Dodge Caravan transmission lock-up
 

I have been watching the SG in my 2003 Caravan 3.3 and noticing the increase in mileage when the transmission locks up. This happens about 42 MPH. This is a little fast for around town driving. Does anyone know what parameters the computer looks at when it locks up and if any of these parameters can be affected by altering sensor inputs without affecting driveability or mileage? This aproach seems to have the potential for significant improvements in city mileage.
Mike

I am not too up on Chrysler. But for my cars, it looks for the following.

-Needs to be in 3rd or 4th gear (there are 3rd and 4th gear switches in the trans). That also dictates the speed to a point, mine is about 33 mph lower limit for 3rd gear.
-Engine needs to be out of warmup mode.
-Throttle position needs to be in a certain middle range, foot off the gas or pushing hard will disable the TCC, it likely reads the TPS.
-Foot needs to be off the brake, there is a switch for this at the brake pedal.

IIRC, if you want to lock the TC, you can just find the TCL solenoid and put a switch on the wires. It shouldn't cause a MIL, but on the 2003 models it might.

Your transmission is electronically controlled, not hydraulically. With a little work and a switch box, you can openly control how your transmission shifts.

Also, if you leave your transmission in L or 1, it will automatically shift at or around 5-5.5k RPM, just so you know. It's a safety feature.

Any mods would have to leave the van driveable by anyone so I am not enthusiastic about a manual lockup switch (but I wouldn't rule it out). I had been thinking about a TPS tweak but am wondering about driveability issues from an altered signal.

A manual lockup switch is a two-way setup... you can just tap the lockup solenoid while leaving it attached to the ECU, so that when the ECU thinks it should be locked, it will still send the signal, even if you've already got it locked w/ the manual switch.

On the other hand, if you engage the manual switch, it will be locked, even if the ECU says "no".

Basically, when someone else is driving, the switch is utterly useless, and they probably won't ever touch it/know what it does. When you're driving, you can impress us all with your extra 2-3 MPG b/c you can lock it manually, without affecting anything. (Except you may get a CEL.. not sure.)

I think I would need to put relays in the circuit to isolate the ECU form the TCL so the ECU wouldn't know about the manual switch. This would avoid the CEL. This would also allow me to wire the relay to either unlock on brake application or maybe on the 4-3 downshift.

Brake application, I can see, but there is an advantage to having the converter locked in any gear - 1:1 operation with the engine and transmission. There are lots of manual valve body options which allow (for race applications, granted) you to have lockup in every gear, without a manual switch. Those applications are for manual lockup though, where the lockup solenoid is controlled by fluid pressure and a valve.

Regardless of the control method, having lockup on demand will allow you to engine-brake (although not in DFCO) in any gear except 1st, which is a sprag-type gear.

If it were me, I'd probably try to find a way to de-activate it with the speed sensor (when it gets below a certain speed), so that I could have engine braking when I actually wanted to slow down, but after it gets into 2nd gear, below, say 22 MPH (where it usually shifts out of first), it would unlock before going into first (b/c it wouldn't make a diff if it was locked in first or not.)

I started thinking about a 3-2 unlock but then it occurred to me that coasting would probably be better for MPG.

the drawback with Autotransmission-"coasting" (not sure if that is the case any more...) is that the hydraulic pumps only work with the car in drive. Coasting in neutral, the tranny fluid isn't being circulated/cooled and the tranny overheats/burns up.

This has never been true, to my knowledge. Back in the day, you could "pop start" an engine w/ an automatic transmission because they still used rear (tailshaft) fluid pumps, but not so much anymore, unless they're designed with that purpose in mind (industrial apps, "other" designs).

When you're coasting in neutral, the engine is still running. Since the pump is driven off the engine's output (transmission input shaft), the pump is still being driven as long as the engine is turning. Putting the transmission in neutral does nothing more than bypass the valve that allows fluid pressure to build up enough to engage the clutches in the gear set, but the fluid is still flowing very much so.

Where the problem comes in is when attempting EOC with an automatic, because then the engine isn't turning, so the transmission fluid isn't being pumped, except in circumstances where the transmission is designed to be towed for long distances in neutral w/ the engine off (most or all Saturns are built this way.)