Colder Weather - Higher Line Pressure?

[ExplorerDMB]

I usually let my Explorer warm up for about at least 3 minutes before I get in it to drive and I usually put it in Neutral as soon as I do (doesn't it bump up the line pressure - but I also thought Park and Neutral were the same -- obviously I'm not a transmission genious).

Anyway, after a minute in Neutral, I'll put it in Drive and it jerks forward (duh, I know it's suppose to but not this violantly). So does colder weather induce higher line pressure during "warm up"? Just to make sure the gears are gotten in to? I know on a lot of chrysler trannys (jeeps) that when its cold and the fluid is a little old it'll have a slow shift. Glacier/Chris - I'm sure i'll hear from you on this subject -- Look forward to reading up on it. Oh, but other than that no other issues with the transmission.



-Drew

EDIT: I don't feel the high line pressure in all the gears - just putting it into Drive. Or am I feeling this cause I'm putting out too much torque haha

 

[BrooklynBay]

The engine revs higher when it is cold. The coolant sensor tells the computer that the temperature is out of range, and revs the engine until the coolant temperature is normal. At this point, the engine revs at a normal RPM in neutral, or in park. If you shift into gear when the engine is reving over 1k RPMs, you will feel it "jerk" into gear. This stresses the mounts, and linkages. It would be a good idea to step on the gas, and rev the engine on your own for a little bit of time. When you let go of the throttle pedal, your RPMs will be normal (800-900 RPMs). You could put it into gear without having that "jerking action" as you are describing.

 

[ExplorerDMB]

BrooklynBay - I don't mean to be mean, but I know what your talking about. It isn't that. When I shift into Neutral the engine is already idling at around 750-850 RPMs. Same with it when I shift into Drive. I know a fairly good amount about carputers (granted thats why I'm monitoring the section) - but that was a good guess and could be right if I wasn't already idling normally.

Glacier - I would love to hear your thoughts about this or any other Transmission Guru!

-Drew

 

[BrooklynBay]

I know some transmissions have temperature sensors inside of them (AXOD FWD for example) that tell the computer what temperature the fluid is. Maybe that might change the shift pattern, and/or pressure, but I'm not sure.

 

[ExplorerDMB]

Glacier? any input

-Drew

 

[Glacier991]

Drew... let me see what I might add to this mix. Colder temperatures obviously (duh) increase viscosity of fluids like ATF. That obviously is why people with blown VB gaskets report SOME operation until warm up, or no really bad symptoms sometimes until their tranny is fully warmed up. Yet the system has a pressure regulator valve, which to some degree moderates that higher viscosity's impact on line pressures.

Now a harsh engagement has several potential components. High line pressures obviously can cause harsh engagement. Just look at what happens when we go into FMEM on the 5R55E's. That is simply a line pressure issue.... computer commanded as part of FMEM. But also remember that most applications of a band have both an apply pressure and a release pressure working against each other to soften the application - accumulator circuits do the same thing in some clutches. So if you had a deficit in that balance you could get a harsh application.

All that blathering said.... let's talk about the 5R55E.... and things that can give you a harsh engagement. It will dovetail into those general areas.

First off... EPC. Since it controls pressure, the inputs to and from the PCM are suspect, as is the harness and the EPC itself. Pinpoint tests are geared towards the EPC as a prime electrical culprit. (Pinpoint test L in particular). For inputs, TP, RPM, and VSS all come into play.

Next we come to control pressure. (Remember in the 5R55 you have a few taps, including EPC and line for testing). You might want to check your pressures. If they are high, we have main control issues. What kind? well... usual list of suspects:

screws out of torque spec... gasket damage.... sep plate damage, EPC solenoid stuck, damaged, EPC O-ring damaged.

Then there are bores and spools... reverse modulator valve (bore 205), forward modulator valve/EPC boost (bore 207) and forward engagement control valve (bore 209).... each could have damaged springs, stuck spools, or bore damage. What if your boost valve was sticky, and you went from reverse to forward with the boosted reverse pressure? Bang... harsh engagement.

Finally and lastly, we have to look at the mechanical. Forward clutch issues. To my mind a low probability.... but a sticky checkball does figure as possible to my mind.... still low probability.

That is my input.

I'd pressure gauge it to start. Every which way.


[ps. This was written for Drew who has a pretty sophisticated level of understanding. FMEM is "Failure effects Management Mode"... or "limp mode" is when the computer senses a component failure, and it defaults to an operation mode that is designed to "limp" you home. It is an "all hands to battle stations" mode, that maximizes transmission pressure to ensure engagement of slipping parts to "get you home". The shifts are bone jarring.]

 

[ExplorerDMB]

haha I like LIMP mode. Very exciting. Anyhow, thank you for the response. I could hook up a line pressure tester gauge and then see what its like in the morning and the different between that and when the fluids warm. It only happens in the 35 or below degree weather...so I'm pretty sure it's the fluid that is cold that'll bump the pressure up.

Now Chris, like I said about the Jeeps/Chryslers transmissions (especially like my dads Cherokee 92) it'll have slow engagement during cold weather. Is this because of a temp sensor not being in the transmission? Or something to tell the PCM (or whatever might control the transmission) how cold the fluid is?

Thank you for sharing your wealth of knowledge.

-Drew

 

[Glacier991]

Actually some trannies have a transmission temperature sensor within the transmission. And in the case of the 5R55 there is a thermostatic valve that inhibits cooler flow until warm, which, in conjunction with a PCM inhibit of TC lockup, speeds warmup. What I do not know is how the 5R accomplishes case lubrication when inhibiting cooler flow, as the return flow comes back as rear, mid and front case lubrication.

And if you routinely see winter temps that can approach -35F, it would be a good reason to consider synthetic ATF - I will assume that it demonstrates superior cold viscosity like other synthetic lubricants.

 

[ExplorerDMB]

You're probably right. Good thing I'm not up north - coldest it gets here is at night at like 19 degrees. No negatives here. Thanks again

-Drew