TSB

[jzaz]

I seem to be one of many getting the P1870 code on my 4t60 transmission. The good news is that it seems like a lot of tranny shops seem to know how to fix it. The bad news is Chevy seems to be ignoring the problem going back to the previous version of the transmission.

 

I can tell there is a TSB 99-07-30-005 issued 03/01/99

 

A/T - 4L60-E DTC P1870 Diagnosis

 

Does anyone have access to this document to see what they say. I do not mind when things break that are supposed to break with use but this transmission has been having this same problem for years and GM gave me the blow off when I talked to them.

[General GM]

I have cut and pasted some comments I have written on this subject on another forum. The TSB's all refer to diagnostic tests that can be performed using the "Tech 2" scan tool, so unless you have access, the TSB's aren't going to be much good to you.

 

Cut and paste comments below...

 

At the first sign of TCC slippage greater than what is specified by the PWM, a code 1870 will be set, and line pressures will be boosted accordingly. However, if the slippage is not constant (meaning that the converter EVENTUALLY locks up) the mil light will not illuminate until the slippiage is detected in at least 2 unique key cycles. Line pressure will return to normal also once the key cycle that the slippage was initally detected in is terminated but the 1870 will remain hidden in memory until the problem is duplicated.

 

I have been through 3 transmissions with my 99 Blazer beacuse of TCC problems, so I am reasonably familiar with how it goes. I posted for help here and on several GM forums and discovered that the whole TCC / PWM relationship in late model transmissions is widely misunderstood.

 

As far as correcting the 1870 specifically in a 98+ trans (which are a bit different and generally better than 97 back), see my suggestions below:

 

In my experience, late model PWM versions of the 4L60E see slippage for one of the 3 following reasons:

 

1. Clogged or displaced filter causing inconsistant or insufficent fluid pressures at the front pump. This may or may not set an 1870 (usually not). This is obviously the easiest and chepest to fix.

 

2. Problems with the PWM valve (pulse width management) that allow the converter to gradually lock up and eliminate the "thump" at lockup seen in earlier 700 series transmissions. A symptom of a failing PWM valve is a transmission that is very soft in lockup, or will not lock up at all until part throttle at high speeds. This will cause a code 1870 in almost every case, and will usually result in an eventual lockup. This is really easy to see if you have a tachometer, as the rpm's will drop very, very gradually as lockup occurs. It is also easy to see it lock and unlock at highway speed by pulsing the brake off and on. This verifys that the converter does eventually lock.

 

3. Lastly, a failing or slipping 3-4 band in the transmission can also cause the 1870 with at first a slow, slipping lock and eventually NO lockup at all. Once it goes it is gone for good in this case, and as this is an internal problem, the only remedy is overhaul. Based on your desciption above, I don't think this is your problem.

 

There are shift kits available to defeat the PWM system entirely without upsetting the PCM. They will result in in the old fashioned "thump" lockup without any slippage at all. This helps to cure many 1870 issues. My latest transmission came rebuilt with the PWM defeated.

 

Certainly, there are other issues that can also cause a 1870. It can be a difficult and frustrating code to diagnose and fix. Most tranny shops will either make valve body modifications or install a shift kit that effectively defeats the PWM system and allows a sudden rather than gradual lockup.