'81 V864 trans switch OK - any other ideas why no 4&6cyl modes?

[fleetwood96]

OK all you V864 "experts" out there, I'm still uncertain why the computer will not kick down to 6 and 4 cylinder modes. I checked out the transmission connector on the driver's side and see a two wire connector (round) where the leads are perpendicular to each other - it is the only wiring in the area so I presume this is the switch which tells the computerto go into 4 or 6 cylinder operation. The wires look OK physically at the connector, and then run to a big wiring loom and become hard to trace where they go.

I even unhooked the connector and probed the wires with a test light looking to see if either wire was a GROUND lead or was +12v hot. Neither wire was ground or hot, so I'm not sure if this is good or bad - maybe bad as another post indicated that this switch allowed ground to go to the PCM when the transmission is in high (3rd) gear.

So.... while I'm waiting for my '81 service manual to arrive in the mail (courtesy of Ebay) does anyone have any other thoughts as to where the disconnect might be (either under the hood or at the computer box itself)?

Thanks!
Doug

[DaveSmed]

Take a look at the wires coming from the valve covers. I believe that if the wires were clipped, the solenoids were locked on.

[fleetwood96]

Yep - checked those wires out too - all 4 valve cover connectors are there, and their wires run to the wiring looms which circle around the top of the engine under the air cleaner. Although, I haven't actually unhooked them or probed them for +12v. Would I find that in either 6 or 4 cyl mode that the cylinders cut off by the CPU would have power going to the cylinder head in question? I'm guessing that power going there would activate whatever it was that shut the valves off, limiting the fuel mixture from getting in there. Thinking along that line, if I manually gave the connector +12v would I be manually cutting off fuel to that cylinder????

On another note, which I'm not sure if this is related or not, occasionally the "check engine" lamp flickers itself at me and then goes off after a few seconds when braking to a stop or when accelerating from a stop (perhaps in 2nd gear or maybe still in 1st gear), but there's no codes registered on the ECC in Diagnostic mode (it just says "88" as a bulb check then goes straight to the "70" decision point). This doesn't seem like a V864 issue as the car shouldn't be in 3rd gear when the check engine lamp is flickering. Also when this goes on, sometimes the "MPG sentinel" goes totally blank or shows me VERY strange displays, but then will return to normal once the check lamp goes out (gremlens, or some sensor or wiring problem? who knows........) Maybe the service manual will give me some help here, too!!

In any case, I'm having a real blast with my new "toy".

Doug

[STS 310]

Originally Posted by fleetwood96

Yep - checked those wires out too - all 4 valve cover connectors are there, and their wires run to the wiring looms which circle around the top of the engine under the air cleaner. Although, I haven't actually unhooked them or probed them for +12v. Would I find that in either 6 or 4 cyl mode that the cylinders cut off by the CPU would have power going to the cylinder head in question? I'm guessing that power going there would activate whatever it was that shut the valves off, limiting the fuel mixture from getting in there. Thinking along that line, if I manually gave the connector +12v would I be manually cutting off fuel to that cylinder????

On another note, which I'm not sure if this is related or not, occasionally the "check engine" lamp flickers itself at me and then goes off after a few seconds when braking to a stop or when accelerating from a stop (perhaps in 2nd gear or maybe still in 1st gear), but there's no codes registered on the ECC in Diagnostic mode (it just says "88" as a bulb check then goes straight to the "70" decision point). This doesn't seem like a V864 issue as the car shouldn't be in 3rd gear when the check engine lamp is flickering. Also when this goes on, sometimes the "MPG sentinel" goes totally blank or shows me VERY strange displays, but then will return to normal once the check lamp goes out (gremlens, or some sensor or wiring problem? who knows........) Maybe the service manual will give me some help here, too!!

In any case, I'm having a real blast with my new "toy".


Doug

Hey Doug,
Listen, i cant help with your problem, but i had an 81 biaritz and it had that variable cylinder deal. My grandfather said it drove them nuts as the system it broke down and cost major coin to fix. Anyway not long after it was fixed it broke again. The dealer didnt want to fix saying warranty didnt cover. To avoid possible "legal issues" from Assault and Battery on the dealer, my grandparents said to hell with it and finally had it wired to 8 cylinders. What it comes down to is that Cadillac is very innovative, however innovation does not always mean something is going to work, or work rite. Consequently Cadillac stopped offering the option and let it die.

