Intermittent total loss of all power (dead pedal but tach still up)
11-27-2016, 03:15 PM
#31
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I'd be more inclined to suspect the DME itself (not the DME relay). Or the coil despite it having been swapped. The coil driver has to do a lot of work so it makes sense it could fail when warm. A fault with the coil driver could maaaaybe damage the coil too and that'd explain why you chased the problem away for a while with a new-used coil.
You should be able to wire a test lamp or LED to the coil and run a pair of wires up and under the rear of the hood so you can see if the coil is going dead. You could get some old plug wires and wire a spark checker up similarly if you really wanted to, or perhaps more easily hook up an inductive timing light with the trigger wired down. Pass the wires though the back of the hood and tape the timing light to the windshield so you can see it flash.
Some multimeters have frequency and voltage pulse to the coil and that would be interesting when the car is not restarting. Again this could be wired up so you can see it in the car.
If that sounds like too much of a pain you could be ready to whip in your other coil and see if it starts right up and runs a while until the other coil gets hot too. You can measure the resistances of the hot coils that are failing as well.
...with that in mind watching coil current could be really interesting also.
You should be able to wire a test lamp or LED to the coil and run a pair of wires up and under the rear of the hood so you can see if the coil is going dead. You could get some old plug wires and wire a spark checker up similarly if you really wanted to, or perhaps more easily hook up an inductive timing light with the trigger wired down. Pass the wires though the back of the hood and tape the timing light to the windshield so you can see it flash.
Some multimeters have frequency and voltage pulse to the coil and that would be interesting when the car is not restarting. Again this could be wired up so you can see it in the car.
If that sounds like too much of a pain you could be ready to whip in your other coil and see if it starts right up and runs a while until the other coil gets hot too. You can measure the resistances of the hot coils that are failing as well.
...with that in mind watching coil current could be really interesting also.
11-27-2016, 03:23 PM
#32
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I'd be more inclined to suspect the DME itself (not the DME relay). Or the coil despite it having been swapped. The coil driver has to do a lot of work so it makes sense it could fail when warm. A fault with the coil driver could maaaaybe damage the coil too and that'd explain why you chased the problem away for a while with a new-used coil.
You should be able to wire a test lamp or LED to the coil and run a pair of wires up and under the rear of the hood so you can see if the coil is going dead. You could get some old plug wires and wire a spark checker up similarly if you really wanted to, or perhaps more easily hook up an inductive timing light with the trigger wired down. Pass the wires though the back of the hood and tape the timing light to the windshield so you can see it flash.
Some multimeters have frequency and voltage pulse to the coil and that would be interesting when the car is not restarting. Again this could be wired up so you can see it in the car.
If that sounds like too much of a pain you could be ready to whip in your other coil and see if it starts right up and runs a while until the other coil gets hot too. You can measure the resistances of the hot coils that are failing as well.
...with that in mind watching coil current could be really interesting also.
You should be able to wire a test lamp or LED to the coil and run a pair of wires up and under the rear of the hood so you can see if the coil is going dead. You could get some old plug wires and wire a spark checker up similarly if you really wanted to, or perhaps more easily hook up an inductive timing light with the trigger wired down. Pass the wires though the back of the hood and tape the timing light to the windshield so you can see it flash.
Some multimeters have frequency and voltage pulse to the coil and that would be interesting when the car is not restarting. Again this could be wired up so you can see it in the car.
If that sounds like too much of a pain you could be ready to whip in your other coil and see if it starts right up and runs a while until the other coil gets hot too. You can measure the resistances of the hot coils that are failing as well.
...with that in mind watching coil current could be really interesting also.
11-27-2016, 03:33 PM
#33
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Joel, all interesting ideas that I have thought about too; except doesn't my tach being up mean the DME coil driver is doing its job? The tach and the DME ground both get their pulse from the coil driver; indeed they share a wire. So, if the tach is operating normally; doesn't that mean the DME is doing it's job of trying to pulse ground to the coil?
