tranny code 87 high ratio error

[gman1]

Hey guys I am new to this forum I have recentley purchased a 95 silverod 1500 6.5L td and I am haveing the check engine light come on over 60 mph I scanned it and transmission code 87 came up high ratio error but there was no real definition on what the problem is I have done some ressearch and there doesnt seem to be much information on this issue if there is anyone here that can help me that would be great
cheers
---Garth

[Robyn]

Hello
The code 87 can have far reaching tentacles.
My first point to check would be the trans range pressure switch.
If this switch is reading falsely the PCM will likely set the code.
The PCM calculates the gear ratio that the tanny is in based on readings from the input and output speed sensors in the tranny. If the PCM is calculating the ratio as being 4th gear and the selected range is otherwise the code will set.
This is all part of the PCM's ability to control everything to such a great degree, it has to have many reference points from the engine and tranny.
You are getting a discrepancy in the ratio seen to what is selected.
You could have an issue with the output or input speed sensors.
I tech tool would be a blessing to figure this out. With the scan tool you can look at what the system is seeing and what readings are happening and where the anomaly is.
The electronic tranny's work so well because they can match the exact ratio needed for optimum performance by comparing all the data coming in and then adjusting accordingly. The only issue is that when all the fancy electronics start going south the gear box stops working right and or the little SES light comes calling.
In short you really need to use a tech1 tool or similar to check the values on the tranny inputs to see whats up. It can be the range pressure switch or could be a speed sensor.
Let us know what you find and we can proceed.
The shop manual has a long list of checks and counter checks to diagnose this. The one thing that popped up was a note that the scan tool will read a ratio of greater than 1.42 in 3rd or 4th if this is present for 2 seconds the code will set.
Robyn

[noelb]

4L80E - Is the torque convertor locking up? Is is going into limp home mode when the light comes on? If the torque convertor is not locking up it when it should it will throw a code.

[gman1]

yes the torque convertor is locking up and the truck seems to run and shift great. I will get a hold of a scanner and run those tests thanks for the input gives me an idea thanks
cheers
---Garth