P1870 – Transmission Component Slipping (Acura, Buick, Cadillac, Chevrolet, General Motors, Isuzu)

By Reinier (Contact Me)
Last Updated 2016-11-15
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1870

Table of Contents

1. What Does Code P1870 Mean?
2. What are the common causes of code P1870?
3. What are the symptoms of code P1870?
4. How do you troubleshoot code P1870?
5. Codes Related to P1870
6. Get Help with P1870

What Does Code P1870 Mean?

SPECIAL NOTE:  Some manufacturers use the definition for this code as stated below, while others, most notably General Motors, define this code as “Transmission Component Slipping”, and apply other codes (and definitions) for switches, shift motors/actuators, and control circuits on their 4WD and AWD systems.

This understandably leads to some confusion about the actual definition of code P1870 as it applies to different manufacturers. Nonetheless, the “Transmission Component Slipping” part of the definition means the same thing for all manufacturers that apply this code to transmission slippage problems. In all cases where the products of any given manufacturer is not fitted with a transfer case, the second part of the definition “Transmission Mechanical Transfer Case 4×4 Switch Circuit Failure” can be ignored. END OF SPECIAL NOTE.

OBD II fault code P1870 is a manufacturer specific code that is defined by some manufacturers as “Transmission Component Slipping/ Transmission Mechanical Transfer Case 4×4 Switch Circuit Failure”. Typically, this code is set when the PCM (Powertrain Control Module) detects a discrepancy between the rotational speeds of the input and output shafts when the transmission is in top gear, or when the overdrive and/or Torque Converter Clutch are engaged.

In the case of some older GM applications and particularly applications fitted with the 4L60E transmission, the root cause of P1870 can almost always be traced to the TCC (Torque Converter Clutch) solenoid valve that causes the loss of hydraulic pressure to the TCC, thus causing amounts of clutch slippage that exceeds design parameters. However, other causes of slippage in GM 4L60E transmissions are possible.

Nevertheless, due to design differences between applications, the causes of transmission slippage in applications other than GM are many and varied, and almost any component from the torque converter clutch itself, to clutch packs, to gear bands, to the valve body, to the pressure pump, to the condition of the transmission fluid, to sensors, wiring, and shift solenoids can cause this code to be set.

In practical terms however, the PCM monitors the relative rotational speeds of the input and output shafts in order to calculate appropriate TCC engagement modulation strategies to make gearshifts smoother. In practice though, the TCC on many applications is designed to slip by certain amounts under some conditions (amounts and conditions depend on the application), to both improve the driving experience and extend the useful lives of some components, especially on high-powered applications.

Thus, when the PCM detects amounts of slippage that exceed design parameters with regard to both the amount of slippage and the timing of the slippage, a code will be stored. Note however that a warning light will not necessarily be set on the first failure; in some cases, more than one failure cycle may be required to trigger a warning light. In these cases, code P1870 may be present as a pending code.

The image below shows typical damage to a TCC that can cause excessive slippage. Note however that there are many possible causes of code P1870, and that the damage in the form of a scored TCC wear surface such as the driven plate shown here is only one possibility. Always refer to the relevant manual for the application being worked on for detailed information on possible causes of P1870 as it pertains to that particular application.

What are the common causes of code P1870?

Due to the general nature of this code, its possible causes are many and varied, and mostly make and model specific. However, some possible causes could include the following-

• Dirty, degraded, or contaminated transmission fluid
• Defective or sticking valve shuttles in the valve body. This includes the possibility of pressure losses in the TCC hydraulic circuit(s) due to excessive wear, or fractures in the valve body casing
• Damage to clutch packs caused by dirty or degraded transmission fluid
• Defective pressure sensors that can influence TCC modulation timing and/or pressure
• Insufficient operating pressure caused by a defective/worn pressure pump.
• Low transmission fluid level
• Towing loads that exceed the application’s rated capacity
• Use of unsuitable or incorrect grade/formulation of transmission fluid

NOTE: In some cases, one or more of the possible causes of code P1870 may be accompanied by one or more manufacturer specific codes. If any codes other than P1870 are present on any application, consult the manual for that application for detailed information on the exact definition(s) of all codes present on that application. Depending on the application, it is possible that resolving the other codes may resolve P1870 as well.

What are the symptoms of code P1870?

One symptom shared by most applications is a stored trouble code, and possibly an illuminated warning light. Other possible symptoms include the following, but note that depending on the nature of the root cause of code P1870, the severity of some symptoms may vary between applications-

• Noticeable transmission slippage
• Harsh shifting between some gears
• Slippage may only occur under some conditions, such as when the transmission is hot or cold

How do you troubleshoot code P1870?

