Transmission Shifting Hard

JJHYukon · January 9, 2005

I have noticed that my Yukon has been shifting really hard. I can drive around town all day and it will shift fine. I only notice it shifting hard when i stop and take off after i have been driving on the highway at a high speed. But if i turn off the vehicle for a few seconds it won't shift hard anymore. I just can't figure it out. I just put a transmission filter in it and it does the same thing. It almost has to be something electrical because i can shut it off for a few seconds then it shifts normal again. If you have any idea of the problem let me know.

Thanks Justin

97 GMC Yukon

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GM-Tech · January 9, 2005

Your truck almost certainly has a DTC P1870 in it. This is a slip code, but not the kind of slip you are probably thinking. This will not necessarily turn the MIL on.

What it is is that at highway speeds, the torque converter is not maintaing full engagement and the PCM sees what's called a high "slip speed". When the TCC is on, input speed vs. output speed of the trans is compared and the PCM can tell if there's slippage, like would be the case if the TCC was not on. If slip speed is not nearly zero for a certain amount of time and under certain conditions, the PCM sets a P1870. This usually only happens on a highway drive where the conditions occur that would make this problem apparent to the PCM.

When this DTC sets, the results is that the PCM commands full line pressure to the trans in an attempt to prevent damage, which you don't notice until you come to a stop and then pull away and it makes it's first shift, which is usually neck snapping hard.

Cycling the key off will clear this problem, though not the DTC. A valve body problem is possible. Pretty common actually. But we also had some input shaft bushing problems back then too. You didn't see any brass looking material in the pan when you changed the filter, did you? If so, that's the bushing.

JJHYukon · January 9, 2005

Yes it is throwing a p1870 code. I didn't change the filter myself so i don't know if their was any brass in the bottom of the pan. If i took it to a local Aamco Transmission Shop could they fix the problem or would i be better off taking it to the dealer.

GM-Tech · January 9, 2005

If i took it to a local Aamco Transmission Shop could they fix the problem or would i be better off taking it to the dealer.

I would hope a dealer would do a better job, but it really depends on the person diagnosing and fixing the vehicle. You'll definitely pay more at a dealer.