11-27-2016, 03:38 PM
#34
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Right. At this point I'm leaning toward it either having been a faulty DME relay, or its an issue with the wire from the ignition switch that provides constant, key-on +12v to the coil. Not really anything else it could be at this point.
11-27-2016, 03:50 PM
#35
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I suppose the ignition pulses go to the tacho and also to a higher power coil driver MOSFET or something. Everything could be working other than the high current part that grounds the coil.
I think a DME relay failure would drop out the whole DME, as I recall the DME relay energizes the DME though one coil and then energizes the fuel pump after a signal comes back from the DME indicating ignition pulse. If true then with no DME then you get no tach pulse. However if it is a fuel issue it could be the fuel side of the DME relay dropping out.
Seems like the DME relay would have been tested and eliminated many moons ago though.
I think a DME relay failure would drop out the whole DME, as I recall the DME relay energizes the DME though one coil and then energizes the fuel pump after a signal comes back from the DME indicating ignition pulse. If true then with no DME then you get no tach pulse. However if it is a fuel issue it could be the fuel side of the DME relay dropping out.
Seems like the DME relay would have been tested and eliminated many moons ago though.
11-27-2016, 03:55 PM
#36
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Can any DME wizards confirm/deny this? I thought I had conclusively ruled out the DME; but if I need to get my hands on a borrowed DME to test with, I need to get the ball rolling so I need to know if the above quote is even a possibility.
it's not as easy as you think to test these things because the issue only crops up on 2+ hour drives, so I cant exactly just swap all these parts and take it for a spin down the street to test. This whole thing is extremely frustrating. My entire experience owning this particular 86 NA has been extremely frustrating..nightmare 944. If not for all the trouble free years of fun with my previous 944 cars, I'd be absolutely 110% turned off from them after the past 2 years with this car.
11-29-2016, 10:42 PM
#37
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Well, I have something to report. I swapped the DME relay with a new one, and it had a way more dramatic effect on the way the car ran than I would have bargained for. My car is set up to run without an IAC and usually idled at ~1100 when warmed up and would sit right on .5 GPH(40 mpg) on the economy meter. After I put the new relay in, I fired it up and it settled into a nice, smooth 950 rpm idle. Before, it wouldn't idle smoothly at that low rpm. It was also now sitting at ~.3 gpm(45 mpg) on the economy gauge. The car felt smoother driving. I took it for a 75 mile trip today and it ran the best it ever has. I'm guessing the old DME relay had a heat related bad solder joint that was causing a crappy connection that was cutting power to my fuel pump; which I incorrectly assumed was an ignition related issue.
Only time will tell if that really was the issue, but it definitely affected the car so I'm inclined to think so. That's what you get with a Uro brand DME relay..
Only time will tell if that really was the issue, but it definitely affected the car so I'm inclined to think so. That's what you get with a Uro brand DME relay..
12-15-2016, 04:11 AM
#38
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so, I think I finally got this one nailed down. I rigged up a test light to the switched 12v side of the coil, to see if I was losing power to the ignition coil when the engine stalled. I drove the car around some and was able to get it to fault, but alas...the light stayed lit. The issue isn't with the wire feeding power to the coil.
What the hell else could it be...well, today I learned that if you unplug the blue DME temp sensor, it stalls the engine instantly, like someone cut the key. The engine will not start with the sensor unplugged, and if you plug it back in and then attemp to start the engine, it will struggle and sputter for a few seconds before it catches and fires properly, even if all you did was unplug it, plug it back in, and try to start the engine. I tested this a bunch of times, and this is always the behaviour I observed, though I can't explain why the car seems to "know" the sensor had just been unplugged, even if you plug it back in before attempting to start. Jiggling the connector on my car, I was able to induce bucking and missfiring as the sensor momentarily lost contact on it's pins; and finally I was able to induce the engine stalling just by jiggling the harness plug on the sensor. This behaviour is 100% identical to the beviour that I've been observing as a fault "in the wild" right down to the reluctant restarting. I do believe I nailed the issue. I had no idea the DME temp sensor was so important to the engine running after it was warmed up.
