TPS, Pot Mod, and overdrive issues

Bersaglieri

Bersaglieri

Ron Swanson's Brother
Suddenly the other day while running around town my truck shifts into overdrive in at little higher RPM than usual. I thought nothing of it until I got back into the highway and it wouldn't go into OD at all. Typically I use the "OD" button to manipulate overdrive, but now even the button won't work. The converter will lock up in 3rd, unlock when I press the "OD off' button to shift into 4th, but 4th never happens.

So I tried adjusting the TPS, I must have made things worse because now it won't lockup or shift to 4th. So I'm contemplating doing the Pot mod and ruling out the TPS as the problem.

But I'd like to request a little help on what else I can diagnose. How can I check the "OD off button?" How can I test the see if the ECM is 'commanding' a 4th gear shift? Anyone have a range of pressures for each of the ports on the trans? Which wires/plugs going into the transmission control OD? I've got a Garmon valvebody.

Last, I've found alot of dead links for the parts for the POT mod. Does anyone have part numbers I could just go to radioshack in town and buy? I'd like to get back to hauling gravel Monday...
 
Hook up a scan tool and see what the tps voltage is at different throttle positions.

For the OD wire, I believe it is the brown wire, can't remember it's number. You can cut it (after making sure it's the right wire of course), and just control it off a grounded switch, just like a lockup switch.

When I tried a POT, I just used a regular POT, as thats really all the TPS is. Hook up the 3 wires in the same order, and you're done. Beware when doing this, especially when towing, it gets old having to adjust it constantly for different driving conditions.

I'm also confused about it locking up in 3rd, that usually means OD is already locked out.
 
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quick way is to just ad a switch to the OD signal line at the trans and connect to ground so that you can manually verify whether or not it is a wiring issue
 
Yeah i know i could if only the nearest radioshack was closer than a 20min drive. Also they barley have anything in there half the time. :lolly:
 
you need a 5k ohm linear potentiometer and i think they are "d type" computer pins if you don't want to cut your original harness, then just some wire, various shink tube sizes, and a knob for the POT because they don't come with one.

for what it is worth, i did the POT mod a couple years ago and loved the way the truck acted with it. then one day it kicked it out of o/d and wouldn't shift back in, or lockup in any gear without using the lockup switch (after the POT was in for about 2 years). Tried ohming all the wires, they were all fine, replaced the pot to no avail, and checked the reference and signal voltages, and the ground (all good). couldn't get it to work again. bought a new OEM TPS, put it in, and got o/d and lockup back. I'll eventually try to get the POT to work again.
 
I stopped at radio shack today and got the Pot [271-1714] and D-sub connector [276-1427]. I'm holding off on a knob until I see where I'm going to mount it. I also got 18/3 wiring from Lowes which was listed as home thermostat or alarm wire, it was 0.52 a foot. Total invested is around $21.

I read somewhere that if you don't vary the voltage once in a while the ECM gets "confused" and sometimes the mod stops working. Is that what you [madmikeismad] mean by sucks after a while? Switch, do you mean the factory OD or a force OD switch like you mentioned is similar to a lockup switch?

Hurley: Which is the OD wire? Brown wire at the trans or ECM? I'm having trouble finding which pins control what.

Locking in or locking out OD would be useful for me while towing, hell most of the time I manual shift my truck when towing, it's the only way for me to get shifts right where I want them. I guess in that case a switch would be better than a Pot mod huh? Could you run them together? I feel dumb because I'm not totally up on the big picture of how OD is controlled, but I'm getting there.

Thanks for the help guys.
 
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I'm not sure about that tricking the computer, I just meant I didn't have to like adjusting it just right for what I was towing, or for Grandma driving, or for driving the hell out of it.

I know it's a brown wire but I can't remember what color is coming out of the trans or the pinout. I can check later if you'd like. I have mine wired up just like a lock up with the wire cut to the PCM, so that I am in sole control of it. sometimes I just leave it on all the time so as soon as i shift into third it shifts straight into overdrive, but that's just for city driving where I never have to get on it hard.

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I'm with Mike, if you're already using the factory switch to manipulate the OD, you may as well hardwire a toggle in. I did it six years ago as part of the many things I had to hotwire when my PCM took a crap and the lockup/overdrive switches stayed after I put a new PCM in. The only thing I'm contemplating doing is going through and pinning out my brake switch to see if there is a normally closed circuit to ground I can hijack for lockup so it will unlock when I hit the brakes. But it hasn't been a big enough problem in six years to push me to do it.

If that doesn't work though, you'll need a pressure gauge on the OD port which is on the drivers side above the rear cooler line. It is possible that the OD took a crap and burned up on ya. I see it a lot, even on "recent" rebuilds. Either high mileage trucks wear the clutches down, over extend the apply piston, and blow out the outer seal. Or recent rebuilds, the builder nicked or rolled the outer seal as they installed the apply piston.
 
I have had a pot in place of the tps for quite a while and had to replace a few of the cheap pots( don't know why they went bad because I rarely change the setting) but now have a 5kohm 5turn from Mcmaster Carr (Untitled) part # 7436k34 which has lasted quite a few years now. I wired in a mini digital voltage meter which makes it very easy to switch back and forth from the setting I use for od or od lock out. Usually I just change it when locking in and out of OD. <IMG SRC=http://mysite.verizon.net/vzeocfdn/sitebuildercontent/sitebuilderpictures/.pond/Dash.JPG.w300h225.jpg>
 
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Whoops. I also have 2 switches to control lock up, one is oem/off so I can l can keep it out of lock up, the other is to manually lock up the converter.
 
I found this for the plug pin out. I have a ATSG manual for the A618, but I don't see a pin out diagram anywhere. The ATSG for the 4L80e does.

1: Red, the transmission control relay output.
2: Orange, 5 volt supply.
3: Black/blue, sensor ground.
4: Light green/white, governor pressure signal.
5: Violet/white, variable force solenoid control.
6: Brown, OD solenoid control.
7: Orange/black, converter clutch solenoid control. (lock up)
8: Violet, trans temp sensor signal.

Does anyone know where the OD fuse is or what it's labeled as?

Right now I'm doing what you guys recommended and force OD with a ground switch to see if there is an internal problem or just wiring. That way I don't waste time wiring up the Pot mod and it be something worse. I'll also be throwing a gauge on the OD port today.
 
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madmikeismad said:
I didn't know there was a fuse LOL
Click to expand...

I was just looking in the ATSG book and first thing on the "No overdrive" list was fuse. I didn't think there was one either, but I figured it couldn't hurt to ask.

List goes like this:

-Blown fuse
-Faulty OD solenoid
-Faulty wiring or connections
-Faulty OD off switch
-Faulty SMEC [Single Module Engine Controller]
-Failed overdrive clutch [Wrong OD piston bearing spacer selected or Low overdrive pressure]
-Lower valve body malfunction
 
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If you wire directly to a switch, I don't think the fuse matters, as there's only one hot into the trans. I know when you do a full manual vb, you wire in a constant hot off an ignition wire so there's no fuses involved.

Have you checked the voltage out of the tps yet?
 
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