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Peanut; correct, there is no ground wire on the sensor itself.
However, DME ground reference used for the sensor reading are taken relative to (from what I recall) MP IX or MP X.
(Now where's that DME schematic.... )
I misunderstood your post sorry.
yes those ECU ground points are critical . Its so easy to dismiss them as unimportant when troubleshooting problems but many problems end up being poor connection issues particularly on grounds
From memory it is the MP1 pump MP2 and MP3 but I think for a 1983 model they may be in a different place than your diagram .I may be wrong .
This is the ground you guys are referring to
so is there a way for me to isolate the ground as the issue or the sensor itself as the. Issue without a scope? Or is it best to schedule to have the car taken to a shop with one?
because the speed sensor and the associated connector and wiring was the initial issue and looks to be likely to still be the problem it would definitely be worthwhile putting a scope on it . You'd need to put the scope on the first connector block and the ECU plug as well . It doesn't have to go to a garage in fact it would be better not to ....anyone that works with electronics radios TVs etc will have a scope and they are portable and the test is very simple and straightforward . it should only take a few minutes .
If it were my car I'd go over the connections with a continuity meter and check the speed and reference sensor wiring from the sensors to the ECU connector first
because the speed sensor and the associated connector and wiring was the initial issue and looks to be likely to still be the problem it would definitely be worthwhile putting a scope on it . You'd need to put the scope on the first connector block and the ECU plug as well . It doesn't have to go to a garage in fact it would be better not to ....anyone that works with electronics radios TVs etc will have a scope and they are portable and the test is very simple and straightforward . it should only take a few minutes .
If it were my car I'd go over the connections with a continuity meter and check the speed and reference sensor wiring from the sensors to the ECU connector first
okay I’ll see what I can do. When I looked up scopes they were like 4 grand that’s as much as I paid for the car! Hahahaha. Alternatives were cheap scopes off of amazon/ eBay or up into the 100$ to 200$ range. Acquiring one might be a bit of a challenge but I’ll see what I can do.
The DME side is literally brand new from the factory. I’ll take my multimeter back to the speed sensor.
The reference sensor is brand new as well lol.
I know there’s a lot between these two threads but I actually did already test the speed sensor and even the old reference sensor with a multimeter to check continuity and they both tested good. None the less I will retest.
so is there a way for me to isolate the ground as the issue or the sensor itself as the. Issue without a scope? Or is it best to schedule to have the car taken to a shop with one?
Not sure if that's proper on the early; The late looks a little like this:
A = MP IX, B = MP X
Take them off, 200 grit then 300 grit sand paper to both sides, the bolt, the housing, clean off with a lint free towel, replace.
Checking the ground requires some decently long scope probes;
Pin 24 and the grounds in the picture you took (if that is the proper ground) should have absolutely 0 ohms between them.
@ Indes
yes its the second image for a pre 85.5 944 it would be best to edit and remove the first image in case it causes confusion for others that arrive at this thread later with similar problem.
The reference sensor supplies the ECU with a TDC crank position to provide correct timing .it will not be the cause of this issue as the car would not start and run so cleanly if the timing was out.
Its the speed sensor we need to concentrate on as that controls the ECU to operate the fuel pump If the speed sensor pulse is absent it can cause the ECU to shut down the fuel pump.
@Jacob you don't need a fancy expensive scope to test the speed sensor . There are plenty of $25 digital scopes on ebay that will do the job but why add extra stress to this ? Ask a local auto electrician what he would charge to test your speed sensor with a scope.
Setting up a scope and testing for the correct sine wave is not a simple job if you've no experience of using a scope. Once you have a scope set up correctly all you need to do is place the 2x probes on the sender connector and start the engine . You would be looking for around 2.5v sine wave as the Porsche test guide states . If that test confirms the sender and wiring is ok then disconnect the ECU and apply the same test to terminals 8 and 27 of the ECU plug (not the ECU terminals )
You cannot test the output of a speed sensor with a multimeter it needs a scope . A multimeter will only test continuity and resistance it cannot tell you the correct signal pulse strength that the ECU needs . The output AC voltage should rise and fall with engine speed but a multimeter is not quick enough to show this
Do you know what brand of speed sensor is in there? If it's the FAE, I'll bet you're having the same issue I did where the sensor's outer shield would have continuity with one of its connector pins. This caused the sensor to display a half sine on the 'scope and the DME to reject its signal. A known problem with some FAE sensors.
Dan the sensors are 100% correctly orientated. I took a sharpie and wrote DG BG on each sensor plug respectively and matched them to the labels on the harness.
Ill order that small scope that you referred me to in that other thread off of eBay.
Until that comes in i wont be able to test the speed sensor output. I can however show you guys the resistance reading I got between pins 8 and 23 on the speed sensor plug.
Seems to def show an issue if I’m not mistaken.
According to Clark’s it should be anything greater than 1M ohm.
In fact, I have zero resistance readings. OL on pins 8 and 27 as well. Should be 600 to 1600.
ok thanks. i just wanted to eliminate the MAF sensor . if the ECU doesn't see a signal from the MAF showing air flow through the throttle body on cold start it would shut down the engine
The only other component I can think of that could also influence cold starts is the Air Idle Control valve If it is closed when starting from cold then that could cause the engine to immediately die but as you applied throttle the engine will have got some air from the throttle body so I guess it cannot be the AIC valve either........
We must be nearly there ....it has to be something really simple now that we are missing ...really frustrating
ok thanks. i just wanted to eliminate the MAF sensor . if the ECU doesn't see a signal from the MAF showing air flow through the throttle body on cold start it would shut down the engine
The only other component I can think of that could also influence cold starts is the Air Idle Control valve If it is closed when starting from cold then that could cause the engine to immediately die but as you applied throttle the engine will have got some air from the throttle body so I guess it cannot be the AIC valve either........
We must be nearly there ....it has to be something really simple now that we are missing ...really frustrating
Tell me about it. I’ve been chasing my own tail on this for over a year now lol. I don’t want to cling onto anything but does the fact that the oil pressure light doesn’t come on after the car stalls mean anything? Only reason I ask is because whenever I have stalled the car before, that light has always come on. It doesn’t now. That’s the only thing I can think of that’s strange/ abnormal.
02-22-2019, 06:49 PM