On a side note, I also confirmed the DME will continue running with only the speed sensor plugged in. Indeed, the engine will not start without BOTH sensors working but once idle is achieved, unplugging the reference sensor has literally zero effect on the motor running. Just thought I'd let everybody know; and I will be doing more research on this subject to better understand the (obviously) vital way in which the DME uses info from the DME temp sensor to run the engine. I think this is important info, because it certainly isn't well known stuff that's been disseminated into the community general body of knowledge. If any of you guys pick up a new car that wont start, or you experience a no start on your car and the normal troubleshooting doesnt solve it, the DME sensor is one more thing to add to the list of things to check out than can definitely cause a no start with tach bounce present.
Anyway, solved. 2 months to the day from when the issue first cropped up.
What the hell else could it be...well, today I learned that if you unplug the blue DME temp sensor, it stalls the engine instantly, like someone cut the key. The engine will not start with the sensor unplugged, and if you plug it back in and then attemp to start the engine, it will struggle and sputter for a few seconds before it catches and fires properly, even if all you did was unplug it, plug it back in, and try to start the engine. I tested this a bunch of times, and this is always the behaviour I observed, though I can't explain why the car seems to "know" the sensor had just been unplugged, even if you plug it back in before attempting to start. Jiggling the connector on my car, I was able to induce bucking and missfiring as the sensor momentarily lost contact on it's pins; and finally I was able to induce the engine stalling just by jiggling the harness plug on the sensor. This behaviour is 100% identical to the beviour that I've been observing as a fault "in the wild" right down to the reluctant restarting. I do believe I nailed the issue. I had no idea the DME temp sensor was so important to the engine running after it was warmed up.
On a side note, I also confirmed the DME will continue running with only the speed sensor plugged in. Indeed, the engine will not start without BOTH sensors working but once idle is achieved, unplugging the reference sensor has literally zero effect on the motor running. Just thought I'd let everybody know; and I will be doing more research on this subject to better understand the (obviously) vital way in which the DME uses info from the DME temp sensor to run the engine. I think this is important info, because it certainly isn't well known stuff that's been disseminated into the community general body of knowledge. If any of you guys pick up a new car that wont start, or you experience a no start on your car and the normal troubleshooting doesnt solve it, the DME sensor is one more thing to add to the list of things to check out than can definitely cause a no start with tach bounce present.
Anyway, solved. 2 months to the day from when the issue first cropped up.
12-15-2016, 10:32 AM
#39
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Although it sounds crazy, you might try changing the fuel filter. I had a similar problem many years ago. The car would run great around town, but out on the open road, at 60 or so, the car would go about 1/2 mile, then die. I'd pull over to the side of the road, with ignition on, and try for a restart. It would do this, run about another mile or so and quit again. Seems that the fuel filter was partially plugged and would allow enough gas for around town driving. But when the fuel demand got higher it couldn't keep up enough flow and would kill the engine. Pull both filters and recheck to make sure. Seems crazy, but that's what happened to me. After reading your last post.. Sounds like that you have the coolant sensor wiring harness/sensor itself to blame on this one.
12-15-2016, 11:59 AM
#40
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For possible future reference I am not sure monitoring B+ at the coil is much of a test of anything but the ignition switch. With a multimeter and long leads you might be able to see the coil primary voltage pulsing. A flashing spark checker is also great to have in the car since you might have high voltage problems even if the coil primary field is working as expected.
12-15-2016, 01:03 PM
#41
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For possible future reference I am not sure monitoring B+ at the coil is much of a test of anything but the ignition switch. With a multimeter and long leads you might be able to see the coil primary voltage pulsing. A flashing spark checker is also great to have in the car since you might have high voltage problems even if the coil primary field is working as expected.