95 3.8l 200K mi, overdrive cancel switch won't light.

Hello everyone,

I've recently moved to a very hilly area, and find that my overdrive cancel switch doesn't work. ( This should not let the engine out of 3rd.) When I depress the button at the end of the shift lever, the light will not come on, and the engine will not drop out of 4th. I've read posts that talk about the switch failing, so I got two switches from a junkyard, and neither of these would light up either. I've probed the three pins in the shift handle to ground, and get no voltage, which leads me to think I have a blown fuse. The owners manual doesn't mention the overdrive cancel switch in the fuse list, and my Haynes book doesn't mention it at all. I see no blown fuses.

The trans was worked on awhile ago, any chance the fellows who pulled it forgot to hook something back up?

Karl

That's the Transmission Control Switch eh, to switch the OD On/Off. The diagram I have for the '95 shows it as being fed via fuse V (10A, engine compartment fuse box). From there, a red wire runs to the switch but there's a connector between the two - the colour after the connector isn't specified so it may or may not change colour at that point.

Regardless of whether anything has been left unhooked further down the line, it wouldn't prevent you from reading 12V at the switch, so I believe your problem is between fuse V and switch, assuming you had the meter negative lead connected to a known good ground point when checking for voltage.

Be careful when probing the switch, because the other two leads connect directly to the PCM.

If fuse V is good, then check that you do have 12V entering that fuse when in Start or Run - it gets its feed via the switched contacts of the PCM Power Relay.

If that isn't shown in your Haynes manual, then it would be worth checking out the first post in this thread (http://www.automotiveforums.com/vbulletin/showthread.php?t=890909) - follow the instructions and select your vehicle, then Wiring Diagrams - Engine Controls - 3.8L - it's on the first of those four diagrams.

Thank you Selectron,

My fresh eyes this morning found fuse "V" in the owners manual, in the engine box. I did a quick check and it looked okay. I'll probe it this evening & see if I have 12 volts to it.

Also, I spotted a photo in the haynes manual of the little switch at the end of the stalk. They call it the "transmission control switch".

I would like to verify, that as long as the ignition is thrown, that switch should work and light up, correct? ( I don't need to be in "D"rive or anything, correct )

I'll keep you posted as to how this turns out.

Karl

I would like to verify, that as long as the ignition is thrown, that switch should work and light up, correct? (I don't need to be in "D"rive or anything, correct)

I'm not sure about that, because the lamp is grounded via the PCM, and it depends how the PCM's internal logic circuitry has been configured. Because the OD On/Off feature only applies to the Drive position, it may have been configured in such a way as to switch the lamp off when you shift to another gear position. Maybe somebody else knows for sure?

I can see that the default setting is 'OD On' (lamp off), and pressing the switch will then switch to 'OD Off' (lamp illuminated), and also when the ignition is switched on, the default setting is OD On (lamp off), regardless of what the switch was set to the last time the vehicle was used.

That really isn't relevant right now though, if you're sure that you don't have 12V arriving at the switch. Any time the ignition is switched to Start or Run, the Transmission Control Switch should be fed with 12V, via the switched contacts of the PCM Power Relay, then via fuse V, to the input terminal of the switch, and that 12V should be there regardless of whether the switch is set to On or Off, and regardless of which position the gear selector is set at.

If the 12V is missing then there is a problem along the path from: PCM Power Relay switched contacts output terminal - to fuse V - to Transmission Control Switch input terminal.

When checking for voltage, be sure that your meter negative lead is connected to a known good ground point. Inside the vehicle I usually verify that my ground point is good by quickly probing the centre contact of the cigarette lighter socket (should be at battery voltage), and under the hood I would probe the battery positive terminal, prior to taking any voltage readings.

Well, that's enough for tonight. Here's what I found (wrong);

1. 12 v to fuse V, Okay.
2. 12 v to a pin at the end of the shifter stalk, Problem.
a. according to Haynes, "in Park, key on RUN", I should get 12 volts to occur at "a pin at the end of the stalk" (Haynes wasn't specific)
b. I got intermittant 12 v. So . . . .
c. I tore down the column, and found . . . .
d. a teflon sleeve out of place. Without this sleeve the shift lever rocked, and caused the intermittant 12 v.
e. I glued, lubed & tightened that sleeve back into place.
f. now I have a steady 12 v to the Pin in the middle of the "L" configuration of three pins in the end of the shift lever stalk.

I've been through the cadavra's in the "Pick-n-Pull" yard, and pulled two TCS switches. None of thesen light up after I hook the power all back up, and plug them in. Tomorrow I'll send 12 v thru each of these switches and see if I can't get the lamp in one of these switches to light.

Hayne's seem to indicate that if the switch is working properly, pushing it once after the ignition is on will leave the od override ON until the igntion is turned off. The system always starts with OD on.

I'll keep you posted.

