TH125 automatic
TH125 automatic refers to the three-speed automatic transmission used in the Pontiac Fiero.
The Fiero automatic transmission is commonly referred to as the TH125, TH125C, or 3T40, depending on naming convention and context. It was the factory automatic transmission available throughout Fiero production.
Overview
The TH125 automatic is a transverse three-speed automatic transmission used with both the 2.5L four-cylinder and 2.8L V6 Fiero engines.
It was designed for front-wheel-drive and transverse-engine GM applications. In the Fiero, it was adapted to the car's mid-engine transverse drivetrain layout.
Compared with the manual transmissions available in the Fiero, the TH125 automatic provides easier driving and simpler operation, but it does not have an overdrive gear.
Location
Image wanted: Photo or diagram showing the TH125 automatic transmission installed in a Pontiac Fiero engine bay, preferably showing its relationship to the engine, cradle, axles, shift cable, dipstick, and cooler lines.
The TH125 automatic is mounted transversely in the rear engine compartment, bolted to the engine.
It is mounted on the engine cradle and drives the rear wheels through the Fiero axle shafts.
Naming
The Fiero automatic transmission may be referred to by several names:
- TH125
- TH125C
- Turbo-Hydramatic 125
- Turbo-Hydramatic 125C
- 3T40
In Fiero owner discussions, “TH125” is commonly used as a shorthand name.
The “C” designation generally refers to the torque converter clutch version.
Specifications
| Specification | TH125 automatic |
|---|---|
| Type | Transverse automatic transmission |
| Speeds | 3 forward speeds |
| Overdrive | No |
| Torque converter clutch | Yes, on TH125C applications |
| Fiero model years | 1984-1988 |
| Fiero applications | Automatic-equipped 2.5L four-cylinder and 2.8L V6 models |
| Common later GM name | 3T40 |
Transmission details, gear ratios, final drive ratios, and calibration may vary by year, engine, and application. For exact service or restoration work, verify the transmission code and model-year information.
Fiero applications
The TH125 automatic was available in automatic-equipped Fieros from 1984 through 1988.
It was used behind:
Related model-year pages:
Operation
The TH125 is a hydraulically controlled automatic transmission.
It uses a torque converter, internal hydraulic circuits, clutches, bands, valves, and governor/modulator control to shift between gears.
Because it has only three forward gears and no overdrive, engine rpm at highway speed is generally higher than it would be with a later overdrive automatic.
Torque converter clutch
TH125C applications use a torque converter clutch, often abbreviated as TCC.
The TCC can lock the torque converter under certain driving conditions to reduce slip, improve efficiency, and lower heat.
TCC-related problems can cause symptoms such as:
- Engine stalls when coming to a stop
- Converter clutch does not lock
- Converter clutch stays locked
- Harsh engagement
- Poor drivability
- Increased transmission temperature
Related topic:
Torque converter clutch control
The TH125C torque converter clutch system is electrically controlled.
The lockup circuit involves several components in the control path. A problem in any part of the circuit can prevent torque converter lockup or can cause the converter clutch to remain applied when it should release.
Common items to check include:
- Brake switch
- Torque converter clutch solenoid
- Transmission pressure switches
- Related wiring and connectors
- ECM control circuit, depending on year and application
TCC problems can cause symptoms such as:
- Engine stalls when coming to a stop
- Converter clutch does not lock
- Converter clutch stays locked
- Poor fuel economy
- Higher cruising rpm
- Harsh or unusual engagement
- Intermittent drivability problems
Because the TCC system uses both internal transmission parts and external electrical control, diagnosis should include wiring, switches, connectors, and the solenoid before assuming the transmission itself is bad.
Fluid and cooling
Automatic transmission fluid is essential for lubrication, hydraulic control, cooling, and torque converter operation.
The TH125 uses cooler lines connected to the radiator or transmission cooler circuit.
Fluid condition and level are important. Low, dirty, overheated, or incorrect fluid can cause shifting problems or transmission damage.
Related topics:
Common problems
Common TH125 automatic topics include:
- Fluid leaks
- Shift cable adjustment
- Delayed engagement
- Harsh shifts
- Slipping
- No upshift
- No reverse
- Torque converter clutch problems
- Cooler line leaks
- Governor or valve-body issues
- Worn mounts
- Axle seal leaks
- Previous incorrect fluid service
Symptoms
Possible symptoms of TH125 automatic problems include:
- Slipping
- Harsh shifting
- Delayed shift engagement
- No movement in one or more ranges
- No reverse
- Transmission fluid leak
- Burning fluid smell
- Engine stalls when stopping
- High engine rpm at highway speed
- Vibration or clunking
- Unusual noise
- Poor acceleration
Diagnosis
Basic 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 TCC-related symptoms.
- Check vacuum or control-related items if applicable.
- Verify that the problem is not engine-related before condemning the transmission.
Shift cable and selector
The Fiero automatic uses a shift cable to select transmission ranges.
A misadjusted, worn, or damaged shift cable can cause incorrect range selection, difficulty shifting, or a mismatch between the shifter position and actual transmission range.
Related topics:
Mounts and cradle relationship
The engine and transmission are mounted on the Fiero cradle.
Worn mounts can cause clunks, movement, vibration, or stress on related parts such as cables, axles, exhaust, and wiring.
Related topics:
Relationship to engine swaps
The TH125 automatic can be retained with some engine combinations, but many engine swaps use different transmissions depending on power, gearing, strength, electronics, and packaging.
Because the TH125 has no overdrive and was not designed for high-output swaps, owners planning engine swaps often consider other automatic or manual transmission options.
Related topics:
Strength and limitations
The TH125 automatic is generally adequate for stock Fiero use when in good condition.
Limitations include:
- Only three forward gears
- No overdrive
- Higher highway rpm than later overdrive automatics
- Limited appeal for high-power engine swaps
- Age-related wear in surviving units
- Heat sensitivity if cooling or fluid condition is poor
Service notes
When servicing the TH125 automatic:
- Use the correct fluid type and service procedure.
- Check fluid level correctly.
- Avoid overfilling.
- Inspect for leaks.
- Keep cooler lines in good condition.
- Check shift cable adjustment after related work.
- Verify TCC operation if stalling or lockup symptoms occur.
- Do not assume every drivability problem is transmission-related.
Notes
A shifting or drivability problem should not automatically be blamed on the transmission.
Engine performance problems, vacuum leaks, fuel delivery problems, ignition problems, mounts, cables, axles, and wiring can all create symptoms that may feel transmission-related.