Could use some help from 944 wiring experts.
10-08-2018, 10:56 PM
#1
Rennlist Member
Thread Starter
Could use some help from 944 wiring experts.
I have a weird issue where after I key off to kill the engine, it stays running.
My car isn't wired like a stock 944, but the issue is in the factory wiring. Here is the breakdown:
- I have a Megasquirt setup, which is fed by a custom relay panel. The main relay is switched by 12v coming from the factory coil +12v, at the location where the coil wiring connects to the stock coil. Coil is gone though. This comes right from the ignition switch with no relay in between. I have had this setup installed for 3+ years and this is a new issue, it was fine before a few months ago when this started.
- With the car running, I see normal ~14v between this wire and ground. When I key off, it decreases to ~5v but this is enough to keep the main relay from closing. This is why it stays on.
- When I key off (and the car stays running) and I break this connection (ignition coil +12v), the engine dies immediately like it should. I have verified with a multimeter that there is still voltage between this wire and ground after key-off. However - this ~5v only remains when the engine is running. As soon as I break the connection (or some other connection that leads to the engine dying) it drops to nothing.
- I replaced the ignition switch, no change.
- A thought was that maybe there's some EM induced voltage on the wiring somewhere? I checked this by running a temporary wire from the coil +12v to my MS relay board so any of my custom wiring would have no effect on this power wire. No change.
- This implies that the factory wiring, downstream of the coil +12v, is getting ~5v somehow after key-off. I traced the wiring up to the firewall and into the cabin but see no smoking gun.
- Even if having some residual voltage is normal, I swapped my main relay with a known good part and it does the same thing. Again.... never had this problem until recently.
- When I key-off the fans stop running. So at some level there is a break in power flowing out of the ignition switch.
Tl;dr something is causing a few volts to stay on the coil +12v even after I key-off. It's not normal because this is a new issue. Ignition switch was swapped with no change so there must be a wiring issue somewhere. Does anyone know where the best place to start looking would be? Because the power wire I chose comes right from the ignition switch this implies a weak path to another switched +12v source (creating a circuit where it always has 12v until you break one connection manually) or some other back-feeding of voltage into the wire.
I may just take switched 12v from another location but my wiring is nicely wrapped and tucked the way it is, I would prefer to actually fix the true issue. Any suggestions are appreciated!
My car isn't wired like a stock 944, but the issue is in the factory wiring. Here is the breakdown:
- I have a Megasquirt setup, which is fed by a custom relay panel. The main relay is switched by 12v coming from the factory coil +12v, at the location where the coil wiring connects to the stock coil. Coil is gone though. This comes right from the ignition switch with no relay in between. I have had this setup installed for 3+ years and this is a new issue, it was fine before a few months ago when this started.
- With the car running, I see normal ~14v between this wire and ground. When I key off, it decreases to ~5v but this is enough to keep the main relay from closing. This is why it stays on.
- When I key off (and the car stays running) and I break this connection (ignition coil +12v), the engine dies immediately like it should. I have verified with a multimeter that there is still voltage between this wire and ground after key-off. However - this ~5v only remains when the engine is running. As soon as I break the connection (or some other connection that leads to the engine dying) it drops to nothing.
- I replaced the ignition switch, no change.
- A thought was that maybe there's some EM induced voltage on the wiring somewhere? I checked this by running a temporary wire from the coil +12v to my MS relay board so any of my custom wiring would have no effect on this power wire. No change.
- This implies that the factory wiring, downstream of the coil +12v, is getting ~5v somehow after key-off. I traced the wiring up to the firewall and into the cabin but see no smoking gun.
- Even if having some residual voltage is normal, I swapped my main relay with a known good part and it does the same thing. Again.... never had this problem until recently.
- When I key-off the fans stop running. So at some level there is a break in power flowing out of the ignition switch.
Tl;dr something is causing a few volts to stay on the coil +12v even after I key-off. It's not normal because this is a new issue. Ignition switch was swapped with no change so there must be a wiring issue somewhere. Does anyone know where the best place to start looking would be? Because the power wire I chose comes right from the ignition switch this implies a weak path to another switched +12v source (creating a circuit where it always has 12v until you break one connection manually) or some other back-feeding of voltage into the wire.
I may just take switched 12v from another location but my wiring is nicely wrapped and tucked the way it is, I would prefer to actually fix the true issue. Any suggestions are appreciated!
Last edited by odonnell; 10-08-2018 at 11:25 PM.
10-08-2018, 11:16 PM
#2
Intermediate
The stray voltage may be coming from the alternator.
