooth, cracked, broken, etc.)
  • The teeth on the output ring shaft gears are broken
  • Something has gone wrong with the manual linkage

No Drives At Any Range

Brace yourself when it happens: the transmission cannot engage or provide corresponding drive across the entire gear range.

Whether the selectors are switched to “Drive” or “Reverse,” your car refuses to bulge or give off even the slightest response.

Below are some commonly-cited culprits:

  • The manual control linkages cannot be adjusted
  • The manual S-link valves are disconnected from the levers
  • The forward clutch is damaged/ does not apply
  • Oil levels are low/ Oil filter gets blocked
  • Fluctuating oil pressure/ Stuck pressure regulator valves/ Valve body falling out or damaged.
  • Damaged drive gears/torque converter hubs/ input shaft
  • Damaged or broken roller clutch assemblies

No 1-2 Shifts

On certain occasions, drivers of GM th350 transmission cars find it extremely challenging to shift to 2nd gear from 1st gear when accelerating.

The transmission staying on 1st gears longer than intended causes your engine to get stuck on higher rev throughout the rest of the trip.

The issue can be blamed for:

  • Stuck governor valves/ Plugged governor filters
  • Damaged governor bores/gears
  • Detent cable bindings
  • Incorrectly-installed sprag-type clutches
  • Low coolant levels/ Stuck 1-2 shift assembly
  • The intermediate clutch does not hold or is incorrectly assembled
  • The intermediate roller clutches are broken or not holding.

1-2 Shifts Only At Full Throttles

As the name suggests, the car experiences extreme difficulties shifting from 1st to 2nd gear when the throttles are aggressively accelerating or fully engaged.

And yet, the shift still progresses naturally without issues in other conditions (where the throttle input is moderate).

Why the difference? Experts put down the problem to: 

  • Stuck/incorrectly-installed detent valve assemblies
  • Stuck/Non-adjustable detent cables
  • Sticking/stuck modulators or modulator valves
  • No back-to-back vacuum modulator

No 2-3 Shifts. Only 1st-2nd Shifts 

Drivers can shift from 1st to 2nd gear as breezily as a piece of cake. Yet, to their frustration, the car cannot continue to shift from 2nd to 3rd gear.

The GM 350 transmission gets stuck forever at 2nd, regardless of your throttle input or vehicle speed!

That can only mean at least one of the following:

  • Stuck control valves for 2-3 shifts
  • Incorrectly installed, damaged, or leaking valve body gasket
  • Missing/damaged sealing ring on the pump hub
  • Missing/damaged direct clutch seal

All-Range Slips

With Turbo 350 trans slips, power loss is only to be expected; even seasoned drivers will struggle to move their cars in the desired manner.

The possible causes of these slips are: 

  • Low oil levels
  • HIgh oil foamed/ oil level
  • Sticking or defective vacuum modulator valves
  • Plugged filter assembly
  • Wrong or misaligned valve body gasket
  • Stuck pressure regulator valves
  • Damaged pumps to case gaskets
  • Slipping forward clutches

1-2 Shift Slips

Although the car can still shift from 1st to 2nd gear, it does so with lots of difficulties; the 350 Turbo trans slips frequently and struggles to perform smooth engagement, resulting in very noticeable acceleration delay or power loss.

The following issues might explain why the slips happen in the first place:

  • Incorrect/low oil levels
  • Defective modulators/ Sticking modulator valves
  • Defective pressure regulator valves
  • Incorrectly installed/ off-position valve body gasket
  • Missing/reversed/damaged intermediate clutch
  • Damaged clutch plates
  • Missing/ damaged accumulator seals
  • Damaged roller clutches/sprag-type clutches installed backward

2-3 Shift Slips

Like with 1-2 shift slips, drivers also experience rough shifts when transitioning from 2nd to 3rd gear. Most professionals put down the issue to:

  • The same reasons behind 1-2 shift slips (refer to the above section)
  • Burnt direct clutch plates
  • Missing or damaged pump hub sealing rings
  • Missing or damaged seals for the clutch apply pistons
  • Lost or damaged accumulator seals for 2-3 shifts

Rough 1-2 Shifts

Reports also note 350 Turbo’s tendency to exhibit jolting, rough, or abrupt transition shifts from 1st to 2nd gear. The jerk/harshness can sometimes get downright uncomfortable.

