No output from reference sensor

mcmahonl · 10-12-2006, 02:28 PM

I've got an 86 944 turbo that won't start. Using a number of valuable post in these forums it seems I'm not getting any output from my reference sensor. I've got an analog volt meter that when I set to AC 15 volts I can read the output from the speed sensor OK. I've replaced the reference sensor (although I don't think the old one was bad anyway) and I still can't read any output. If I remove the sensor and tap it against something metal the meter will see the signal.

So my question is does anybody know what could cause this?

It would seem like if something happened to the gap the speed sensor wouldn't be signaling anything either. I've not tried to reset the gap because I've no data to indicate it would have changed

Lawrence Coppari · 08:51 AM

Seems to me that if the sensor produces a signal when it nears a ferrous metal, then something has to be wrong with the clearance or the union of the leads in the engine bay.

I've never had to remove the sensors but I have had problems with the union that is several inches from the sensor. Do the sensors get actuated by the same cog?

todd951 · 10-14-2006, 11:45 AM

I second Lawrence's opinion. If you're confident that gap is ok, and you get a signal when it's connected to a meter, then the likely cause is a bad connection between the sensor and the DME. I would try putting the sensor back on the car, then looking at the output on the meter while cranking the car. If you can still see the signal on the meter, then it's probably the connection.

mcmahonl · 10-14-2006, 02:38 PM

I'm not sure what activates the sensor. I can't seem to find my Haynes manual and I've not had any luck finding any pictures of the mechanism on the flywheel that the reference sensor picks up.

I wouldn't say I'm confident about the gap just that I have no evidence anything would have changed it.

I'm measuring the sensor output at the sensor. I can read a signal from the speed sensor which has it gap set in parallel to the reference sensor which isn't sending a signal. This while cranking the engine.

If I switch the speed and reference sensors I have the same problem. Speed sensor produces a signal, reference sensor doesn't. This while cranking the engine. Actually, I have three sensors and they all produce a signal when they're used as the speed sensor.

951and944S · 10-21-2006, 01:09 AM

The "pickup" is a set screw (4mm iirc) raised above the surface of the flywheel. The speed and reference sensors pickups are on two different planes so your test result is certainly very possible.

On non turbo cars with rubber centered clutch discs, my guess would be that a piece of rubber was blocking the sensor tip but since you have a turbo this is not possible.

Short of removing the sensors and/or mounting bracket to inspect the set screw presence or height (8mm iirc), maybe you can remover the starter and inspect from there.

I've never tried it.

If you want a picture of the sensor pickup set screws, email me at 944t at bellsouth dot net.

TS

soxnail · 10-21-2006, 08:50 AM

I think even if your ref sensor is good you won't see it on a VOM because the duty cycle i.e repetition rate is much less than the speed sensor. So there is not enough signal to show on the VOM (it works by RMS (root mean square) ) so it has to have a signal that is large enough in amplitude and repetition rate, which in this case depends on the speed of the car cranking, and is not enough. Long story short you need a scope or you could use a sound card (mic input) you will need to check the spec on your sound card. the two signals are posted below.

thedge · 10-21-2006, 11:24 AM

VR sensors can be tested with multimeters. Connect the multimeter to the two leftmost pins according to the pic on clarks garage (the ones that get 600-1600 ohms). Then you need a wrench or pliers or something, bring the wrench close to the tip of the VR sensor then jerk it away. Youll get a pulse on the multimeter. You have to move the wrench closer and further fairly quickly to get a good pulse. Nothing will show if you just touch the two together and leave them there.

http://clarks-garage.com/shop-manual/ign-02.htm

Edit: Yes ive done this myself on 951 VR sensors.

soxnail · 10-21-2006, 06:31 PM

I agree that the pulse will make the VOM register, but what you need to do if you want to be certain is to look at the input at the DME of the sensor, and unfortunately no substitute for a scope. My sensor was showing a signal at the connector next to the fire wall engine side, and it was showing a signal at the DME, but it was just a little weaker 1.volt peak to peak (some of the time ) drove me nuts

I finally just replaced the connectors at the fire wall for both sensors. No more problem.

mcmahonl · 10-24-2006, 06:53 PM

Update so far...

I bought an oscilloscope and have verified the reference sensor is actually OK. I've even checked the signals at the plug to the DME and both speed sensor and reference sensor are OK at the DME plug.

However, I have no no spark. Here's what I've verified (or had verified)

1.) DME relay works. I applied voltage to both sides and checked that if completes the corresponding circuit
2.) Coil works - completing grounding circuit manually causes spark
3.) DME rebuilt and tested. I had previously found a burnt trace and actually sent the first replacement back and had the DME reverified.
4.) Ignition switch works. I have power at the coil with the key 'on'
5.) Started solenoid signal OK. I have power at terminal 4 of the DME plug when cranking.
6.) Distributor cap was removed and visually inspected.
7.) Ignition wires are relatively new and tested.

