Automatic transmission
Automatic transmission refers to the automatic transaxle option used in the Pontiac Fiero.
The Fiero used one factory automatic transmission during production: the TH125 automatic, also known as the TH125C or THM125C. It was a three-speed transverse automatic transaxle with a torque converter clutch.
Overview
An automatic transmission changes gears without the driver manually operating a clutch pedal or moving a manual gear selector through individual gears.
In the Pontiac Fiero, the automatic transmission is mounted transversely in the rear engine compartment and is bolted to the engine. It drives the rear wheels through axle shafts.
The Fiero automatic was available with both the 2.5L four-cylinder engine and the 2.8L V6 engine.
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Fiero automatic transmission
The only factory automatic transmission used in the Fiero was the TH125 automatic.
| RPO code | Transmission | Common name | Factory Fiero application |
|---|---|---|---|
| MD9 | 3-speed automatic | TH125C automatic | 1984-1988 automatic-equipped Fieros |
The TH125C was used throughout Fiero production from 1984 through 1988.
Because all factory automatic Fieros used the same general automatic transmission family, automatic transmission identification is usually simpler than manual transmission identification. However, final-drive ratios and transmission codes still varied by year and application.
Operation
The Fiero automatic transmission uses a torque converter instead of a manual clutch.
The torque converter allows the engine to keep running while the car is stopped in gear. Inside the transmission, hydraulic pressure, valves, clutches, bands, and other components control gear engagement.
The driver selects ranges using the automatic shifter, such as Park, Reverse, Neutral, Drive, Second, or Low.
The transmission then shifts automatically based on vehicle speed, throttle position, hydraulic control, and transmission calibration.
Torque converter
The torque converter is located between the engine and automatic transmission.
It transfers engine power to the transmission through fluid coupling. This allows the car to stop without stalling the engine, while still allowing power transfer when the driver accelerates.
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Torque converter clutch
The Fiero TH125C uses a torque converter clutch, often abbreviated as TCC.
The TCC can lock the torque converter under certain driving conditions to reduce slip, lower heat, and improve fuel economy.
TCC-related problems can cause symptoms such as:
- Engine stalls when coming to a stop
- Converter clutch does not lock
- Converter clutch stays locked
- Bucking or harsh behavior while slowing
- Poor fuel economy
- Unusual drivability at steady cruising speed
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Gear ratios
The Fiero automatic transmission has three forward gears and no overdrive.
| Gear | Ratio |
|---|---|
| 1st | 2.84 |
| 2nd | 1.60 |
| 3rd | 1.00 |
Final-drive ratios varied by year and transmission code.
For detailed information, see:
Fluid and cooling
Automatic transmission fluid is used for lubrication, hydraulic control, cooling, and torque converter operation.
The Fiero automatic transmission uses cooler lines connected to the radiator or transmission cooler circuit.
Fluid level and condition are important. Low, dirty, overheated, or incorrect fluid can cause shifting problems, slipping, overheating, or transmission damage.
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Automatic shifter and shift cable
The Fiero automatic shifter is connected to the transmission by a shift cable.
The shift cable selects transmission ranges. If the cable is worn, damaged, misadjusted, or binding, the shifter position may not match the actual transmission range.
Possible shift-cable-related symptoms include:
- Difficulty selecting Park, Reverse, Neutral, or Drive
- Car does not start in Park or Neutral
- Shifter feels loose or vague
- Selected range does not match the indicator
- Transmission does not engage the expected range
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Common problems
Common Fiero automatic transmission topics include:
- Fluid leaks
- Incorrect fluid level
- Burnt or contaminated fluid
- Delayed engagement
- Harsh shifts
- Slipping
- No upshift
- No reverse
- Torque converter clutch problems
- Cooler line leaks
- Shift cable adjustment problems
- Worn mounts
- Axle seal leaks
- Previous incorrect service
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Symptoms
Possible automatic transmission symptoms include:
- Slipping
- Harsh shifting
- Delayed engagement
- No movement in one or more ranges
- No reverse
- Burning fluid smell
- Transmission fluid leak
- Engine stalls when stopping
- High engine rpm at highway speed
- Vibration or clunking
- Unusual noise
- Poor acceleration
- Poor fuel economy
Not all symptoms that feel transmission-related are caused by the transmission itself. Engine problems, ignition problems, fuel delivery issues, vacuum leaks, mounts, cables, and wiring can create symptoms that may feel like transmission trouble.
Diagnosis
Basic automatic transmission diagnostic checks include:
- Check the transmission fluid level using the correct procedure.
- Inspect the fluid color and smell.
- Check for external leaks.
- Inspect cooler lines and fittings.
- Check shift cable adjustment.
- Check engine and transmission mounts.
- Check axle seals.
- Check for torque converter clutch symptoms.
- Verify that the problem is not engine-related.
- Identify the installed transmission and transmission code if exact gearing or parts information matters.
A shifting or drivability problem should not automatically be blamed on the transmission.
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Relationship to manual transmissions
The Fiero was also available with several manual transmissions.
Factory manual transmission topics include:
Automatic-equipped cars do not have a clutch pedal or manual shifter, and use different related parts such as the automatic shifter, shift cable, torque converter, cooler lines, and automatic-specific transmission mounts or brackets where applicable.
Relationship to engine swaps
The Fiero automatic transmission may be retained in some engine-swap cars, but many swaps use different transmissions depending on power, gearing, electronics, strength, and packaging.
Because the factory automatic has only three forward gears and no overdrive, it is not always preferred for high-output or highway-oriented swaps.
Engine-swap transmission choice can depend on:
- Engine torque
- Intended use
- Desired cruising rpm
- Strength
- Electronics
- Mounts and brackets
- Axles
- Shift cable arrangement
- Parts availability
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Service notes
When working on a Fiero automatic transmission:
- Verify the installed transmission before ordering parts.
- Check fluid level correctly.
- Use the correct fluid type and service procedure.
- Avoid overfilling.
- Inspect for leaks.
- Check cooler lines.
- Verify shift cable adjustment.
- Check transmission mounts.
- Verify torque converter clutch operation if stalling or lockup symptoms occur.
- Do not assume every drivability problem is transmission-related.
Identification
Factory automatic Fieros used the TH125C automatic transmission, associated with RPO code MD9.
Useful identification clues include:
- Automatic shifter
- No clutch pedal
- RPO code MD9
- Automatic transmission dipstick
- Cooler lines
- Three forward gears
- Transverse automatic transaxle mounted to the engine
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Notes
The Fiero automatic transmission is simple compared with later electronically controlled overdrive automatics, but condition, fluid level, cooling, shift linkage, torque converter clutch operation, and previous service history all matter.
Because the transmission is now decades old, diagnosis should consider age-related wear, previous repairs, and possible transmission replacement.