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Old February 5th, 2007, 14:59
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TCU "manual shift" test & "limp mode" questions

So Ive been having some trouble with my AW4 lately and have been testing a variety of things. My TPS checks out okay but I have the feeling it's not working "as it should".

Today I tried doing a "manual shift test" by unplugging the TCU fuse and seeing how it would react. It shifted as it "should", meaning it stays in 1st in 1-2, stays in 3rd in 3 and stays in OD in 4.

What can I deduct from this? Is my TCU fried? If it's unplugged and everything is working a-okay, does that mean it's hooped?

Heres the other question: The shifting problems have been causing me alot of grief lately, and I have taken it off the "highway" because the shifting was getting bad. Can I safely drive it on the highway in "limp home" mode as long as I watch the RPM's and manually shift it at the right times? If I start out in 1st, run to around 2400RPM and shift to 3rd after that, I don't see any problems but I thought I'd better ask.
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Old February 5th, 2007, 15:31
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Re: TCU "manual shift" test & "limp mode" questions

Oh, and one other question:
The fuse that was in the TCU is a 10amp fuse. I have read elsewhere online that it should be a 15amp fuse, and the glovebox manual says a 7.5amp fuse.
which is it?!?
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Old February 5th, 2007, 16:35
88xjlover 88xjlover is offline
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Re: TCU "manual shift" test & "limp mode" questions

When you unplug that fuse it disable's the tcu and you have to shift manually starting in 1-2 and working your way up. I have some shifting problems w/my 89, it sometimes wont come out of 1st. I unplug the fuse and shift it myself.
This will get you home but I wouldn't daily drive it that way.
How did you test your tps? My friend the jeep tech says no other way than to hook it up to the jeep tester.
My fuse is red, can't remember what size.
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Old February 5th, 2007, 17:08
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BrettM BrettM is offline
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Re: TCU "manual shift" test & "limp mode" questions

Doing that manual test tells you that the problem is in the electrical system, but doesn't tell you where. Bad TCUs are very, very rare, and probably not the problem, but if you have access to a known good one it wouldn't hurt to try it. Most AW4 shifting problems are the TPS. I can't confirm the previous post saying that only the Jeep TPS tester works, but that would make a lot of sense because many people have tested their own TPS fine but have problems go away after replacing it.
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Old February 5th, 2007, 17:12
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Re: TCU "manual shift" test & "limp mode" questions

All good information..

Now, can anyone tell me if driving my XJ in "limp mode" on a regular basis will cause problems as long as I'm careful? Basically it does the same as your AWSHIFTING device but doesn't quite give as good a selection of gears. Any insight?
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Old February 5th, 2007, 17:31
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Re: TCU "manual shift" test & "limp mode" questions

I don't think so. The TC makes up for a lot. Your MPG might suffer.
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Old February 5th, 2007, 18:56
lawsoncl lawsoncl is offline
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Re: TCU "manual shift" test & "limp mode" questions

How did you test your TPS? Best way is with an analog meter and making sure it changes smoothly with no hiccups or bad spots. You didn't say what year, but a bad TPS is very common on older Jeeps as they do wear out. What year is this anyway?

The manual shift test verifies the mechanicals are good and any shifting problems are on the electrical side of the house. If you have a meter, check the TPS as I mentioned and check the solenoid resistance at the TCU connector. In order of probability I'd vote for a bad TPS, bad solenoid, and very unlikely a bad TCU.

You can continue with the TCU fuse pulled and shifting manually if you need to, just keep in mind you're skipping 2nd gear and the torque converter won't be locking up.
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Old February 5th, 2007, 19:09
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Re: TCU "manual shift" test & "limp mode" questions

thanks for the info. I tested the TPS using my analogue multimeter and it looked to go through the voltage with no hiccups. I also tested the voltage ouput at 4.70v, and adjusted it to 82% of this number when connected to terminal "d" and terminal "b". I used my digital multimeter for the second test.

I also suspect a bad TPS but I'm trying to eliminate everything possible before I drop $120 CAD on a new TPS. Mine is a 1990, and for some reason the TPS units are expensive as hell for them!
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