When this happens, make sure you check out:

  • Possibly damaged or broken accumulator pistons for 1-2 shifts
  • Blocked accumulator feed holes for the 1-2 shifts inside the body plates
  • Blockage inside the 1-2 accumulator passages

No 1-2 Shifts, Only 3-1 and 1-3

The title says it all: your car has to wrestle with the shift from 1st to 2nd gear. However, it can engage 1st to 3rd gear or vice versa just fine. 

Needless to say, the GM Turbo 350 transmission literally ignores your 2nd gear during the entire shifting process.

Why does that happen? Either one of these two scenarios must have happened:

  • The intermediate roller clutches are defective does not hold
  • The aftermarket clutch is installed backward

No Engine Brakings in Second

In cases like these, cars cannot brake at all at the vehicle’s second gear, even after drivers have already released their accelerator pedals.

As a result, they face immense trouble slowing down the car promptly at demands.

Here are the most likely reasons: 

  • Missing or damaged servo-sealing intermediate rings
  • Stuck intermediate servo in bores or on pins in case
  • Broken accumulator rings/ stuck accumulator piston/ damaged accumulator bores
  • Broken, burned, or damaged intermediate overrun bands
  • Stuck boost valves/ low oil pressures/ stuck pressure regulator valves

No Engine Brakings in Low

At times, the Turbo 350th transmission keeps the engine from providing efficient braking at low gear ranges (ex: 1 or L). The problem might have something to do with:

  • Stuck manual control low valve assembly
  • Low oil pressures
  • Stuck boost valve or pressure regulator
  • Missing, broken, or damaged reverse/low piston seals

No Downshifts At Part Throttles

Does your car’s transmission refuse to downshift when you apply the throttle partially? “Yes” to this question means your acceleration and speed control will be quite a challenge, and it’s time you need to inspect:

  • Low oil pressures
  • Sticking modulator valve/ defective vacuum modulators
  • Malfunctioning pressure regulator valve trains
  • Sticking or stuck detent valves/ sticking or damaged detent cables
  • Stuck 2-3 shift valves

No Detent Downshift

Detent downshifts are expected to take place when drivers perform quick accelerator releases or apply forces on the selectors, shifting the transmission to lower gears.

Seeing no signals of them on your vehicle? Then chances are:

  • Your 2-3 valves are stuck
  • Your cables, linkages, or detent valves are broken/ damaged/ disconnected/ sticking.

High- or Low-Shift Points

Excessive upshifts or downshifts result from at least one of the following problems (or, in the worst-case scenario, all of them):

  • Incorrect oil level (too high or low)
  • Defective vacuum modulators
  • Sticking or stuck modulator valves
  • Sticking or stocking pressure regulator valves
  • Blocked governor filters
  • Restricted feed holes
  • Sticking 2-3 or 1-2 valvetrain
  • Stuck detent valves

Slow Engagements When Cold

Does your car take much longer to shift or engage gears after extended idle periods? This usually occurs in winter months or when: 

  • The oil pressure is low
  • The piston seals are old/ not pliable
  • Common problems in old units

Noises

Noisy transmissions are among the most annoying problems in the world, likely caused by:

  • Low or high oil level
  • Pump noises
  • Water in the oil
  • Damaged housing and pump gears
  • Damaged or worn planetary gears
  • Worn or burnt clutch plates
  • Burnt band lining

How Much Does It Cost to Fix Turbo350 Problems?

symptoms of a bad transmission governor th350

Transmission issues are very expensive, so do not be surprised if their prices can fluctuate between $300 and $1500.

And complete replacements charge even higher, reaching $3400 in extreme cases!

That’s all the more reason to pay extra attention to your transmission system.

Conclusion

350 Turbo’s discontinuation does not stop devoted car owners from employing it in their classic cars.Still, the lack of upgrades means there are tons of 350 Turbo transmission problems to look out for.

Keep an eye on them, and bring your car to a mechanic shop immediately should anything feel out of place.

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