Chrysler Repair: Chrysler LHS transmission: limp mode with no codes, chrysler lhs, bias voltage

Question
My transmission goes into "limp mode" but sends no code to the computer.  I have changed the sensors in the transmission, transmission oil to Chrysler's oil, and have no idea why it continues to go into limp mode when it is hot.  On a hot day in heavy traffic it will act up rather quickly, on a cool day it takes longer, but will still act up.  Chrysler says there are no codes.
Is it possible that the transmission is not causing the limp mode, but another sensor is doing it?  The engine does not run hot, it only has 60,000 miles and has been doing this off and on for about 5 years.

Answer
Hi Bob,
The "manual valve" in the valve body of the trans is what will produce the limp-in condition. So it is possible that there is a problem with that valve which is producing the limp-in behavior. So if you are absolutely certain there are no trans codes stored in the memory the next step would be to remove the valve body and examine it for a malfunction of the manual valve. That is about the only suggestion I have. Fortunately that sort of servicing doesn't require removing the transmission.
I would be inclined to try and find another dealer or a trans shop that has a DRB II and seek another readout via the blue data link connector that is under the dash to the right of the steering column.
If you have a voltmeter you could verify that there is a bias voltage of + and - 2.5 volts on the two sockets that are diagonally opposite one another on that plug when key is in the "on" position, both before and after the trans is limping. One of those sockets is adjacent to the "ear" on the plug, the other is diagonally opposite. That would at least establish that the data bus is functioning.
I just see nothing to suggest why the trans would go into limp and not give a trouble code.
Roland
PS Thanks for the evaluation and nomination.




Hi Bob,
You didn't say what year your LHS is but if it is pre-'96 or so it may be that they are not reading the codes correctly. Before the present OBD-II system which has a single 16-pin plug under the dash there were two plugs: one under the hood for the engine and one under the dash for the trans and body computer readouts. And they had different plugs and needed the DRB II readout box to get the codes. Many times shops read the one under the hood but don't know or have the plug for the one under the dash. So that is one reason why you aren't getting any codes.
You should get a code because the trans controller always sets a code in its memory if it puts the trans in limp in.
I suppose there is the possibility that the trans controller has gone bad but that should be evident too.
So check that possibility out about the under dash plug, called the data link connector and find a place that has a DRB II readout box.
If you have the '96 or later model then there is only one plug and with the proper reader it should be able to get a trans fault code.
Because the it seems to be tied to heat, you might get the trans cooler core cleaned out and also check that there is not a bunch of leaves, mud, debris blocking the air from passing thru the fins by putting a light behind it and viewing how well you can see the light from the grille side.
The trans filter must have been changed when you did the fluid change, but if not were that to be clogged it would cause a reduction in pressure which would also affect the cooling of the trans fluid. So check that idea too.
Let me know what you learn, please.
Roland