I recently purchased a 96 2500 4x4. It has an ATS stage 3 with racing edition valve body. The issue has been intermittent, but now recently it is constantly doing the same thing. It sifts fine into 3rd and OD, but once the trans. is warm and reaches 3rd, it will not downshift when coming to a stop, not even manually. Have to turn off the key and restart, then shifts fine, does this every time now. Any ideas?

 

does the pcm have any troubles codes and have you done basic leak and fluid level checks?

 

No, havn't pulled codes yet. I have fixed all transmission leaks and fluid level is right where is should be, it has new fluid and filter.

 

Check your governor pressure, if it is not dropping all the way back when you stop it will not downshift.

 

Sounds like Limp Mode. Check codes and gov pressure.

 

Yeah, its definitely limp mode, but my question is why. I am just going to take it to a transmission shop to pull codes and check pressures, i don't have any gauges or a code reader.

 

You don't need any fancy gauges to read from the test ports and a lot of the codes can be read for you at a local advance autoparts or perhaps autozone ect for free.

You better contact ATS and see what their correct pressures are for what you have, your average tranny shop won't know what the correct settings are for your setup they will only have the stock specs.

They will also not likely know how to handle the engine mods.

 

Thanks for the reply! A buddy of mine has a tranny shop, was going to take it there to check the pressure. I did call ATS and got the pressure they say is correct, 50-80 PSI in drive at idle. I might just do that though, go to the parts store and use their scanner and buy my own gauge.

From reading other posts and forums, i think i know what the problem is. With aftermarket high performance valve bodies, the line pressure is higher than stock so the PCM thinks something is wrong and locks it up in limp mode. A fix would be a voltage regulator which can be adjusted or a resistor soldered in the governor sensor wire to lower the sensor voltage.

Does anybody know what size resistor to use or where i can get a voltage regulator?

 

Oh, its a friend of a guy i know who has the transmission shop, did talk with him and seems i know more about the problem then they do, i guess its true, those shops are only good for pulling and rebuilding stock gear boxes. They saying its probably a shift solenoid. Is it possible something in the valve body is sticking, not allowing downshift? I know very little about the internal workings of automatic boxes.

 

It could be a bad governor solenoid,pressure imbalance issue, the governor itself hanging up or a sticking shuttle valve but always the 1st place to start is with pressure testing while driving.

I picked up a OBDII scanner 2 years ago from advance autoparts hot online deal for $169.00 a model cp9580 made by actron. It can also do ABS brakes, is USB updateable and has what they call code connect which is their data base from repair shops of the most common fixes for a particular error code. You can read a live data stream as well = plug it in and then drive the vehicle while watching what it is actually doing.

Never seen it for that cheap since but then again i am not looking for one anymore

 

Yes i do need to get a good scanner. I bought a snapon scanner years ago, which was a huge costly mistake.

Code P1763, governor pressure sensor voltage too high, which convinces me even more i need a resistor. The previous owner told me he replaced the pressure governor. I see no resistor in any of the wires at the PCM nor near the plug on the transmission. I do not know which wire is for the gps. Running in park, the code was active, vehicle off the code would clear, which tells me even at idle the gps is sensing a higher than normal pressure.

Still need to find a gauge so i can verify correct pressures. The my friend's buddy at the "trans" shop turns out to be a total shade tree mekinik, didn't even have gauges or a scanner and told me it has nothing to do with the higher pressure of ATS valve body.

Anybody know where i can get one of these voltage regulators and which wire i need to test?

 

I cannot remember how many ohms the resistor needs to be, but it seems like it was 100 ohm up to 150 ohm.... not sure, but they are available for pennies at electronic parts stores such as Radio Shack or eBay. I have the ATS resistor in my truck so if I have to, I'll rip it open again and measure the resistance for ya.

As far as which wire to tap into:
PCM is on the fire-wall behind the air breather
It has three plugs
Pin 29 on the center plug is the Gov Press Signal wire
Should be Light Green with White stripe or Light Green with Red Stripe
Splice the resistor into that wire

 

I think i will first check the pressure at transmission, and if that checks out according to what ATS guy told me, i will measure the volts at the wire and ask my electronics buddy what size resistor i would need to drop to the correct voltage, that is if the voltage is higher than needed.

 

If you get to the point of adding a resistor, be SURE to post up the pre and post voltages as well as what size of resistor you used. That info seems stinking hard to find!

 

I'm not even sure which wire is the "before" and which is the "after" sensor wires, my repair manual doesn't even show the schematics for transmission circuits. I've read pin 31 and also 29 at PCM middle plug is the correct wires. Light green wire??? am gonna do some probing and find out, anybody know for sure which is sensor feed and return?