3.2 Tough Electrical Problem Assistance Needed

dsmyth · 05-08-2008, 01:06 PM

Heres the problem. My 85 911 will not stop running when you turn off with key. Only disconnecting DME relay or battery will shut it off. If you pull the back of the key electric portion off the car will keep running. This problem happened off and on until it was permanent.

I have replaced the electric portion behind key, the DME relay and the Diode behind the generator light on the dash. I am now scratching my head at what to do next.

Of particular interest is after spending many hours with a generator/alternator specialist ,we discovered that if you remove the alternator belt temporarily ,the problem disappeared which drove us to replace the diode behind the alt. light. It then turned off with the key. All was perfect .

This solution lasted for a few tries then the problem returned. We then retested the diode and it was ok and disconnecting the alternator belt did not have the same success as before????

Also of note , the radio and speedo go off when you turn the key off but the engine and some dash lights keep functioning. Notably the brake warning light and the generator light stay lit even when you pull the DME relay.(though the alternator light stopped staying on after the diode was replaced)

Once you disconnect the battery everything goes out and nothing comes back on when you reconnect it. Then everything is back to square one.

This winter I took out the alternator to paint the fan, replaced the steering wheel, replaced the shift rod bushings and took some of the top of the engine off to put a new heat/sound pad in. It was all done carefully and it all ran well for several days before some symtoms of this started to appear. The alternator guy says that the alternator is working perfectly and I was very careful with the reinstallation.

I am weak on the electrical side so any help is appreciated. The shops here in Ottawa say that days could be spent on this without any solution. Should I consider a battery cut off switch installation inside the car? The car runs perfect and it is a very low mileage widebody. Would be nice to solve. thanks Doug

Scott R · Scott R's Garage

I would pull the ignition switch and disconnect it form the electrical, if that stops the car you may have a bad switch. If you look at the wiring diagram you an see that the radio in particular is fed from a different pin on the switch than other items. It could be the mechanicals in the switch that are bad, thereby not closing the correct connections.

xlr8 · 05-08-2008, 04:08 PM

Pelican offers a fully electronic master battery cut-off switch.
It is found on this page- http://www.pelicanparts.com/catalog/shopcart/911M/POR_911M_ELchrg_pg3.htm#item15
This could be a solution for a convenient driver's seat remote cut-off to use while you trouble-shoot the underlying problem.
Good luck!

dsmyth · 05-08-2008, 04:40 PM

Scott I was thinking the mechanicals of the switch as well so I pulled the back of the switch of when it was running and it kept running???? Something must be overriding the switch and only shutting down when you disconnect the battery. On reconnection of the battery everything powers off as it should without the key in so it must be some sort of relay thats stuck and releases when you disconnect the battery ,then sticks again when you turn the key and allows the power to bypass the DM relay. That makes no sence>>>>>

Thanks for the tip on the remote battery disconnect Russ, at least I wont have to open the hood and unhook the battery everytime I start the car.

Anyone have any other Ideas?????? thanks for looking Doug

Scott R · Scott R's Garage

Quote:

Originally Posted by dsmyth

Scott I was thinking the mechanicals of the switch as well so I pulled the back of the switch of when it was running and it kept running???? Something must be overriding the switch and only shutting down when you disconnect the battery. On reconnection of the battery everything powers off as it should without the key in so it must be some sort of relay thats stuck and releases when you disconnect the battery ,then sticks again when you turn the key and allows the power to bypass the DM relay. That makes no sence>>>>>

Thanks for the tip on the remote battery disconnect Russ, at least I wont have to open the hood and unhook the battery everytime I start the car.

Anyone have any other Ideas?????? thanks for looking Doug

Lets start looking at other things, do you have an aftermarket alarm? Or even a factory alarm? Look for anything spliced into the ignition wiring harness. Next I would find the pin on the ignition switch that should be off with the key off and look for voltage. If you find voltage start systematically removing fuses and relays until you find the one thats supplying power to the switch.

dsmyth · 05-08-2008, 06:09 PM

Thanks for your input Scott. I do have a factory alarm but it has been disabled before I got the car. I have not disturbed it. When I pull the plug from the back of the key the car continues to run by itself,I can even take the key out and throw it out the window and keep driving. Somehow all of the systems are continuing to get power on their own with the exception of the speedo , radio and headlights? Where would the starting point be for the power search?

Scott R · Scott R's Garage

Quote:

Originally Posted by dsmyth

Thanks for your input Scott. I do have a factory alarm but it has been disabled before I got the car. I have not disturbed it. When I pull the plug from the back of the key the car continues to run by itself,I can even take the key out and throw it out the window and keep driving. Somehow all of the systems are continuing to get power on their own with the exception of the speedo , radio and headlights? Where would the starting point be for the power search?

Start basic, start removing fuses until the car shuts down, after that move to relays. Also there is obvisoulsy a constant draw, so without even starting the car get a voltage tester that has a 10 amp test port. Plug one side of the tester to the positive batter post, and the other to positive cable. Look for the amount of amps draw and record the initial number. Remove the clock fuse and the radio first, that should get you a good baseline. Now, if you still show a good amount of amps say 2 or 3 pull more fuses and see which one drops it to 1 ~0. The one that drops it is probably supplying power when it should not be.

911quest · 05-08-2008, 06:50 PM

I think you are on the right track when you refer to how the car turns off correctly if you remove the Alt. belt. Why I say this is the car acts like what happens if you install a battery disconnect incorrectly....if you don't shut of the alternator the car will continue to run. I would remove the Alternator and see if you have any melted wires.Or evidence of a problem.

Scott R · Scott R's Garage

Quote:

Originally Posted by 911quest

I think you are on the right track when you refer to how the car turns off correctly if you remove the Alt. belt. Why I say this is the car acts like what happens if you install a battery disconnect incorrectly....if you don't shut of the alternator the car will continue to run. I would remove the Alternator and see if you have any melted wires.Or evidence of a problem.

Good advice +1

rick-l · 05-08-2008, 09:14 PM

Looks very similar to the problem discussed here
ENGINE WON'T SHUT DOWN !! Need Help

dsmyth · 05-09-2008, 04:53 PM

There was one relay that ticked with the key off and you disconnected the battery. It was the windshield washer relay behind the guages. Believe it or not with that relay sticking the car would not shut off and you could pull the electrical plug from the back of the key and throw the key out the window and keep driving. It also played with my alternator diode. It makes no sence but that was the problem. Thanks every

one for your valuable assistance. Doug