I had the same issue (key off engine stays running) when I installed a new Digital MSD 6A to replace the 35 year old MSD 6A that finally failed.
To test this possibility, turn the key off s l o w l y to verify if the engine stops running as it should.
If it stops as it should with this test, install a diode between the charge light and the alternator (doesn't hurt anything at all).
Wiring Diagram: charge light end (anode) -->| -- (cathode) alternator / you install this in the existing little blue wire
This diode will block any residual EMF generated by the alternator as it attempts to wind down. As MSD notes in their troubleshooting instructions, this residual EMF may be enough to keep electronic ignitions powered, or as in your case, your relay.
I had the same issue (key off engine stays running) when I installed a new Digital MSD 6A to replace the 35 year old MSD 6A that finally failed.
To test this possibility, turn the key off s l o w l y to verify if the engine stops running as it should.
If it stops as it should with this test, install a diode between the charge light and the alternator (doesn't hurt anything at all).
Wiring Diagram: charge light end (anode) -->| -- (cathode) alternator / you install this in the existing little blue wire
This diode will block any residual EMF generated by the alternator as it attempts to wind down. As MSD notes in their troubleshooting instructions, this residual EMF may be enough to keep electronic ignitions powered, or as in your case, your relay.
10-09-2018, 11:57 PM
#4
Rennlist Member
Thread Starter
Alternator exciter wire turned out to be a dead end, I tried it, but I greatly appreciate the suggestion.
Ealoken, no relay is here between the ignition switch and the +12v switched power to the coil. Going to just find another switched +12v source and hope it's cleaner.
Ealoken, no relay is here between the ignition switch and the +12v switched power to the coil. Going to just find another switched +12v source and hope it's cleaner.
10-10-2018, 09:43 AM
#6
Not an expert, per se, but I have spent a lot of time buried in the schematics!
What year 944 is the wiring based on? If early, here's a few suggestions - The coil (normally) is powered from the 15 bus within the fuse / distribution panel. The other items on the 15 bus are the Fuel Pump Relay, Fuse 8 - Hazard Flasher/Radio/Sunroof/Cruise Control, Fuse 9 - Brake Lights, Horn Relay (own fuse), and Electric Window Relay (G3). I would pull fuses / relay to see if you are getting some kind of back feed from one of these components. (Fuel pump relay may need to be swapped. Can't see pulling it to test!) It's worth asking - Are you getting any voltage from the coil lead with key off / engine off?
If it's a later model, I'll dig in once I have an idea what year to look at.
What year 944 is the wiring based on? If early, here's a few suggestions - The coil (normally) is powered from the 15 bus within the fuse / distribution panel. The other items on the 15 bus are the Fuel Pump Relay, Fuse 8 - Hazard Flasher/Radio/Sunroof/Cruise Control, Fuse 9 - Brake Lights, Horn Relay (own fuse), and Electric Window Relay (G3). I would pull fuses / relay to see if you are getting some kind of back feed from one of these components. (Fuel pump relay may need to be swapped. Can't see pulling it to test!) It's worth asking - Are you getting any voltage from the coil lead with key off / engine off?
If it's a later model, I'll dig in once I have an idea what year to look at.
Last edited by jmc1590; 10-10-2018 at 09:46 AM. Reason: Update info
10-10-2018, 10:30 AM
#7
Mike, the #15 buss line is hot while cranking. The other hots while in the on position goes dead while cranking. In the meantime a 10 ohm, 20 Watt resistor on the #15 buss line away from anything that could burn should clamp down on any stray voltage until you find the source of the back feed.
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10-10-2018, 11:14 AM
#8
Rennlist Member
Thread Starter
Thanks guys. This is my 1983 944, so I'll investigate that #15 bus. Must be something backfeeding.
I have indeed checked is the ~5v is present with key off and the car off, and it is not. So something that also only receives power when the engine is running must be the cause here. My fuel pump is run off a separate relay as well (DME relay fully removed). The ECU triggers the relay to open or not just like a stock DME, and the relayed power comes from outside the original fuse block. The power then goes from the relay into the stock FP wiring harness (tees in near driver side door sill). However because the DME relay has been physically removed there should be no connection from this wire to the rest of bus 15. I have also tried pulling the fuel pump fuse (again, on my custom relay panel) to see if that was backfeeding but the car just slowly sputtered and died like you would expect.
