TDC "pointer" sensor on 3.2 (did you know?)

75Carrera · 75Carrera's Garage

If you have a 3.2 you might be interested in this information.

I have a '86 3.2 in my 75 911. The TDC or "pointer sensor" just failed and the car wouldn't start. This is one of two sensors on the flywheel housing. After replacing the sensor it fired right up, my top end increased by 15-20 MPH, throttle responce is amazing, cams are kicking sooner and no more back fire or crackling on decelleration... Who'd a thunk it? Apparently that sensor tells the DME when the #1 is at TDC and is function is sort of a timing sensor. I guess they can fail without going completely out and send you searching for hours to resolve the issues I listed here. Have you heard this before?

der Mond · 09-27-2006, 01:40 PM

Never knew that but this will be another saved piece of data for the library. Thanks!!!! Hey, do you know my old friend Steve LeFleur who owns Prescott Import Car Service?

75Carrera · 75Carrera's Garage

Interesting isn't it? I wonder if there are many cars out there suffering from reduced preformance or with owners chasing "no start" issues when it could be something like this sensor? The sensor is a magnet and it only makes sense it could wear out or get weak over a 20 year period, eh? I've been a Porsche owner for many years and never heard of anything like this. Even my wrench was amazed.

I don't know Steve but know the shop. Never heard anything +/- (that's good in a small town).

KTL · 09-27-2006, 02:18 PM

Those sensors are a VERY important part of the car's fuel injection and timing system. They fail, the car doesn't run.

The way the sensors typically fail is for the protective covering on the wires dries and cracks from the heat of the exhaust system. Moisture then gets into the wire and corrosion starts.

der Mond · 09-27-2006, 02:46 PM

Can one of you provide a photo or description of where the part is located and how hard it is to replace? Also, how to get a new one? Not an engine drop I hope...

75Carrera · 75Carrera's Garage

The actual sensor is titled a "reference sensor" and mounted to the left side of the flywheel housing. New from Pelican it's $141.00. But I got a used one from yard for $30 that tested perfectly. It is hard to get to but certainly no engine drop required. It is one of those vital parts we seldom hear about.

Dtrain475 · 09-27-2006, 04:18 PM

I'd be interested in a pic too. I have my flywheel housing out right now and I'd like to check it out.

Pelican no longer has it in stock and there is no pic associated with the part number-

Thanks

Jeff · 09-27-2006, 04:48 PM

It is not hard to replace this sensor. The hardest part is getting the gromet on the firewall out/in. Tape the old wire from the sensor to the new sensor wire and pull it thru.

Sorry, I don't have any pictures but, if you pull off your driver's side rear wheel you will see the reference and speed sensor by the fly wheel.

Also if you replace both sensors, make sure you connect them to the correct plug.

Jeff · 09-27-2006, 04:52 PM

AND, The other end of the sensors connect in the driver's side of the engine bay. There will be a 3 plug connector that houses your Reference, Speed and Cylinder Head Temp. sensors. If you have the one wire CHT replace it with the 2 wire unit, it will save you down the road.

ianc · 09-27-2006, 06:56 PM

There are actually two sensors there: the reference sensor and the speed sensor.

The reference sensor is only used when the car is first starting up. It keys on a small allen screw sunk into the side of the flywheel and tells the DME where TDC on #1 is.

More critical is the speed sensor, which gives the DME information about engine RPM and gets its input from the toothed wheel.

Both senors are the same part # and mount in the same bracket. It is a bit of bear to do it with the engine in the car. I would recommend replacing both at the same time if you're in there; for the amount of work it takes (unless you have the engine out), it doesn't make sense to use a used part or just do one unless you're really hard up for cash. Static testing of these parts will not always reveal a failure either; they really need to be checked on an oscilloscope.

When mounting be sure you route the cable housings so they are not touching the hot intake manifolds. The gap between the sensors and the flywheel must be adjusted with a feeler gage. There is some play in the mounting bracket to allow for this.

Here's a pic of mine when I was finishing up my top end a couple months ago:

HTH,

ianc

der Mond · 09-28-2006, 08:27 AM

Thanks, gentlemen! I just had my shocks out for replacement and put new grommets in with the new head temp sensor. I wondered what those other two wires were. The OEM wires from the sensors you show are each labeled with an identification band on each side of the 3-wire wire connector harness on the upper left intake manifold that also holds the head temp sensor wire (white wire shown) so they cannot be mistaken. Maybe the new ones attached to the sensors do not have these bands. My motor runs great at present and I don't have the heart or the money to do a proprietary chaneout at this time, but this is really valuable information to keep for the future. Thanks again!!!!

