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Speed Reference Sensor and Tach Bounce

I've checked the speed and reference sensors per Clark's and got the following:

DG: 8-27: 35K ohm
8-23: 0.9M ohm

BG: 25-26: 154K ohm
25-78: 153K ohm

I guess I thought the car wouldn't run, or would run a lot worse at these values. This might explain the random cutting out of the engine I'm experiencing! Just wondering what kind of number anyone has gotten on their failing sensors.

Also wondering what magnitude of Tach bounce I should see when cranking the engine? The Tach moves but is barely noticeable.
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Last edited by 944_science; 02-06-2009 at 09:01 PM..
Old 02-06-2009, 08:54 PM
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While checking the continuity or resistance of the flywheel sensors may give you an indication, the only real test of the sensors is by using an oscilloscope, and comparing the resultant output to that in the factory service manual.

Any other test is not definitive.
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Old 02-07-2009, 03:41 AM
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I am going through this also. Well at leats your car runs, haha. I just tested mine, Clarks garage, with an ohmmeter and got nothing. 0 Ohms, and no voltage. This mean that they are done? Or like you said, the only real test is with an oscilicope.
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Old 02-11-2009, 01:32 PM
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8(25)-27(26) should be 0.6 to 1.6k Ohm, and
8(25)-23(78) should be more than 1M Ohm.

Replace your sensors.

Beamer ones are half the price.
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Old 02-11-2009, 01:44 PM
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I've been thinking of "borrowing" an oscilloscope from my lab to test the voltage output as 944 ecology suggested but don't think my boss would like that very much. Besides it's pretty large and not very portable. I hate to replace the sensors without knowing for sure but at this point I tested/replaced pretty much everything else. This would explain the tach dropping to zero/car dying and just random shutting off of the engine.
Outback Porsche, during troubleshooting I ran into a post about using BMW sensors same Bosch part # and as you said half the price, thanks for the reminder!


Any of you guys resolder the DME? I took it out and the solder joints looked OK but a few were dull-gray in color as opposed to a shinny silver. I would rather not mess with it if it's not the problem but just wondering. I guess first step is replacing sensors, man I hope they come out without a fuss!
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Old 02-11-2009, 08:56 PM
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I gave my DME the once over with a soldering iron when I was having my intermittent start problem. I'm a IPC certified operator/inspector(equiv JSTD-001 for the mil spec lot out there) with a soldering iron so I'm supposed to know what to do with it.

I found some joints I'd consider suspect and cleaned them up but it made no difference to my fault. I also replace the electrolytic capacitors on the cct boards - After 26 years it's a good chance they've dried out and are no longer the correct value.

Just gave me peace of mind - probably worth it.
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Old 02-11-2009, 09:15 PM
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My sensors gave me grief initally, I sprayed some CRC (light machine oil) at them then left them for a day. They came out with no effort.
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Old 02-11-2009, 09:20 PM
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I'll start with the sensors and if I still have issues I will clean up the DME, I guess it's better than dropping big bucks on a rebuilt one! Replacing capacitors/resistors on the board might be a little beyond my skills, but so is everything until you get in there and do it! What's a good soldering iron? I have a relatively inexpensive electric iron type, solders come out OK, not the best.
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Old 02-11-2009, 09:29 PM
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Can understand about the component replacement but have a look at the electrolytic caps. If the ends of them have weeped or shrunk, then they need to be replaced. If you do replace them, remember they are polarized (take a photo so you remember the correct orientation). Find someone to help if your unsure, and when in doubt - leave it be.

Best if you can get a temperature controlled iron (@ around 300-320DegC). Make sure you use 63/37 or 60/40 solder to add to the solder joints if your just going to 'freshen' them up. Do not use LEAD FREE (Pb Free) solder to 'touch' up your joints. You will create a weak solder joint from the alloy it'll make.

Because the solder joints are old, you should use a liitle liquid flux to help the solder to flow (it removes contaminants, you'll get a better joint and it'll be easier to solder). If you can't get any flux, choose rosin cored solder (it's got flux in it - but not much).

Another tip - Dont spend more than 5 seconds with the iron tip on the joint/lead. You'll damage the circuit board and the pad will begin to de-laminate from the board.

Have fun.
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Old 02-12-2009, 12:50 AM
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Well, I can't seem to get the speed sensor (closest to the firewall) out. The reference sensor came right out, no problems, so I thought I might get lucky.