I have indeed checked is the ~5v is present with key off and the car off, and it is not. So something that also only receives power when the engine is running must be the cause here. My fuel pump is run off a separate relay as well (DME relay fully removed). The ECU triggers the relay to open or not just like a stock DME, and the relayed power comes from outside the original fuse block. The power then goes from the relay into the stock FP wiring harness (tees in near driver side door sill). However because the DME relay has been physically removed there should be no connection from this wire to the rest of bus 15. I have also tried pulling the fuel pump fuse (again, on my custom relay panel) to see if that was backfeeding but the car just slowly sputtered and died like you would expect.
10-10-2018, 12:09 PM
#9
Rainman
Rennlist Member
Rennlist Member
maybe related...
on my car i had intermittent dying problem, turned out that the actual pin socket in the electrical board for the black wire from ignition switch to coil (it does run through this board) was burnt and making spotty connection.
my theory was that the MAF kit i had been running off the black wire maybe pulled a little too much current versus what that circuit was originally designed for, because i made so much power with the MAF
i ran a new black wire from the ignition switch directly to the coil and problem solved.
there are other wires tied into the black wire at the ignition switch - i think they go to the gauges, and maybe the FP relay. so you might have some backfeeding there.
on my car i had intermittent dying problem, turned out that the actual pin socket in the electrical board for the black wire from ignition switch to coil (it does run through this board) was burnt and making spotty connection.
my theory was that the MAF kit i had been running off the black wire maybe pulled a little too much current versus what that circuit was originally designed for, because i made so much power with the MAF
i ran a new black wire from the ignition switch directly to the coil and problem solved.
there are other wires tied into the black wire at the ignition switch - i think they go to the gauges, and maybe the FP relay. so you might have some backfeeding there.
10-10-2018, 09:46 PM
#10
Mike, how did you determine that it wasn't the alternator excite wire feeding back? From what I'm reading it sure sounds like it. Did you try removing the charge light bulb?
10-11-2018, 01:14 PM
#11
Thanks guys. This is my 1983 944, so I'll investigate that #15 bus. Must be something backfeeding.
I have indeed checked is the ~5v is present with key off and the car off, and it is not. So something that also only receives power when the engine is running must be the cause here. My fuel pump is run off a separate relay as well (DME relay fully removed). The ECU triggers the relay to open or not just like a stock DME, and the relayed power comes from outside the original fuse block. The power then goes from the relay into the stock FP wiring harness (tees in near driver side door sill). However because the DME relay has been physically removed there should be no connection from this wire to the rest of bus 15. I have also tried pulling the fuel pump fuse (again, on my custom relay panel) to see if that was back feeding but the car just slowly sputtered and died like you would expect.
I have indeed checked is the ~5v is present with key off and the car off, and it is not. So something that also only receives power when the engine is running must be the cause here. My fuel pump is run off a separate relay as well (DME relay fully removed). The ECU triggers the relay to open or not just like a stock DME, and the relayed power comes from outside the original fuse block. The power then goes from the relay into the stock FP wiring harness (tees in near driver side door sill). However because the DME relay has been physically removed there should be no connection from this wire to the rest of bus 15. I have also tried pulling the fuel pump fuse (again, on my custom relay panel) to see if that was back feeding but the car just slowly sputtered and died like you would expect.
Fans might be your source of 5v as they wine down?
10-21-2018, 05:33 PM
#12
Rennlist Member
Thread Starter
Thanks guys for the ideas. Now to rig up a diode segment for that wire. I'm wondering if this implies something has failed within the alternator or voltage regulator. I don't want to put a bandaid on it but for now the car still seems to charge fine.
10-21-2018, 05:43 PM
#13
Rennlist Member
Thread Starter
To follow this up: I noticed that when I removed the exciter wire, my charging/warning light in the dash didn't come on. A few years ago I accidentally ripped off the exciter wire when I changing motor mounts, and the warning light came one when this circuit was broken. Is it possible that the light itself is causing an issue here? I know it's a "special" bulb and maybe there's some diode inside it that failed leading to my issues.
10-21-2018, 08:40 PM
#14
Mike, a diode won't be bandaid fix, infact one should have been installed at the time when you replace the coil with a relay. Why the run on just started now, who knows. Maybe the spring in the relay just got a bit weaker. I don't know if the charge light is any kind of special bulb. I doubt anything special is needed for that circuit. I also doubt a diode is in the excited circuit. It's not the wiring diagram and the ignition coil loaded down the #15 buss circuit enough not to be a problem.
10-27-2018, 08:39 PM
#15
Rennlist Member
Thread Starter
Very happy to report that the diode trick has solved this problem! Thanks again edalenta! I added the diode near the brake booster, where the exciter wire goes into the 2-pin connector.