I'm not sure what to check next.... any ideas?

soxnail · 10-24-2006, 07:17 PM

Check that there is a ignition input to the KLR on pin 9 and if so then is there an out put at pin 16 KLR. Let me know what you find out.
ps. worst case there is a limp home mode to test your system pull the KLR and run a jumper from the pin 9 klr to pin 16 klr this should let you start up and go into a low power "get home mode".

mcmahonl · 10-25-2006, 12:04 PM

When you check for ignition input on pin 9 of the KLR I assume you mean I should I should unplug the KLR and use one probe on pin 9 and ground the other.

What am I looking for and when?

I only had a few minutes this morning. When I unplugged the KLR and put one probe in pin 9 and grounded the other I was getting a 60Hz sin wave of about 3.5 volts. And the key wasn't even in the ignition.

The only other thing I did was jump pin 9 to pin 16 with the KLR unplugged and still got no spark.

How do I test output of pin 16? What am I looking for?

Thanks for the suggestions!

soxnail · 10-25-2006, 12:19 PM

Sorry I should be more clear..
Check the input to the KLR from DME (ignition signal) Should be a square wave if the car was running. If it is not running as is your case the the frequency will depend on the crank speed. The 60hz sine wave sounds like an AC hum being picked up by a bad ground or if you have a battery charger hooked up. If you see no signal in to the KLR then of-course you will get nothing out on pin 16. Try looking at the pin 9 without the KLR hooked up. (just in case the input of the KLR is shorted) see pic. try that and let me know. My understanding is :
1) you have the right signal from the speed & reference sensor. (amplitude and frequency).
2) The DME relay (fuel pump) comes on for a few secs when you turn on the ignition.
3) no signal on pin 1 of the DME connector.

soxnail · 10-25-2006, 02:47 PM

Also see if your tach needle bounces when you crank the car...

mcmahonl · 10-26-2006, 05:17 AM

OK more data and clarifications...

1.) The tac bounces while cranking
2.) I've not verified anything about the fuel pump but I have pulled the DME relay and applied voltage to both circuits and verified that it completes the corresponding circuits using the instructions from clarks-garage
3.) According to my oscilloscope the signals from the reference and speed sensors fine when tested at the plug to the DME
4.) I don't know how/what to test for at pin 1 of the DME

I put the scope on pin 9 of the KLR when unplugged and got a flat 9 (ish) volt signal when cranking (after removing the trickle charger)

mcmahonl · 10-26-2006, 06:20 AM

The KLR may be bad but jumping pins 9 to 16 which, according to SK, should allow it to start did not work so it would seem like there is something else wrong or am I missing something here?

soxnail · 10-26-2006, 07:36 AM

"1.) The tac bounces while cranking
2.) I've not verified anything about the fuel pump but I have pulled the DME relay and applied voltage to both circuits and verified that it completes the corresponding circuits using the instructions from clarks-garage
3.) According to my oscilloscope the signals from the reference and speed sensors fine when tested at the plug to the DME
4.) I don't know how/what to test for at pin 1 of the DME

I put the scope on pin 9 of the KLR when unplugged and got a flat 9 (ish) volt signal when cranking (after removing the trickle charger)"

Pin 1 of the dme is just the output to the ign coil. THis is very weird because if your tac is bouncing then pin 9 on the klr should have a signal, same connector. Also if nothing on pin 9 klr then jumping it won't do any good, as there is no signal in so there will be no signal to jump to pin 16. I am confused, if tac is getting a signal but pin 9 klr shows flatish 9v some thing is no computing, I know you have done this already but humor me...
Remove the klr connector, connect the scope probe to pin 9 and crank the car (trickle charger disconnected) should be a square wave of 9 to 12 volts. If not then connect the probe to pin 21 of the dme and do the same test (remeber not to connect the klr). This will eliminate the connection from the dme to klr. If no signal then the tac is getting some funky stuf from somewhere else...
good luck and patience

soxnail · 10-26-2006, 07:41 AM

Sorry also connect the scope to fuse 34 in the fuse box. Set it to trigger on a positive going signal. turn the ignition on there should be a positive going pulse which will go away in a few hundred mili secs.
Let me know what you find out.

mcmahonl · 10-26-2006, 03:43 PM

User error on the scope. Here is what I got from pin 9 on the KLR.

Still no start when I jumper 9 to 16.

Should I still do the fuse 34 thing?

soxnail · 10-26-2006, 05:15 PM

Ok now we are geting some where...
put the scope on pin 16 KLR with the KLR in and tell me what the scope does. (nice scope btw)
if there is a signal similar to the one pin 9 then check pin 1 dme same way. the fuse 34 is just to confirm that the fuel pump is comming on momentarily. let me know I will leave my computer online so if you post it will let me know. Lets see if we can knock this out tonite..
luck

mcmahonl · 10-26-2006, 07:19 PM

SK,

$99 USB scope should've got it sooner.

I looked at one of your other threads and saw how to test pin 16 with the KLR plugged in. I was missing the part about removing it and taking it apart.

Anyway, that and dinner were a bit of a delay. The output on pin 16 was flat 9 volts ish