Speed sensor moves back and fourth about 20 degrees but I'm having trouble putting upward force on it to remove it. I'm using a long pair of needle nose pliers, coming in from the right top side of the engine, basically right next to the oil fill tube. Any suggestions besides brute force?
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Old 02-14-2009, 11:43 AM
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Take your time and try to use some PB Blaster on it and let it soak. When I pulled mine out there was enough of an edge built up around it from gunk inside the engine that made it difficult to move. If you can move it round & round & round with some upward force it will eventually end up coming out. Time, lubrication & pressure.
I would not try replacing components until going across the board with the soldering iron and reheating the joints. Then test the DME inside the car. Do make sure that you add a little clean solder to the end of the iron repeatedly and keep the end of the iron clean with a wet sponge. A hot iron will pull the solder up off the board and the junk at the end of the iron will clutter and join together any tight areas where you are working. A little flux will make it a LOT easier without heat buildup.
If you are worried about messing up the board, strip the cover off of an old piece of 8 gauge (or similar) wire about a half inch long in several places, leaving pieces of cover in between those areas. Imagine a road line such as the passing lane. Take the iron and practice soldering and covering up the wire along those places until you can do it smooth and have a good judgment of the iron and heat control. Then you will have a good idea of solder flow and it will be easier to just touch the board and melt the solder quickly.
Take a look at the chip that is in sockets. You can use a chip puller or small flat screwdriver to lift it up and re-seat the chip. It can be pulled out and put back in, BUT if you are not used to working with chips the legs are tiny and very easily bent. A small amount of pressure can bend a leg to the point that it breaks when you are placing the legs back into the sockets. To re-seat it only takes a small amount of movement. This will hopefully knock away any corrosion that may be stopping a good connection.
PM me if you have any questions. I have about a third of the schematics from http://www.the944.com/files/951digit.pdf drawn out with colored pencils and any questions would most likely help me as well as you. Good Luck!
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Old 02-14-2009, 12:56 PM
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I hit it with PB and am letting it soak, is there any tool you found best when removing it? The long needle nose pliers I'm using only allow me to move it slightly back and forth, and its too tight to move by hand. I don't want to destroy the black top of the sensor and end up with the magnet lodged in the bracket/bell housing. I guess time/solvent/force are my best friends at this point.

As for the DME, I will resolder it if my problems don't subside, otherwise I will just leave it alone. A schematic would help, I'm a grad student here at I.U. and we have a pretty good electronics shop that I have access to, if needed those guys can probably help me out too.
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Old 02-14-2009, 02:28 PM
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Unhappy Using pliers on the sensors.

The only schematic's I have are from the PET diagrams or http://www.the944.com/files/951digit.pdf . I am trying to map it out the best I can just to gain knowledge about it since mine whacked out, regardless of exact part match, an electronic theory type of thing. Outback & another guy gave me some feedback that knocked one of the "why didn't I think about that?" bells in my head when they mentioned the ignition timing and fuel injectors.

I don't know of any tools that may be used. That plastic top looks like it would break fairly easily with a lot of pressure twisting it on opposite ends, and even "wide" {16 Metric} pressure evenly applied on each side would most likely crush it and leave the magnet like you stated earlier.
See if you can use a tube like those on the 10W-40 cans and spray the PB as close as you can directly down on that "O" ring. It may be stopping the lube because the crap underneath has formed a tight bond.

Another thing you might try is using high pressure air like one of those cans you would use to blow out a computer. It would create a lot more force and push the lube down without having a gallon of lube sprayed all over the top of the sensor.

You don't have a lot of options if you are having to turn it with the pliers. Of coarse you know that since you have already taken the other sensor out. When I took mine out it took a long time while I was killing my knees and trying to turn the sensor. If you can get it to turn with your hand that will be a good sign and it shouldn't take much more force to get it out.
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Old 02-15-2009, 01:05 PM
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Thanks for the diagram, I will try to make sense of it when and if I get to the DME. After laying across the engine trying to pull the sensor I decided to remove the fuel rail and intake manifold/oil fill tube. I noticed some cracked hoses under the intake and was interested in the condition of the injectors.


This is the general condition of the injectors, the caps look good but everything was a bit dirty. You guys try cleaning them on your own or should I just send them to witchhunter? I was thinking of sonicating them in mineral spirits but wasn't sure if that would do much.
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Old 02-15-2009, 05:23 PM
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Well, with intake/fuel rail off getting at the speed sensor is a lot easier, plus I found some cracked hoses to ICV that I will replace. I've been reading a thread by h8tr_ who is having the same problem My new 944 and the reference sensor dillema need some advice from the Porsche guru's

My question is can I remove the bracket with the speed sensor still in it? I seem to be finding some conflicting info that with the speed sensor still in pace, the bracket wont move back towards the firewall enough to remove. Is this correct? It seems like just removing the bracket to get the sensor out might be a lot easier. I've hit it with PB a few time and let it soak but it's still impossible to move by hand, I really don't want to break it off. Any thoughts would be great.
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Old 02-19-2009, 10:36 AM
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Anyone?
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Old 02-19-2009, 02:40 PM
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from my own personal experience the last few days the taking out the bracket seems a no brainer but its hell unless you remove the bell housing that seems the easiest way lol alot easier than going through what I'm doing i don't have the means right now on going that route so I'm taking the bumpy road, tomorrow I'll try going at it again i'll let you know how it goes
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Old 02-19-2009, 02:53 PM
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Great, thanks, good luck to you.
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Old 02-19-2009, 03:53 PM
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Well I got the bracket out and ended up with two pieces similar to h8tr_ there is one issue though, I chipped the bell housing where the pivot sleeve goes while trying to get it out!! I'm not too happy about that I got the sleeve out and it looks like when I put a new bracket back the sleeve should go it far enough to hold OK. With the bracket out I rotated the sensor top and saw the metal body wasn't moving. Therefore I'm glad I didn't spend a bunch of time lubing and twisting to just end up with a broken top and having to break the bracket anyway.
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Last edited by 944_science; 02-21-2009 at 11:39 AM..
Old 02-21-2009, 11:24 AM
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As for the injectors, check locally for a heavy truck repair shp that does fuel injection service; they'll have the same type of equipment that whichhunter uses. You're looking for a place that can bench clean injectors. Call a couple of truck service places and I'm sure they can point you to someone locally (if they don't do it themselves)
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Old 02-21-2009, 11:51 AM
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