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I have a 1984 944 NA that has recently been put back together and I cant seem to get a reliable fuel injector pulse for either injector set. When the ignition is turned on, I have tach bounce and my oscilloscope says that the speed and reference sensors are doing what they are supposed to. I also have spark and fuel pressure. Moreover, when the ignition is turned on I have 12v on both the power and ground/DME terminals for all 4 injectors, which is correct, but when cranking, the DME/ground terminals never pulse to 0V. I tested the injector DME/ground wires and both have <0.25 ohm resistances from the injector plug to their respective DME pins. I have tried to get a pulse using both a F9Tech sport DME as well as the stock 1984 DME, and I have tried to use both a solid state DME relay as well as the OEM electro-mechanical one to no avail. Getting the same result with both types of DME and relay leads me to believe that the issue is not with either of them. My next plan of attack is to check all of the DME pins marked 'ground' and see if they are actually grounded while I crank. I suspect that the DME switches the injectors from their 12v state to one of these ground pins to get a pulse, which my schematic says would be pins 16, 17, and 19. If anyone has any other potential suggestions, I will happily entertain them.
If it's injector negative pulse, which would be unusual, you should be able to get it to start and run a bit with a squirt of fuel or starting fluid. Then you could try clicking the injectors with a wire to ground and starting. If you can get it to run by manually firing the injectors that verifies that you have everything you need other than an injector pulse.
If that works I would try ohming out the injector harness and then the DME-side harness. Maybe the injector harness is crumbling and shorted.
Great diagnostic work. I agree that the ground side of the DME needs to be investigated. I believe the injectors have a separate ground path from the electronics or coil, but absent a DME schematic I cannot tell you specifically which pin or pins. Grounds to the DME are on on pins 5, 16, 17 and 19.
I gave each injector a couple squirts using a 12v drill battery and I was able to get a couple coughs of life from it. All of my resistance tests have been showing low/no resistance between the engine side of the harness and the DME plug, but I am going to do some more investigating by the 9 pin multiplug located above the brake booster which has a bunch of engine management wires.
t I am going to do some more investigating by the 9 pin multiplug located above the brake booster .
The 12V to the injectors runs through this and is a known source of issues. Check the 12V at the injectors as you are cranking. If it dips significantly using your oscillosope, this is a likely issue. If it doesn't, then back to the DME grounds.
A 12v drill battery is a monster source of current, I test injectors with a 9v smoke detector battery or a small wall-wart power supply and a momentary switch.
It would be easy to pop an injector like a fuse with a drill battery.
A DMM can be confusing for problems like this as it might show 12v in a circuit where it might actually drop to nothing if there was a meaningful current draw. A test lamp that draws a bit of current like a 1156 bulb and a pigtail would be good. A DMM would be good for looking for a shorted injector harness or the like though, but you would need to disconnect all the injectors as they look like shorts.
I am not crazy about only getting a couple of coughs out of the engine having given it fuel. Makes me think there is a different issue. I suggest getting it to start and run on alternative fuel, starting fluid, carb cleaner, gas in a squirt bottle, etc.
During cranking, none of the DME ground pins, 5, 16, 17, 19, and 28 showed a voltage more than 0.3v, so I do not think it is a grounding issue. However I jumped them straight to the negative battery terminal anyway to see what happens, and I was able to get a faint illumination out of a noid light on one of the injectors, and the light got dimmer with every pulse. The light was more dim than I have seen on the injectors of other cars, so I don't know if it is enough to pulse them. Does anyone have experience with how bright the noid light is on their 944 when testing an injector?
The scope was showing no less than 11.2 V during cranking on the red/blue hot injector wire. I believe that correcting some groundings has solved my fuel injector issue as the scope now shows the voltage dropping from 12 to 0 momentarily, meaning they should be firing.
I have now determined that my incompetent reassembly has led to another issue. While the timing mark on the flywheel at TDC appears in the bell housing port, the flywheel pin which triggers the reference sensor, indicating TDC is not appearing through the sensor hole concurrently. I timed the cam marking to the flywheel marking, but I forgot to make sure that the Hall effect pin is also correctly positioned. By my guess, the pin is about 30 degrees ahead of where it should actually be. The photo shows the cam positioning when the pin appears under the sensor. I don't have a photo of the other time it appears, but the cam gear should be rotated 180º for the second time. I'm not really sure how I managed this, to be honest, but if there is a way to put the flywheel on incorrectly, I did it. Time to drop the transmission!!
I thought the flywheel was keyed making such a mistake impossible?
Its normal for the cam gear to be off 180 rotation every other rotation of the crankshaft. The crank turns twice for each rotation of the cam. This is why finding TDC requires both the cam and the crank mark line up.
Try rotating the crank one more revolution, my guess is the cam mark should line up. Based on your photo it would appear you are 180 out from TDC.
Last edited by walfreyydo; 06-20-2024 at 10:58 AM.
. Based on your photo it would appear you are 180 out from TDC.
He's saying it looks 210 out. It's probably 225 out, with the flywheel bolted on 45 off, since there are 8 bolts. I wonder if the locating pin can be pushed back on the crank if you do this. I also wonder about the valves at this point.
On second thought, the valves are likely OK if the crank was locked at TDC before disassembly, and camshaft set from that. Maybe the locating pin was missing from the crank.
orig944 is right, I’m saying that it looks to be about 210° off which would make it about 30° off when it comes back around. The reference sensor stud is not missing, just misaligned I don’t have valve contact when I turn the engine and at “TDC” the borescope shows piston 1 at the top and both valves closed. I also set the timing belt before I did the clutch, so my guess is just improper flywheel installation. I’ll have more information once I get the bellhousing off. My initial guess was that i put the flywheel on upside down and just flipped along the axis of the TDC mark, since the bellhousing TDC hole is about 15° off of the reference sensor hole, but after looking at flywheel photos I don’t think that is possible because the clutch is properly installed and functioning correctly.
Does your crank not have a dowel/key to align the flywheel correctly as shown in the below photo? This is why I am confused as there shouldn't be any way you can misalign the flywheel unless you are missing this dowel
Ignore the leaky rear main, it was an image I found on google and unrelated
Last edited by walfreyydo; 06-20-2024 at 02:15 PM.
06-18-2024, 08:13 PM
No Injector Pulse