I wasnt trying to be a kill joy,but u are porbably better off sticking to V8 mode and forgetting about the variable. You may save headaches, time, and the most important thing, COIN!!!

Unless you got lots coin and time and dont mind migraines.

Those 80's Eldos are nice rides. ROOMY!

Happy Trails
SLS LUV

[Elvis]

I don't remember where I saw it, but somewhere on this Internet, someone posted an explanation of "how to" bypass the 8-6-4 mode. I read it about two years ago, but don't recall the link. Try a Google, and you might get lucky.

Anyway, my suggestion is to find that and maybe you'll be able to figure out whether it was done to your car or not.

[Casey1]

I currently drive a 1981 Coupe deville with over 400,000 miles logged. Last week the teeth on the cam sproket stripped . Only time the motor has been touched.

I apply 12v to the computer line coming from the backup lights when I want to disable the 8, 6, 4. When in reverse the computer will not command any changes.

The two wires on the transmission send 12V to the computer informing that
the transmission is in 3rd gear and now the computer can cane to 6 or 4.
You can cut the12V orange wire and insert a dashboard switch which allows you to control turning on and off this function.

There's something wrong if the "Check engine" light does not come on. the
computer decides when to change the engine to 8, 6, and 4 when in 3rd gear.

The copy of the original Cadillac shop manual from Helm shows how to use
the front panel controls to diagnose the 8, 6, 4 priblems.

[ssstealth]

The wires at the transmission are a loop through. It is a ground circuit that is completed when the transmission gets to 3rd gear. If either of those wires are cut or do not form a continuous circuit, the system will be OFF.

When you get the manual you will find a procedure that will FORCE manual cylinder shutdown while at idle in your driveway. This will answer the question about the cylinder selenoids devices actually working.

Good Luck

[fleetwood96]

OK gang, I have the wiring figured out! With the help of the service manual, I was able to figure out that one of the two wires at the transmission has nothing to do with the 3rd gear switch to the ECM, rather, it is a "kick-down" switch that's attached to the gas pedal which feeds +12v to the transmission when you have the gas pedal at (or near) the floorboard. That part works fine, but the Ground wire out of the transmission when it is in 3rd gear mode wasn't working (and still isn't working). I take it to see what's wrong you would have to drain the transmission & remove the trans pan and filter to see what's going on. I also discovered another problem, someone had cut the same wire at the ECM (under the glove box) to disable the 6 & 4 cylinder modes.

So... what I decided to do is attach a relay to the dash cruise control switch. With the switch ON, ground is fed to the ECM (like the 3rd gear switch mode would). When crusing around town, etc, with the cruise switch off, I get 8cyl mode all the time. Not a bad compromise, in my opinion...

One thing I noticed about the 8-6-4 operation, around 35mph at steady speed or small accelerations around town (and the system ON), the computer wants to switch back and forth between 8 and 4 cyl modes and won't even use the 6cyl mode. This makes the car feel "jerky" at these low speeds when it switches modes. Around 50mph or so, the computer will take the car into 6cyl mode and then eventually 4 cyl mode, which is less noticable, but like another member has stated, the 6 cyl mode definitely feels "nervous" (could it be because one side of the engine has all 4 cylinders going while the other side just has 2, therefore it's unbalanced?). Anyhow, I can tell when it's in 6cyl mode just by the feel of the car, without even looking at the MPG readout.

Another thing that's pretty clear is that 4cyl mode has next to no power whatsoever. It works fine if you're cruising along level at 55mph with the cruise set, but anything above 65 or so, or with any sort of wind and you're constantly back & forth between 4 and 6 cyls (or 6 & 8cyl depending on wind, speed, and going up hills) with the cruise set.

Anyhow, that's my experience so far.... I'm still glad I got her - she's been a blast to work on and drive!! I've also heard that GM is definitely going to try this "Displacement on Demand" technology on some 2005 (and beyond) products, so it will be interesting to me how they do with it 24 years since the '81 attempt! Doug