Cliff: Created page with "'''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 individ..."

2026-07-12T15:05:04Z

Created page with "'''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 individ..."

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'''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.

Related topics:

* [[TH125 automatic]]
* [[Transmission]]
* [[Transmission identification]]
* [[Transmission gear ratios]]

== Fiero automatic transmission ==

The only factory automatic transmission used in the Fiero was the [[TH125 automatic]].

{| class="wikitable"
! RPO code
! Transmission
! Common name
! Factory Fiero application
|-
| MD9
| 3-speed automatic
| [[TH125 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.

Related topics:

* [[Torque converter]]
* [[Torque converter clutch]]
* [[TH125 automatic]]

== 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

Related topic:

* [[Torque converter clutch]]

== Gear ratios ==

The Fiero automatic transmission has three forward gears and no overdrive.

{| class="wikitable"
! Gear
! Ratio
|-
| 1st
| 2.84
|-
| 2nd
| 1.60
|-
| 3rd
| 1.00
|}

Final-drive ratios varied by year and transmission code.

For detailed information, see:

* [[TH125 automatic]]
* [[Transmission gear ratios]]

== 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.

Related topics:

* [[Transmission fluid]]
* [[Transmission cooler lines]]
* [[Radiator]]
* [[TH125 automatic]]

== 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

Related topics:

* [[Shift cable]]
* [[Interior]]
* [[Transmission identification]]

== 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

Related topic:

* [[Automatic transmission problems]]

== 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.

Related topics:

* [[TH125 automatic]]
* [[Torque converter clutch]]
* [[Transmission fluid]]
* [[Transmission identification]]

== Relationship to manual transmissions ==

The Fiero was also available with several manual transmissions.

Factory manual transmission topics include:

* [[Muncie 4-speed]]
* [[Isuzu 5-speed]]
* [[Getrag 5-speed]]
* [[Manual transmission]]

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

Related topics:

* [[Engine swaps]]
* [[TH125 automatic]]
* [[Transmission]]
* [[Parts interchange]]

== 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

Related topic:

* [[Transmission identification]]

== 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.

== See also ==

* [[Transmission]]
* [[TH125 automatic]]
* [[Torque converter]]
* [[Torque converter clutch]]
* [[Transmission fluid]]
* [[Transmission cooler lines]]
* [[Automatic transmission problems]]
* [[Transmission identification]]
* [[Transmission gear ratios]]
* [[Manual transmission]]
* [[Muncie 4-speed]]
* [[Isuzu 5-speed]]
* [[Getrag 5-speed]]
* [[Engine swaps]]

[[Category:Fieropedia]]
[[Category:Transmission]]
[[Category:Diagnostics]]
[[Category:Service procedures]]

Automatic transmission - Revision history