Adjusting Overdrive Shift Point

[Stepperado]

I'll probably need more infer some someone to help me, but at least this will get the ball rolling………

My '84 truck originally came with a TH350 and a PO put in a 4speed auto. I haven't had the truck long and I would actually have to crawl under the truck and ID the tranny to tell you which trans I have. What I do know is it has an electrical connector on it which I believe is for a lock up converter.

Anyhow, in my continued effort to increase gas mileage, I was going to try adjusting the point at which the trans shifts into overdrive. Currently it seems to spend a lot of time in 3rd gear when it could be in 4th. Don't get me wrong, it appears to work flawlessly, but I would like it to shift sooner. What I don't know is, what inputs does the transmission consider before shifting?

If anyone can guide me in the right direction, I'm all ears.

Thanks in advance,
Stepperado

 

[350runner]

I'm going out on a limb here, but here's what I'd do first. I have a toggle switch inline (12v) that unlocks the torque converter. Overdrive isn't really that bad it's the lockup feature that drops the rpm. My 2wd gets 17.5 regularly, but I use the switch regularly as well.

Sent from the dust in front of you!

 

[chengny]

The TCC engagement point is controlled by the ECM. The ECM monitors the following sensors to determine whether the TCC should be energized:

1. Coolant temperature (engine must be warm to engage the TCC)

Note: Coolant temperature is the only permissive that "allows" engagement. The following is the list of inputs that "deny" (or interrupt) engagement of the TCC.

2. The throttle position sensor (TPS). The ECM will disengage the TCC if the TPS indicates that the vehicle is being rapidly accelerated/decelerated.

3. The brake switch. If the brakes are applied, the ECM opens the power supply circuit to the TCC

4. Pulse switch. This is an electrical signal generated within the transmission during a 4-3 down shift. If the ECM senses this signal the TCC is disengaged.


Unless you could somehow "flash" the ECM - HAH - there is no way to adjust the TCC cut in point using the existing circuitry.

Just do as Ceasar suggests and install a nice neat switch under the dash. Connect one side to a IGN power source and run the other lead out through the firewall. Then just splice it into the power supply for the TCC.

Then, if the ECM isn't locking up the torque converter quickly enough for your requirements - just flip the switch and BAM!. As soon as the contacts close you are mechanically coupled.

Be aware however, with power supplied via an external source you will have by-passed all the "safeties" and be in full manual control. Since the TCC will be unable to disengage, you - in effect - will have to become the ECM and release the TCC when required. The two most critical times to not have the TCC engaged are:

1. Speed is less than 30 mph
2. While hard braking

Whether you release the TCC in the event that the other permissives are not met (coolant temp, 4-3 downshifts, rapid speed changes, etc) ... whatever - those are up to you.