Tow Haul switch on trucks

[Guest]

I would like to use the tow/Haul functionality of the 2000 5.3L engine that I have swapped into my landcruiser. The tow/haul switch is connected to the BCM (Body Control Module) which sends a message to the PCM when the switch is pushed. Unfortunately I do not have a BCM in my swapped setup. Could someone with 2000 truck or suburban that has 5.3L or 6.0L engine sniff the messages that are sent when the switch is pushed and post them? I am hoping that with the version 2 ELM chip I will be able to send the correct 3 byte header and message onto the buss to enable and disable the tow haul mode.

Thanks for any help,
Vortec62

[RoboGeek]

A feature I never heard of! What exactly does that do?

[Vortec62]

I switched browsers and no I can post as someithing other than a guest...

The tow/haul switch is used on automatic trucks. I believe it selects some alternate shift tables to use when there is a heavy load on the drivetrain.

I haven't ever driven a chevy/gm that had it but my dodge truck has one too. When you enable it the transmission drops out of overdrive and the torque converter locks up in third gear. I have noticed that if you just select third with the shifter the torque converter will not lock up. I don't know if it works like this on the chevy/gm trucks but it is probably similar.

I am planning on using my landcruiser with the swapped in vortec to tow a camping trailer so I want to be able to use the tow/haul function so I don't overheat the transmission.

/Vortec62

[beerman]

Kind of like performance shift mode on the F-bodies?

I looked at my Helms manual PCM pinout for my 1997 Camaro. I found one PCM pin that controlled on and off (by suppling ground to the pin). Another pin after the first was grounded supplied ground for a lamp/LED.

So, you may want to check the PCM pinnout not the BCM.

[Vortec62]

I wish there was an input on the PCM for it but on the trucks it was wired into the BCM which sends a message to the PCM.

I've been told the truck PCM's are the same as the Car ones but with different software. I think there may be some differences though. For instance I used LS1edit to enable the electric fan function in my truck PCM, which doesn't normally have electric fans, but I don't get any output on the pin that is supposed to control the fan relay. Other people have gotten it to work 2001 and up truck PCMs. Mine is a 2000. Thats what got me started with messing with the buss. I plan to hook my elm chip to a microcontroller and just read the engine temp and have the microcontroller turn the fan off and on. I might also rig up some LED's to tell me what gear I'm in and if the lockup clutch is engaged. It's just fun to hack around...

Vortec62

[pswired]

I have dumped a 6L engine into my 95 Chevy pickup and have the same situation. I use EFILive FlashScan and there is an option in the tuning software to enable an unused PCM input pin to trigger the tow/haul mode. There is also an option to use one of the PCM inputs for manual shift mode but I'm not sure if that is supported on the trucks. I haven't tried either yet but eventually I will.

[Vortec62]

This sounds promising. Please post again if you are able to get it working.

[BigBlackAv]

Anymore info on this...i would like to reprogram my tow/haul switch to soften up the shifts (for like that rare occasion i'm on a date :P)

[Main_One]

When the tow/haul switch is engaged, the system uses the "Performance" tables. Therefore, soften up the "Performance" transmission table settings.

[stone]

I have dumped a 6L engine into my 95 Chevy pickup and have the same situation. I use EFILive FlashScan and there is an option in the tuning software to enable an unused PCM input pin to trigger the tow/haul mode. There is also an option to use one of the PCM inputs for manual shift mode but I'm not sure if that is supported on the trucks. I haven't tried either yet but eventually I will.

did you ever figure it out?

[airdeano]

I
The tow/haul switch is used on automatic trucks. I believe it selects some alternate shift tables to use when there is a heavy load on the drivetrain.

like you said prior, the message from the t/h switch is a trigger signal in
the BCM. the BCM passes the trigger for use in the serial data line.
due to the serial network and what all is incorporated ABS,door locks and
modules, seat wamers, etc... by default all these errors will allow the BCM
to allow no use of t/h. kinda like protection of itself.
ive tried wiring 60% of the BCM for the t/h trigger to no avail.

airdeano

[jeffro1234]

I have dumped a 6L engine into my 95 Chevy pickup and have the same situation. I use EFILive FlashScan and there is an option in the tuning software to enable an unused PCM input pin to trigger the tow/haul mode. There is also an option to use one of the PCM inputs for manual shift mode but I'm not sure if that is supported on the trucks. I haven't tried either yet but eventually I will.

It would be nice to also be able to use this function for the cruise control. On the 07 LS2 I have, the cruise enable signal comes from the BCM which I do not have on the retrofit car. It would be nice to reprogram an unused input or have some sort of module that could communicate these extra features to the ECM on retrofits.

[rpmextreme]

Use the "cruise" table. ground the input to the pcm that tells the pcm that the cruise is on and it will use the cruise tables to shift. looks like C2 pin 13 light blue/black wire. I did this on my 2000 silverado and it worked great....3 shift tables lol

[aknovaman]

So what's the pin on the PCM that runs the light? Grounding pin 71 on the blue connector enables the Tow/Haul mode.

[G-Body]

[QUOTE=rpmextreme]Use the "cruise" table. lol

Where is the cruise table located? All I can find is the cruise table for part throttle shift speed, but nothing for the shift properties, only tables for performance and normal