Hi Folks, hoping I can get your guidance on a weird issue with my 3500S. Car is running well, but when hot, the engine will not switch off with the ignition switch; it just keeps running until I press the brake pedal, then it stops.
I recently replaced the ignition switch so it’s not that (hopefully!). I’ve checked the coil with a meter and with the engine off, the live feed works as it should based on the ignition switch on/off. With engine running however, I switch off the ignition and the coil feed is still live.
Fuses seem ok and the only thing I can find wrong is a broken wire on one of the front brake pads.
Any suggestions welcome and thanks in advance!
Scotty
First, what do we mean by saying that the engine keeps running? Is it just as if you left the ignition on, or does the character of the engine noise seem to change after shutting off the ignition switch? let's eliminate the ignition switch. If the ignition is off, all power to the coil and distributor should be shut down. If not, then the power must be getting through to the coil., and thence on to the distributor.
If we find that source we need to determine if something is amiss with the wiring or the involved unit itself. The wiring diagram shows two possible sources for the ignition coil. Wire one is the power leg of fuse #62. This is the one that is active under normal driving conditions. But, there is another source, and that would be the wire that runs to the ignition from the solenoid. During starting (starter motor active) a line from the starter solenoid bypasses the ballast resistor and gives its full power to the coil. This is because when starting, the load on the starter lowers all voltage in the system, and it fails to fire the coil properly. Again, the purpose of this secondary source is to bypass the ballast resistor, thus giving full battery voltage to the coil and 'reviving' the needed voltage/amperage to fire it and the distributor. Hmm. Two possible sources...so far.
The car will run fine on either source, but when you release the ignition switch to the 'run' position, the boost from eliminating the ballast resistor should drop away, leaving the ballasted coil to run the show. You should also understand the ballast principle of coil function. Coils for 12 volt ballasted systems are usually made to run on 6 to 9 volts. When the resistor is temporarily bypassed, the required voltage is still present, despite the load imposed by the starter. When the starter has done its job and you let go of the key, the starter load is gone, and the normal driving mode is attained at that 6-9 volts (through the ballast resistor). So, how does that relate?
Well, it could be that the ballast resistor wire from the solenoid stays hot, even if you let go of the key. This could mean a wiring fault, or contacts within the solenoid are stuck. Generally, this is not the cause of your problem, but I'm not there.. Let the engine run up to warm or so and touch the ballast resistor to see if it's excessively hot. It may still be active and running at increased voltage. If so, we can be pretty sure it's getting voltage when it should not. This sounds just fine until you realize that the resistor will burn up eventually. How long? Who knows? Lots of things factor in here, and I've seen a car run for months or even years on just the hot voltage from the solenoid. Others will burn out within a minute. Like I say, lots of factors.. There's a way to check that.
While the engine is running, disconnect the 'boost wire' ( NOT the normal running wire!) that runs to the ballast (distributor end) from the solenoid. If the engine continues running on ballasted power after disconnecting that wire, the starting up system is likely working, but if it stalls immediately, it has been running on 'boost power' (no ballast resistor involved) If this happens, you most likely have a wiring problem or a bad solenoid.
But, to get into that, I will need to explain a lot of stuff that relates to the wiring diagram itself. Please answer the question about the character of running sound after switching off. Also, What idle speed does the tachometer show? Does it or other instruments change when you shut the key off?
You indicate that the engine continues to run, but then you write later that the problem begins only when you start the car. So, when does the engine 'recover' from this condition? Only when you step on the brake? Or, eventually by itself? How long does the engine continue to run if you leave the brake pedal alone? The brake pedal thing seems possible, but not a certainty.
Do you have a workshop manual or access to one? I found one online through Google. Many a non-professional mechanic has no clear idea of what goes on electrically in these cars. They are complex, and frankly, needlessly so. I worked in the late sixties and early seventies for one of the first dealers of these cars, and I have seen some mighty strange things go on with them; too many to list! I've loved and owned several of them (2000s) and had a 'flock' of customers which I inherited when that dealer went out of business. I also have a dealer's tool board with all the special tools present, plus a few duplicates! (not pertinent, but nice to recall)
One strange example of starter/ run issues: We had in a 1970 3500s that would sit quietly for fifteen minutes or so and then suddenly start up and sit there patiently waiting for someone to drive it or shut it off! It drove us crazy untill we discovered that when the clock winder triggered, the car would start. I cannot remember the remedy, but we did eventually discover its curse. (Incidently, this can be quite dangerous, because the steering lock stays engaged.) The point is that because they are so needlessly complicated, it becomes a mind numbing experience trying to figure out the seemingly endless permutations and possibilities. If you're working without a paper manual, you are truly handicapped. See if you can at least get a printed out version of the diagram and the key. We'll do better that way, I'm sure. And don't worry, we'll get it!
A sudden thought occurs: How are your brake lights working? One of the things that runs off of that #62 fuse is the brake light switch. You indicate that stepping on the brake shuts the engine off. How quickly? You may have a short in the brake lamp wiring or switch that causes the voltage to drop precipitously or completely, and that may be enough to stop the engine. Have someone stand behind the car with it running. Now shut the igition switch. If things are as suspected, the brake lights may come on dimly and briefly or not at all. Then the engine stalls. If so, you have a short in the brake wiring or switch, and that just may not be the cause of thee running 'continuation'. Try disconnecting the brake lamps at the switch and see what happens. Unfortunately, there are other possibilities. But we need to bring more information to the table. You also say that you recently replaced the ignition switch. Was the problem present before you installed the new one? Check those connections again!
Yes, these electrical systems are full of gremlins. Such are those days between standard carburation and fuel injection. The electrical engineers went nuts!
I'll help if you like, or leave you alone like any other annoyance. Just tell me..