KTL · 09-28-2006, 09:15 AM

Quote:

Originally posted by der Mond
..................Maybe the new ones attached to the sensors do not have these bands................

You are correct. Since the sensors are interchangeable, a replacement sensor is not labeled. But it's not a big deal. If you replace them (or simply unplug them) and plug them in incorrectly? The car won't start and no damage is done. Switch the plugs and it should then start.

Great description and picture, Ian. I would also add that it is important to keep the wires routed without sharp bends. The heat from the exhaust system is more likely to cook the sheathing than the intake manifold- which incidentally stays cool or only mildly hot to the touch (only on hot days or track days). So i'd recommend rotating the clamp holding the wires to widen the bend in the black sensor wires?

goat · goat's Garage

Very interesting.
The sensors are wrapped much like a coaxle cable. an outside sleave that protects it. I know last time I looked the housing was cracked on mine but figured there was no problem, With you first remarks on the replacement I may have to rethink this and replace it. good to know.
But a good used one will on my shopping list those new ones are pricey.

75Carrera · 75Carrera's Garage

My wrench said it's easy enough to check the sensor with a meter (out of the car) to determine if it's functioning properly. Sorry I don't recall what the range was exactly so I won't offer any unsubstanciated numbers. I believe the Bently addresses it if you have one. A clean, used one with no breaks or wear on the wiring leads should be very servicable. Everyone I talked to said these are "rare" to fail but when they go you're not going anywhere. You should be able to find used sensors in good shape in the $25-40 range as opposed to $141 each from Pelican and probably more from our friendly competers.
Great pictures from ianc too...

MBEngineering · 09-28-2006, 11:28 AM

HI do'not forget to set the gap correct between the teeth and the end of the sensor or the bad starting/missfire/iratic running will be back.

regards mike

ralphedo · 02-16-2021, 10:06 AM

I appreciate the idea of using the Bosch BMW part in place of the Porsche Part number for the Flywheel position and RPM sensors, and based on all the posts it will work fine, BUT can anyone confirm the difference in the length of the cable between the original part and this BMW substitute? I am certainly happy to save $45 bucks for the identical sensor ($90 for two) but not sure I want an extra foot of cable coiled up in my engine compartment just to save the money. If the Bosch Part number is different I am guessing the cable length must be the reason. I am getting ready to order them but just wanted to confirm the lengths??

GH85Carrera · GH85Carrera's Garage

Just the lengths of the cables is the difference.

Hnichols · Hnichols's Garage

Quote:

Originally Posted by ralphedo

I appreciate the idea of using the Bosch BMW part in place of the Porsche Part number for the Flywheel position and RPM sensors, and based on all the posts it will work fine, BUT can anyone confirm the difference in the length of the cable between the original part and this BMW substitute? I am certainly happy to save $45 bucks for the identical sensor ($90 for two) but not sure I want an extra foot of cable coiled up in my engine compartment just to save the money. If the Bosch Part number is different I am guessing the cable length must be the reason. I am getting ready to order them but just wanted to confirm the lengths??

As mentioned above, the cables for the BMW sensors are a tad longer, which in my view is actually an advantage over the Porsche part. Getting the BMW sensors is a no-brainer.

Quasimoto · 12:47 PM

Curious what the purpose of the 3rd sensor is...the one mounted at 12 o-clock on the flywheel?

(more like 11:30)

In the fiche it's called 911-606-108-00 "Timing Mark Sender"

wjdunham · wjdunham's Garage

Quote:

Originally Posted by Quasimoto

Curious what the purpose of the 3rd sensor is...the one mounted at 12 o-clock on the flywheel?

(more like 11:30)

In the fiche it's called 911-606-108-00 "Timing Mark Sender"

That sensor is rather strange, it's not connected to the DME, there is a connector mounted on the throttle body to which one can hook up a piece of diagnostic equipment (O-Scope?). It's only purpose would be to verify that the screw in the flywheel is in place so one knows if the reference sensor isn't sending a signal to the DME that the screw fell out?

I can't think of another use for it.

Bill