Which body wire is for the Crank speed sensor?

[RichHawk]

1987 911 3.2L Stock Motronics system. Getting ready to put the engine back in.

I had a lousy loose label on the wires that connect to the two crank triggers and the head temp sensor. When I pressure washed the engine bay last week, I blew the label off.

I know the engine won't run if you mix up these sensors. I'm hoping someone can guide me on how to identify which wire from the body goes to which sensor?

I kept the wires on the engine side in the correct positions on the bracket (Pictured) that bolts to the intake, so I'm hoping that Porsche kept the orientation correct AND hoping that the body wiring is somehow identifiable without tearing things apart.
The closest I can hope is that the two white wires shown below are always in the same orientation, left to right, in the factory wiring loom.






Scanned in from my Bently manual:


Can anyone tell me which body wire is the speed sensor, and which is the reference sensor?

Thanks,
Richard

[Tim L]

I would just plug them in, if it doesn't start switch the plugs.

[86 911 Targa]

Originally Posted by tim l

i would just plug them in, if it doesn't start switch the plugs.

+1

[big911fan]

If you didn't disconnect the plugs from the bracket in the engine compartment this may help. According to Bentley both sensors are electrically identical. The wires going to the engine compartment bracket should be labeled. The speed sensor is labeled 'DG' and the reference sensor is labeled 'BG'.

The bell housing should also be marked 'D' for the speed sensor and 'B' for the reference sensor.

The white connector on the engine compartment bracket is the CHT sensor.

I couldn't find a picture that actually showed which of the bottom two connectors were which. Hope this helps.

[RichHawk]

I didn't want to just do that, for fear of hurting the brand new cam shafts from extended cranking. (Per John D's advice)

I want to minimize the cranking time:
I have to crank to build oil pressure,
I have to crank to get the injectors to fire
I have to crank to get the spark to ignite.

What I don't want is to crank for 5 minutes as I try to figure out if the sensors are backwards.

Bently says the wires are the same color all the way back to the DME, so unless I ring out the wires with my meter via pin number from the DME to the body connector, I'm stuck with just guessing.

So I am hopeful someone else with a Carrera 87 or earlier can give me at least an educated guess.
Look at the my wires where all three sensors join at the sheath under the fuel filter area:
My wires go in order White, White, Brown. lets call them White 1, White 2, Brown 3.
Brown is the head sensor. easy.

If you have a Carrera and can check for me, does White 1 go to the bottom plug bolted to the intake, or the middle plug?

Thanks,
Richard

[RichHawk]

Pictorially, this is what I'm hoping you can look at for me..


Does this wire (White 1, not in the middle of the loom) go to the bottom connector or the middle connector bolted to the intake?

Thanks for taking the time to do this. Richard

[RichHawk]

Originally Posted by big911fan

If you didn't disconnect the plugs from the bracket in the engine compartment this may help. According to Bentley both sensors are electrically identical. The wires going to the engine compartment bracket should be labeled. The speed sensor is labeled 'DG' and the reference sensor is labeled 'BG'.

The bell housing should also be marked 'D' for the speed sensor and 'B' for the reference sensor.

The white connector on the engine compartment bracket is the CHT sensor.

I couldn't find a picture that actually showed which of the bottom two connectors were which. Hope this helps.

Thanks. Unfortunately the wire markers DG are on the engine side. and it matches to what you said, and what I drew with the yellow and red lines. There are no labels on my body side wires. I went as far as to examine every cm exposed to find any difference. There is none. They are identical on the body side. if there was a DG or BG on the body side of mine it's long gone.

Still looking for someone to look at their car as a best guess...

[big911fan]

I looked at my '89 Carrera if that'll help. The reference sensor is the BOTTOM of the three connectors in the engine compartment (labeled BG).

It looks like the yellow line on your diagram. If you think the '87's and '89's are the same, maybe this helps.

Since your brown wire has a white connector I would think it is the #3 CHT sensor.

All three of my wires are light grey. Only the CHT sensor has a white connector.

[Geronimo '74]

Cyl head temp is the white one
Speed sensor in the middle
Ref sensor is the bottom one

[big911fan]

I agree with Geronimo '74. I found a diagram in the Bentley manual that shows the sensors. P. 101-14 if you have the manual.

Top= CHT, Middle= Speed, Bottom= Reference

[RichHawk]

Yep I agree with all of that. Yellow line and red line are definite knowns. it's the blue lines that are the question..

Imagine you disconnected all three wires from that connector, How would you know which one fit where? The Body wires are not labeled. White plug Brown wire is definitely the Head temp sensor. That's easy. it's the small 2 prong plug it only fits in the top location. But what about the other two?

it's a long shot, but can you look at the orientation of the wires as they disappear in under the fuel filter. What is the order of the three sensors left to right as they enter the loom.

Pic again...

[big911fan]

Maybe I still don't understand it but can you "ping" each wire with a 12v current to isolate each one and plug them into their respective connectors? There's a fourth wire coming up near the fuel filter and it's the oxygen sensor wire that connects to a wire in that same area.

[steely]

Rich,
the orientation could have been changed on both sides on some vehicles, so even if they're clean and marked and visible, someone elses' could still be 'swapped'.

Last year I replace mine, and the one remaining label fell off of the dme side - I deduced what I had, and still boo-boo'ed, but as soon as I swapped them - she started right up - and it didn't take long. Honestly, you have nothing to lose and nothing'll get hurt.

[steely]

Here's a pick showing DG in the middle.
This is before cable replacement. The corresponding mate had a piece of tape I think.
If the pic doesn't turn out look for the blob of yellow label - it's marked DG at the rez I can see it with under the intake - speed sensor cable side.
HTH

PS..in German...(from Wil Ferch)
BG = Bezugsmarken geber ( reference mark identifier or "giver")
DG= Drehzahl geber ( revolutions identifier or "giver")

[RichHawk]

Thanks Dan. your picture is the closest to an answer. I'll plug my White #1 wire into the bottom connector as the reference connector and cross my fingers.

So It's still just a guess, 50:50 in my unfortunate experience means it WILL be wrong the first time. and if it's not wrong the first time, I'll swap it to be wrong in error of something else not letting it run.

richard

[steely]

Richard,

yeah - let us know how you made out - you can always blame me

[Por_sha911]

Have you tried using continuity on a multimeter to check which one goes where?