3.2 sensor question

[RickM]

I'm a bit confused on the description of sensors.

Below is a pic of the TDC sensor and the Speed sensor. Both identical parts used for different purposes. The TDC sensor used during start-up (the shiny one) and the speed sensor (positioned over the teeth) used for determining RPM for the DME while running.

(Pic credit to Ianc)






Now for the question: On the top is the TDC/speed sensor part (A) I just mentioned and below it is another sensor also described as a TDC sensor (B) for the 3.2. Is this correct? If not what is this second sensor used for?

[Wil Ferch]

It could be the "other" sensor that you find when peeking into the engine compartment... directly in front of you...under the throttle body. There is a "capped" connector going nowhere...and its a sensor connection that gets hooked to Porsche dealer diagnostic machines. I believe it measures TDC but is in no way hooked up ( actively) to the car's on-board systems...

Others here confirm ??? .....

- Wil

EDIT...see also this, for sensor names/locations ---> Which connector is which?

[RickM]

Interesting bit of info. Thanks.

Will a bad CHT sensor also result in a crank/no start condition?

[Wil Ferch]

I *think* so Rick.....If you look around here or on Rennlist....you are advised ( in a pinch) to fab a hook shaped wire with a a specific-value resistor ( 1000 ohms ?) to mimic an "in between" limp-home value...and gets the car to run. You supposedly unplug the CHT and jumper the connections with this wire/resistor...

Again...anyone with more direct experience confirm ?? I'm away from my notes....

- Wil

[Wil Ferch]

Rick:
Here are two more links with pics of this sensor...

What's this test port for?

Quick question on engine plug

- Wil

EDIT--> here's another with "no start" troubleshooting chart
Stranded! DME Ignition Guru Help needed

[DonMo]

The CHT will definately keep the car from starting as well as the speed sensor. The speed sensor tells the motronic that the flywheel is moving and the engine needs fuel. No movement, no fuel. The CHT when it fails tells the engine it is warm and makes starting almost impossible. Once warm, you can jumper the CHT with a paper clip and the car will start and run fine.

[RickM]

OK, great links and testing procedures. Thanks Wil and Don.

BTW, I recall the "What's this connector for?" post but I didn't connect that with the pic of the unused TDC sensor....good catch, Wil.

[Lorenfb]

"Will a bad CHT sensor also result in a crank/no start condition?

ABSOLUTELY NOT! It just causes an over rich condition (open) when hot.
A shorted one causes hard cold starting, but NEVER no-start.
Come-on guys, think about it.

Check here for more info: www.systemsc.com/diagnostic.htm

[RickM]

Originally posted by Lorenfb
"Will a bad CHT sensor also result in a crank/no start condition?

ABSOLUTELY NOT! It just causes an over rich condition (open) when hot.
A shorted one causes hard cold starting, but NEVER no-start.
Come-on guys, think about it.

Check here for more info: www.systemsc.com/diagnostic.htm

Loren,

Others here have posted that they solved a crank/no start condition by replacing the CHT sensor. Then reading one of Wil's links it states a too lean or rich condition is the result of a bad sensor.

This is why I ask the question.

Are you stating that these other observations are inaccurate?

Also, can you shed some light on the unused TDC sensor?

[Wil Ferch]

I also don't understand Loren's remark..the "no-start" troubleshooting chart refered to via link above.....apparently from Bosch/Porsche....shows all the sensors and sensor condition faults that may cause "no-start"....

?????

- Wil

[RickM]

Wil,

I guess the "Engine does not start or starts hard" depicted on the Bentley chart can be interpreted different ways.

But if I read right others have what appears to be first hand experience with a no start relating back to the CHT sensor.

[rick-l]

Is any one else sick of Loren contributing nothing and sitting there like a vulture ready to pounce on DME and electrical problems that may generate income?

Contributing??? You would expect some one with all that experience to occasional contribute something like "when there is no spark it is typically cracked solder joint at xxx or yyy". Info like that must be considered a trade secret.

Although it usually results in a "do a search of Pelican for ignition repairs" to me it is obvious where it will steer you (systemic.com) and blatant and offensive as someone posting "I'll sell you that for $$$

[Lorenfb]

"Are you stating that these other observations are inaccurate?"

YES! The Bentley manual has MANY errors.

"Is any one else sick of Loren contributing nothing and sitting there like a vulture ready to pounce on DME and electrical problems that may generate income?"

Typical of most on this website! Always have all the "answers", but NONE of the correct ones.

"Flip the coin" again, & may be:
1. A missing +12 volts on DME pins 1, 18, 35, (bad DME relay) or
2. A missing ground on DME pins 5, 16, 17, or
3. A bad speed sensor on DME pins 8 & 27 (1000 ohms/2.0VAC), or
4. A bad reference sensor on DME pins 25 & 26 (1000ohms/.100VAC), or
5. A speed sensor plugged into a ref sensor connector or opposite, or
6. A bad ignition coil, or
7. A bad ignition switch (no +12volts on coil), or
8. A missing flywheel pin, or
9. A bad +12 volts on the injectors, or
10. A bad DME ECM.

Start flipping or use a proper troubleshooting methodology, i.e. versus
listening to everyone who guesses and has "learned" from "searching" the internet.

Bottom line: Let's just solve the no-start problem which in all likelihood is NOT #10.

If it is a bad part, check with Pelican Parts for all your parts needs (new/rebuilt),
as they have multiple sources for most/all Porsche parts. The key is to ALWAYS
call (John) & ask if it's not shown on the on-line catalog.

Note: Once the problem is solved, disconnect/short the temp sensor and
see how the engine won't start - NOT!!!!!!!!!!!!

[JTL]

Rick,
I was a little confused with all the replies to this post. There are 4 sensors involved here: First, the 2 in the pic located at the flywheel with black wires, flywheel reference sensor and flywheel speed sensor. Both are part A in the second pic. Second, TDC sensor located at 12 o'clock position above the flywheel. Part B in the second pic. Third, cht sensor, which you can't see in the first pic but has a white wire and is located behind the black rubber plug.

jt
-84 targa

[Wil Ferch]

JTL:

Yes..you described them correctly...

....3 active ( speed and reference sensors near the starter/flywheel teeth, plus white wire CHT screwed near a cyl hd.)..
...plus the fourth TDC ref.."not-hooked-up" ( inactive) unit that has a cap-plug front and center under throttle-body.

- Wil