Defeating the torque limit request on the Volvo 850 Turbo

[northernlights]

Everyone who owns a stock turbo 850 knows that they are soft off the line, but much faster from a roll due to the Volvo computers limiting power at low speed/in low gear. I saw an old posting where someone commented about attaching some wires to a dash switch to defeat this (and Matt offered a MVS party for this information ) so I thought I'd share what I had found. Note that I have only written this up because after seeing the old post it reminded me of another thing I had read, and one thing lead to another...

So, here's some information I've compiled about what I believe is the way the TCM (transmission control module) signals the PCM (powertrain control module) to limit power and the various sources of this data.

There is quite a bit of information, so in a nutshell, the TCM appears to have three instances where it makes a request to the PCM for a reduction in torque production:

1) When the vehicle is in Reverse
2) When the vehicle is in 1st gear and the engine speed is below 3000 rpm
3) Immediately before a shift (plus timing is retarded)

What I have found is there are three paths for this information.

a) Torque limit request TCM (connector B pin 2) to PCM (connector B pin 2)
b) Torque limit request TCM (connector B pin 3) to PCM (connector B pin 3)
c) Boost pressure limit signal TCM (connector B pin 1) to PCM (connector B pin 42)

That is how they are identified in the literature, so I am not sure (yet) which is which. I am posting this info in case someone out there has a breakout box or OBD II tool that allows them to monitor outputs while driving, as that should demonstrate which channel does what based on the change in the output signal, i.e. stick the vehicle in reverse and see which channel is active. Each of the above TCM output signals are identified as being 1-2 V less than battery voltage, so ultimately a Rube Goldberg direct measurement may be possible with a voltmeter.

Of course, one could start cutting wires and hope for the best...but I am trying to determine how to lose the 1st gear torque limits without losing the shifting torque limits, as I would really hate to grenade the gearbox for no good reason. Tires, on the other hand, are big rubber fuses, so a little extra spin won't hurt...much

Eric

[matthew1]

OMG Northernlights, first post, you hit it out of the park.

This is great stuff.

So you're saying the ultimate result of this research -- given there are no "roadblocks" -- is that someone with a Motronic 4.3 Turbo car could stop the signal that limits torque from a starting position? For example, in caveman terms, cut the wire responsible for this signal?

And you're also saying we need someone with a breakout box to look for this ~10v signal upon throttle application at rest, so we know which wire is responsible?

Thank you.

[northernlights]

So you're saying the ultimate result of this research -- given there are no "roadblocks" -- is that someone with a Motronic 4.3 Turbo car could stop the signal that limits torque from a starting position? For example, in caveman terms, cut the wire responsible for this signal?

Effectively, yes, assuming the TCM uses a single channel to relay the torque limit request when in 1st gear under 3000 rpm. It looks like there are three channels from the TCM to the PCM, but as there are also three limiting conditions listed in the literature (with the other two being selecting Reverse gear and just prior to a shift) it seems reasonable to assume a discrete channel is used for each specific request.

I'm not a fan of cutting signal wires without knowing what the actual signal is, because it might be bad for the equipment. If the TCM 'flips a 12V switch' and the wire is cut the current has nowhere to go. My initial thought (assuming the concept works) is to shunt the signal to one of the TCM or PCM grounding pins/points through a resistor with the same value as the PCM loop. This would give the current (and signal) somewhere to go without the PCM having to know it's on.

And you're also saying we need someone with a breakout box to look for this ~10v signal upon throttle application at rest, so we know which wire is responsible?

I think a breakout box or equivalent would provide the answer, by monitoring the specific pinout locations for a voltage change that would signify the TCM is essentially 'throwing an electronic switch' under certain conditions. The breakout boxes usually have a way of backprobing specific pin locations, so that's why that came to mind. The OBD II stuff I'm not as familiar with as far as the capabilities.

Thanks for the kind words. You have a great site here.

Eric

[jimmy57]

The two pins out of TCM are the request for the 4 levels of shift torque reduction 0, 1, 2, 3 are the levels based on the combinations available by grounding (0V) or not grounding (5V) the two pins. both at 5v is 0 or no torque reduction.
The third wire is just acknowledgement by ECM that the request is done.
The torque reduction for low gear, reverse, some faults detected by TCM where trans is in FMEM mode ("limp home" mode) and the engine power should be reduced for reduced stress on trans,and also for Winter mode is another wire.
I used to cut and splice that wire to a power antenna switch (only SPST) switch available for the blanks next to steering column) on my company cars so I could kill the reduced boost when I wanted to. The car will have BAD torque steer and will spin fronts when you are easy on throttle and do not wa nt it to happen. It is best for it to be done with a switch so you have the control.
I will have to look at the Mot 4.3 or Mot 4.4 manual to know the wire as I have forgotten it since I last did it on a 98 in 1997 when I got it.
Iwill have to

[jimmy57]

I found it. It is B1 of the TCM. It is described as, " U low on torque limiting request (ist gear, Winter mode, and high oil temperature)"
There is no mention of reverse here but in Volvo training info it was described as being for Reverse also. I have not verified the reverse gear torque limiting.
It is not listed on M4.4. I recall that being the case that it is omitted in some manuals. I think it is described on Mot 4.3 but I don't have that here. I found M4.4 and TCM signal description lisitngs here at home.

[matthew1]

So if I have this correct: I want Connector B pin #2 and pin #3 to get 5v when I want to override the factory TCM programming and provide un-regulated torque to the drive wheels?

[jimmy57]

no, you want to intterrupt the TCM pin 1 wire that does that job. Pin 2 and 3 of TCM do timing retard for shifting.
Pin 1 of TCM signals ECM to not operate the TCV so the engine is limited to the base (low pressure) setting of wastegate.

[matthew1]

Ah, gotcha. Sounds easier. Thus your simple on/off switch on the dash of that company car -- you were interrupting TCM pin 1 signal.

[boosted5cyl]

[kcjones]

Is this a concern for too much torque off the line, as described in posts about manual boost controllers and improper wastegate adjustments? Wouldn't boost also rise quickly in low rpm 1st gear?

I'd hate to see a bunch of guys wiring in switches just to end up bending rods..... Please correct me if I'm wrong.