I'm running a ground wire for my Innovate Auxbox to the same ground point used for the dme's various sensors (aka the dme ground). After studying the wiring diagrams for the '86 951, I'm still unsure which grounding point is used for the dme sensors (collectively) - MP VIII or MP IX.

According to the ground point diagram:
MP VIII --
Engine Block
DME Electronic Ground (or "DME Electronics Chassis", as described in another similar diagram, below)

MP IX --
Clutch Bell Housing
Batterie Cable LHD
Also DME Cable to Ground

I believe that one dme ground is for the sensors and the other for the dme enclosure (the box). From the wiring diagram, it looks like the sensors ground at MP IX which, despite its location in the diagram as being further forward than MP VIII, shares its ground point with the larger battery ground wire on the larger of the two bolts ("B" in the photo below), which is further rearward on the bell housing.

So...
Can anyone confirm that the dme sensors (via dme connector pins #16, 17, and 19) ground at MP IX (larger bolt "B" in the photo)? And that pin #28 grounds the dme enclosure at MP VIII (smaller bolt "A" in the photo)?

Thanks in advance for your replies!

 

use either one- both are grounds. One is a 10mm head bolt and one is 13mm head. Pick the one that fits best!

 

Yes, both are grounds, but I need to attach to the dme sensor ground point so that my auxbox grounds share the same ground reference as used by the car's factory sensors.

On the auxbox, there are differential ground inputs for each external sensor used. The ground reference for any particular sensor needs to come from that sensor's ground wire. Since all of the sensors ultimately ground at one of these two points, I'm running a ground wire from there to a ground bus where each channel of the auxbox will have its ground reference attached. These ground references don't actually ground the auxbox unit or the sensors themselves. They are ground references, which only reference each sensor's ground, so as to eliminate ground offsets. Most of Innovate's equipment (including the auxbox) are extremely sensitive to ground offsets. There are hundreds of posts on Innovate's forums about the neccessity of correct grounding. Just picking another ground point on the engine will likely result in erroneous data. Even sharing the same correct ground point with the wideband O2 sensor's heater ground will skew data results, as the heater ground carries more current than any of the sensor ground references, and will create a ground offset. Innovate equipment is very accurate, but must be installed correctly.

Anyone know for sure which ground point is used for the dme sensors? I believe it's the larger one ("B" in the photo").

 

Is it correct that point "A" in the photo in my first post is MP VIII and that point "B" is MP IX?

 

try this.

pull the dme plug and disassemble it, and then undo one of the two BH grounds and test for continuity at the pins for the sensor grounds. if that doesnt work try the other one.

or if you can wait a few days i can go home and look at my spare wiring harness for you.

 

I was hoping to avoid the extra task of doing the continuity tests, but it's probably best if I do. That way I can also check the individual sensor grounds to confirm where they ultimately terminate. (Really I was originally hoping that ground point "A" was the one, so I wouldn't have to pull the IM again. "A" is accessible, but "B" would require intake removal.)

I hope to have time tomorrow to remove the intake.

Thanks for the offer to check your spare harness. If it's not too much trouble, it would be nice to have the info, especially if I can't get the time tomorrow. (I always heard that "You can't go home." )

 

Chris,
If you needed to pick up a sensors specific ground would you not have to tap it at that point? The grounding points shown are common for other items, so would that not give an inaccurate reading? And if not then why would it matter which one as they are muddied currents at that point anyway?

Just wondering, let me know what you find.

 

This is the way it is shown in the diagram for the '86 engine wiring harness, and when I disassembled mine I didn't note any discrepancies in this area.

The circuit diagram I have for the DME shows all the ground pins (5,6,16,17,19 & 23) routed to a common circuit ground. For example, the filter circuit for the O2 sensor draws its reference from the common circuit ground, with no indication in the diagram of any special routing of the ground from pin 23 (which connects to the sheath surrounding the O2 sensor signal wire).

That said, I've never opened up a DME to see how its actually wired on the inside. So reality may differ from the diagram I have.

 

Yes, that is ultimately the most accurate ground reference. I was looking for a way to get nearly the same accuracy without having to tap as many wires. I could just ring-terminal at the bell housing/block and add as many grounds to the bus as needed. Innovate's forums stress that the grounds should connect to "the same piece of metal" (i.e., block) and on the same lug as the sensors. The wideband O2 sensor heater ground should be grounded also to the block, but on a separate lug, as it draws significantly higher current. The only extra piece in this scenario is the ground bus, but a bus was also mentioned by Innovate's head developer (Klaus) as being acceptable. I also get the impression that it might depend on the signal type and a variety of other environmental factors.

In the end, I think that anything short of tapping each sensor's ground wire may involve some trial and error (which I'm trying to avoid). I may just do this, after all.


Thanks very much for confirmation.

I'm curious about a dme schematic. I've never seen one anywhere.

 

Here you go, download 951digit.pdf and 951power.pdf
http://www.the944.com/ml31dwg.htm

You will find that the sensor ground connections are connected to a common ground plane at the DME board connector. The reason for a signal, ground and shield connection is related to noise. Running a twisted pair of signal and ground with a outside shield gives the lowest noise level in most cases.


You don't need separate grounds for each sensor since they are low current devices. Devices that pull high current do need separate grounds. Devices like WB heaters pull many amps and you will have a small voltage drop in the wire since it has resistance. If the sensor (like a WB) has a small signal, and you use a common ground wire for the heater and the sensor, you will get a small error in your readings. The large gauge wire, the lower the resistance.

 

Does anyone have descriptions of where all the ground points are located the map above provided by cb951 shows there location but not where exactly they are or how to get to them

 

I'm reviving this post becuase I have the same question. I'd like to clean all the grounds but I don't know how to get to all of them. anyone know?

 

The diagram points to the locations and the diagram gives a description of were to find the grounds. You just need to look and find them. Now, maybe a question would be I can't find point x, but they really are were they say they are. Not trying to be rude here, it is really easy to find the main ground points if you look for them.

 

Fair enough - As I read my post this morning I agree with your assessment! I should have been more clear. I'll see if I can find them all this morning...

 

Just wish to revive this thread, as I am trying to find the earth for my 944S2, and the link to pdf above as expired. I have tried looking down behind the engine block from above, but can't see any wires... guessing I need to lift the car?