SPECIAL NOTES: In cases where the full definition, “Transmission Component Slipping/ Transmission Mechanical Transfer Case 4×4 Switch Circuit Failure”, is present on an application that is fitted with a mechanical transfer case, the best course of action would be to start with an inspection of all wiring, connectors, and switches fitted to the 4WD system.

Refer to the manual to locate all components and wiring: if no visible damage is found, follow the directions in the manual to perform resistance, ground, input voltage, as well as continuity tests on all associated components. Only when it is certain that all components are in proper working order and that all electrical values fall within the manufacturer’s specifications, should a diagnosis of the transmission be attempted.

WARNING: Non-professional mechanics, and/or non-dealership technicians that do not have access to all relevant technical information for a transmission being worked on should NOT attempt any diagnostic procedures on transmissions that have “life-time” fills of transmission fluid. It is inevitable that some fluid will be lost during the diagnostic/dismantling process, and since it is generally not known how much fluid “filled-for-life” transmissions require, it is easy to either over-, or under fill such a transmission. Both conditions are equally bad, which means that diagnosis and repair of this type of transmission is best left to professional dealership technicians.

Step 1

If repairs to wiring, switches, and/or connectors were made, clear all codes and rescan the system to see if the code returns. Note that a complete drive cycle may be required to clear this code. If the code persists, record all codes present, as well as all available freeze frame data. This information could be useful should an intermittent fault be diagnosed later on.

Step 2

If the transmission is fitted with a dipstick, consult the manual on the correct procedure to check the fluid level, and top off the fluid level if required.

NOTE #1: At this point, it is a good idea to check the condition of the fluid. Apart from the level, check the color, and smell of the fluid; not all transmission fluid is red, but regardless of the color, it should be bright and almost transparent.

NOTE #2: Transmission fluid that is dark, or has a thick, tarry consistency is degraded, and apart from the fact that such fluid cannot provide proper lubrication, it can actively contribute to all manner of transmission problems because it can cause moveable parts such as valve shuttles in the valve body to stick. Sticking valve or solenoid shuttles that are related to the TCC can cause the TCC not to engage properly, which will almost always set code P1870.

NOTE #3: Transmission fluid should also not have a “burnt” odor. If it has, the fluid has been subjected to abnormally high temperatures, which can also cause it to degrade and break down. High heat breaks down some of the additives in the fluid, but it also causes the formation of harmful sludge, varnishes, and gums that can cause moving parts to stick to the point where the transmission must be scrapped.

Step 3

If the transmission fluid is in less than perfect condition, and it is possible to replace the fluid on a DIY basis, consult the manual on the correct procedure to replace the fluid. Be sure however to use only the recommended grade and formulation of transmission fluid for the application to prevent possible damage to the transmission.

Once it is certain that the transmission is filled to the correct level, clear all codes, and operate the vehicle with a scanner attached to monitor the operation of the transmission in general, and the TCC in particular. In some cases, a simple fluid replacement will resolve the issue, but if it does not, the wiser course of action might be to refer the vehicle for professional diagnosis and repair.

Step 4

If the code persists and you have access to the proper test equipment, prepare to test the transmission’s operating pressure. In most cases, the manual will provide systematic instructions to do this test at various points on the transmission, but bear in mind that unless you are a skilled mechanic with experience in diagnosing and repairing automatic transmissions, there is not much to gain by performing this test. Due to the many possible reasons why the operating pressure may not be sufficient, a proper diagnosis requires disassembly of the transmission and/or torque converter, which is an operation that is best left to specialist technicians.

WARNING: If it is found that, the operating pressure of the transmission does not fall within specification, but you are not comfortable with the idea of tearing down and reassembling an automatic transmission, the wisest course of action is to refer the vehicle for professional diagnosis and repair, or transmission replacement.

NOTE: While it is perfectly doable to replace a transmission or torque converter clutch on a DIY basis, bear in mind that the replacement(s) may have issues of their own, and that the replacement part or component may have to be “matched” to, or integrated with the PCM or TCM (Transmission Control Module) on the vehicle.

Codes Related to P1870

Apart from the GM specific definition “Transmission Component Slipping”, there are no known codes that are directly related to P1870. Below are some examples of definitions of P1870 used by some manufacturers to describe issues that are not related to transmission slippage –

• Daewoo – SOL/EDS PWR SPL (GND SHT/OPEN)
• Honda – Problem in CVT Speed Change Control Valve Assembly Circuit

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