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Limp mode suddenly, tire size or other issues?

7K views 4 replies 3 participants last post by  MopaR/T 
#1 ·
2005 awd 300c procharged

I've had limp mode creep up on me and I'm becoming very concerned about it from a safety perspective. Once it happens, I lose all ability to accelerate and basically have to be able to cruise to a spot to turn car off and then back on and then it's fine. Or I'll have to clear the codes and then its fine. But I'm concerned because there are a number of places when driving on the highway around chicago where there is absolutely no shoulder or exit and if I get stuck on one of those, I and my passengers could be in serious danger from other highway speed cars.

I am running slightly smaller tires (27" tall vs 29" standard) with compensating adjustment made for tire size in my diablo tuner. If the tires are part of the equation and hal LX just can't handle it, I'll get a new set of standard factory height tires all around (awd). I'd rather keep the little bit of gear if there is some other variable I'm not thinking of though.

The codes it threw last night were:
P0700: Transmission control
P0730: Incorrect gear ratio

I'm running a procharger so much more than stock torque/hp and that could be a contributor of course.

I haven't tried running with fuse 17 pulled to see if its a torque management issue. I suspect it is or that is part of the combined issue.

Last night it happened when I let off the gas after a very aggressive acceleration. Basically I let off at 5k and it hovered there then went into limp mode.

What do you guys think?

Thanks in advance.

-Erik
 
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#2 ·
My magnum was doing the same thing. Basically your tcm is seeing it as a higher rpm and sending it into limp mode to save your car. Even after I adjusted my rear gear ratio and tire size, if I went above a certain rpm it would still do it. I actually had to go into the tcm and change the rpm limit.

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#3 ·
Thanks, StrokedRT

Hadn't thought of that! My rpm rev limit was maybe too close to my shift points. With an occasionally laggy trans shift when tranzformer is off, the rpms do end up climbing higher than the set shift point so that lines up that it could be my main issue!

I reset the in drive rev limit a lot higher and will see if that solves the problem for good now.
 
#4 ·
You need to also raise the tcm rev limit. If I remember right the 5.7 tcm limit is 5xxx +/- 500. And when you hit that point it puts the trans into limp mode to keep from blowing your motor.

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#5 · (Edited)
Saw in your signature you're using a TranzFormer...? I had mine connected using t-taps and I believe it lost connection at the taps to TCM once and caused a limp mode on the interstate and those same faults. I turned the TF off and restarted the car and was fine.

Hardwire the TranzFormer. Cut, Bare, Solder, Splice, whatever you can do to get rid of the t-taps! They'll lose connection at a critical moment and could screw things up badly. I noticed this first with CAN controls. ANything that used the CAN-BUS controlled TF functions would flake out... Until the transmission did what it did. Since I've hardwired (cut/spliced in new wires) all my connections, never had an issue again.

FWIW.

EDIT: And I know we are using 2 different variants of the TranzFormer... Gen 1 vs Gen 2. But Joe at ZAutoTech even told me t-taps aren't good for this application.
 
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