500, 472, 425, 368 Discussion, Code 25 on a 81 Eldorado V-8-6-4 in Cadillac Engine Technical Discussion; First let me say thanks for this great board. Lots of invaluable information here.
My grandmother passed away a few ...
First let me say thanks for this great board. Lots of invaluable information here.
My grandmother passed away a few weeks ago and left me her 81 Eldorado Biarritz. The car has been driven very little in the last two years. I have done several small maintenance things to it (plugs, wires, oil and filter). I am going to replace the fuel filter next. It has 88,000 miles on it.
Now on to my real question. This car has the V-8-6-4 engine. After running the diagnostic program I get a code 25 (modulated displacement failure). I know a lot of V-8-6-4 cars had a wire from the transmission cut to force the engine to run in 8 cyl mode all the time. I do not know if my car has had this done or not. Where would I need to look to find this out?
I assume that this car has not had the V-8-6-4 disabled because, when I first crank the car up it runs in 8 cyl mode with no check engine light. However as soon as I go maybe a 1/4 mile and slightly let off the gas, the car tries to go to 4 cyl and I can feel a slight hesitation. The active cylinder display very breifly changes to 4 on the MPG sentinel. At that time the check engine light comes on and the active cylinder display changes back to 8. After the check engine light comes on it will never try to switch to 4 or 6 cylinders again until the car is restarted. It does this same thing every time without fail. In fact I have never seen it try to switch to 6 cylinder mode at all.
One of my main reasons for wanting to clear the check engine light is so that my cruise control will work. I was told that the check engine light is what was probably causing the cruise control not to work. Also can anyone tell me how much the actual MPG would increase if I could get the V-8-6-4 to work properly vs. just running with 8 cylinders all the time?
Thanks STS, sounds like my best bet is to disconnect the variable displacement. Does anyone know the procedure for doing this? I have been told that I need to disconnect a black wire from the tranny by one person and I was told to disconnect an orange wire by my local dealership (but he said he wasn't completely sure).
To the people that have already done this, do you still get a check engine light? I wouldn't think so but I just want to verify. I miss my cruise control and I would really like to have it back.
Is there anyone out there that would think I should try and repair that variable displacement? I would prefer that it did work but I'm curious how much it really improved fuel economy when working properly. With the current cost of gas this may really be beneficial.
Ok I have an update for anyone who is interested. While cleaning up the engine today, I found two seperate wire harnesses that had came unplugged. Both of these were located on top of the valve covers. I plugged these back in and took it for a spin and sure enough the variable displacement worked as it should.
The check engine light did not come on, however my cruise control still didn't work. Oh well, something else to tinker with.
By the way when the engine shifts to 4 and 6 cylinders the loss of power is quite noticeble and I get a lot of "clatter" that I don't get when it is running on 8 cylinders. The "clatter" noise sounds kind of like an exhaust leak but I would think that it would make this noise when it is running on 8 cylinders also, so I guess it must be something else??
Also if anyone knows which wire I need to unplug to force the engine to stay running on 8 cylinders, please let me know.
Automobile(s): '80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
Re: Code 25 on a 81 Eldorado V-8-6-4
You want to remove the 3rd gear switch wire. This switch is located inside the transmission and works exactly like an oil pressure idiot lamp switch. When the trans is in 3rd gear the switch will make a connection to ground. I have no idea where the plug would be located on a FWD car but the RWD cars had a 2 terminal plug right above the trans pan on the side of the trans. If you look around the plug will appear. If it has more than 1 terminal then pull it out and take the car for a ride with an Ohmmeter hooked to one terminal on the trans and chassis ground. The terminal that makes a connection when you hit 3rd gear is the one that you want to disconnect to eliminate 8/6/4. It will not set a code when you disconnect the wire. Some people have used a toggle switch for this. A shop manual would save all the guesswork here.
Ape Man The plug is on top of the transmission housing almost directly under the brake master cylinder. single contact just pull it off black plastic covering is misleading there are only 2 wires black and orange.
Now I still have my electrical problem:
1. Interior courtesy lights do not come on.
2. Under dash courtesy lights come on when brake pedal is depressed and they flash when turn signal is turned on.
3. Electric door locks will not work when door is opened.
4. Car runs normal with 8-6-4 disconnected.