Karl

I'm not sure how many wires you have at the end of that switch assembly, but you're interested in the three which connect to the red, tan/white, and white/light green wires at the harness. If they change colours at the switch, you should still be able to identify them from that. With the switch out-of-circuit (i.e. not plugged into the harness), and with the meter on its resistance range, you should be able to verify correct switch operation by checking for continuity between the wire which goes to red at the harness, and the wire which goes to tan/white at the harness, when the switch is pressed.

Again, with switch out-of-circuit, and still on the resistance range, you should be able to measure the resistance of the lamp filament between the wires which connect to red, and white/light green. That resistance should be unchanging, regardless of switch position.

When you say: "Tomorrow I'll send 12 v thru each of these switches and see if I can't get the lamp in one of these switches to light.", if you mean you'll plug them into the harness and try them then that's fine - no problem.

If you mean you intend injecting 12V via a jumper lead to either the tan/white or the white/light green wires, while the switch is plugged into the harness then there are problems with that. You absolutely must not apply 12V to the white/light green wire, because the PCM switches that wire to ground when it wants the lamp to illuminate. It's expecting to have the resistance of the lamp filament to limit the current, but if you inject 12V at that wire then the PCM will try to switch it to ground and the PCM isn't capable of sinking the current which would then flow, and it would fry itself. I don't think that's what you had in mind, but if it is then absolutely do not try it, because you'd lose the PCM.

You might also mean that you'll apply 12V across the lamp terminals (the wires which connect to red, and white/light green) with the switch out-of-circuit. That should be ok, assuming that the lamp is rated at 12V. It isn't specified on the diagram though so rather than apply a voltage to it, I'd settle for just measuring the resistance of the lamp filament - it's the safer option.

I've no interest in randomly sending power into the PCM, I had your last suggestion in mind, testing the switch removed from the vehicle.

I pried apart one the switches last night and found . . .
a. The button contacts the two ends of the "L"
b. The point of the "L" (the pin where I now detect 12v steady) goes to the lamp.
c. The lamp grounds via a tiny resistor (gold, tan red, green) to the "top pin of the L configuration".

From what I experienced last night, It seems that the lamp should be lit. No?

I'm going to put 12 v to the lamp (the middle pin), ground it from the top of the "L" and see if it lights. If it doesn't, I suspect all three switches have burned out LED's.

Well, back to work. ( Comfy chair, telephone & keyboard work )

The lamp will only illuminate if the PCM decides that it should, and it makes that decision based on whether or not it receives a 12V input at pin 29, from the TCS. There will be other factors involved, but if the expected conditions are met then it will ground the low end of the lamp, which will then illuminate (actually, it grounds the low end of the resistor, now that we know there's a resistor in series).

It isn't clear whether that's a conventional filament lamp (that's what's shown on the wiring diagram) or an LED. What you describe is consistent with an LED though. The resistor value sounds like green - red - brown - gold, which is 520 ohms, with a 5% tolerance.

When the vehicle is running, supply voltage will be in the region of 14.4V, and around 2V will be dropped across the LED, depending on what colour it is. That leaves 12.4V to be dropped across the resistor, and 12.4V divided by 520 ohms gives a current of approx. 24mA, which is about right. Actually, it's just slightly on the high side - if it's a regular LED then I don't like to run them above 20mA, because that tends to shorten the life.

Anyway, if it is an LED then it's polarity-sensitive, so only apply current in the one direction when you test it, and if you can clearly see that 12V is fed to the point of the L, which goes to the indicator, then observe that polarity, making the negative connection at the far end of the resistor, like so:

http://i180.photobucket.com/albums/x64/Selectron/Misc/led.png

Well, that hot pin showed 11.65v actually . . .
Hope I don't pop the thing when I put current from the battery throught it.
The color of the lamp "was" orange.(ish)

Well now,

I pulled the switch.
I ran leads from the battery.
I put red to the pin that goes straight into the bulb, black after the resistor, nothing
I switched it. I put 12v into the resistor, grounded the pin going straight to the bulb and the bulb glows orange.

Hey, how'd you get that nice diagram in your post? A picture says 1000 words . . .

Karl

Guess it's time to check continuity back to the PCM.

Solution, broken wire at the bottom of the shift stalk. Just under the rubber next to the column.

How do I post a picture.

Karl

Phew, that's a relief; I'm pleased that you found it.

For a quick and easy, no-frills upload of pics, go to http://imageshack.us/ (doesn't require registration, and it's free). From the homepage, click on Browse, then find the photo on your hard-drive, then click on 'Host It'. The photo will then upload and the page will refresh and give you a list of links. Copy the second link on the list - 'Thumbnail for forums (1)' and paste it into your post here in the forum.

Also free but with a lot more features, is www.photobucket.com - allows you to create folders etc. to store and manage your images (registration required). It will also give you a link suitable for using on a forum.

There's also a 'Gallery' feature for the members here on AF but I never did learn how to use it so I use my Photobucket account instead.

I think I've figured out how to post a photo.

I went into AF via the Gallery tab, and uploaded an image of my broken overdrive wire to it.

http://files.automotiveforums.com/gallery/showgallery.php?ppuser=235037

Thank you all.