'84 911 Coupe Won't Start

[Cal44]

I 've been reading some of the posts about no starts, must be something in the air...........my '84 will not start, period. Turns over with gusto. There seems to be no spark. This is what I have done so far.

New fuel pump, What the heck, it was 25 years old anyway.
Traded another Motronic brain then traded back to see if the other car started, it did. So in my mind it can't be the brain.
New coil
New DME relay

Tested resistance on cap, rotor, new coil.

In the Bentley book it states to test the Reference and speed sensors.
The table for the ohms is on the same page. So, I removed the connectors on the senors went from one to two, one to three and two to three.
My results are "0". My Fluke reads nothing. It shows no resistance on either one.
I tried this with battery disconnected and connected, ignition off/ignition on.

What am I doing wrong? I hate to admit defeat and have it taken to the shop

[rick-l]

Originally Posted by Cal44

There seems to be no spark.

Is there actually no spark or it just seems that way?



The speed and reference sensors are just a coil of fine wire and a shield to isolate the signal on the wire from electrical fields in the engine compartment.

You should disconnect the sensor and measure the resistance looking into the sensor not into the DME. Ignition on or off in this case does not matter. The Bentley says ~960 ohms between 1 and 2 and open between these two wires and shield (pin 3).

[Cal44]

No spark when engine is cranked. I have one of the inline lights made just to check spark. With the speed and reference sensor unpluged and with the test leads touching on the terminals on the reference sensor side I get no resistance reading on the ohms, no matter which of the three terminals I touch. I find it odd that there is no reading of resistance. I am sure it is me doing something wrong. Hope my writings make sense. I would have posted on the other guys "won't start" but his won't start after it is warmed at least I think that is what he stated. Mine just won't.

[175mph951]

i just replied to another post like this....its usually not the coil inside them that goes bad....its usually the magnet.this cant be tested by resistance.ive remove numurous crank triggers from 944's and 911's that have no magnatism at all...they loose it over time..the older they get the less they are able to pick up the trigger on the flywheel...if they are original change them!!!

[rick-l]

I'm sure you tried just touching the leads of the meter together right?

[Cal44]

Yep. I touch the leads together and I get some numbers so I reckon it is fine. I have been testing my rotor and cap and they check fine. All of this is according to the Bentley specs. This is a new Fluke multimeter so I am thinking it is fine.

Now to the next reply. The magnet is in the sensor right? I don't need to change out the little nut on the flywheel....right? I just want to be clear. I was a pipeliner and not a mechanic but I need to learn.
So I need to change them both (sensors) I suspect. I think our host is getting about $142.00 each. Ouch, this just never stops.

Aaaaaaaa, the kids don't need shoes anyway.

If you guys can add anything helpful to my delima info then please chime in. Funny, there is another post where the persons 911 won't shut off...go figure

Thank you for taking your time to help me.
Mike

[175mph951]

you can use bmw sensors...bmw # 12 14 1 708 618 they are cheaper and work the same..yes only need to change sensors not the pin on the flywheel

[rick-l]

Something sounds wrong. What are the odds both sensors failed open at exactly the same time?

When you go between pins 1 and 2 does the meter says OL or 0.00 ohms?

With your high impedance Fluke you could get on pins 1 and 2 of the speed sensor with the meter set to AC volts and you should get a small reading when you crank the engine. If the sensor is open it is a moot point however.

Before you shell out the cash for sensors you should check the DME connector (harness) for power in the appropriate places.

[jlex]

Check the other posts on this forum after searching for sensor information. My understanding is that only one of the sensors (speed or reference) will result in a no start problem & that mostly only one of the sensors usually gets replaced. I forget which it is...check it out before ordering two...

regards,

jlex.

[jlex]

The reference sensor can be checked at the ECM harness using pins 25 & 26. The speed sensor is checked at pins 8 & 27. Readings for each should show resistance of .6 to 1.6 kOhms.
You can also check the sensors at the vertical connector strip on the left side of the engine compartment (my preference). It's just beyond the black blower tube. The white one at the top is the head temp sensor connector. The lower two are the reference & speed connectors. You may want to take the blower tube out to have more room. Disconnect the lower two wires from the connector strip & check pins #1 & 2 (the two lower pins in each connector) for resistance. They should each read 960 ohms + or - 96 ohms. The wires are supposed to be marked so you know what you're testing: DG is the speed sensor and BG is the reference sensor. For no start problem, BG is probably the culprit to watch out for.
Have fun!

regards,

jlex.

[jlex]

Read the following post for some helpful hints on running down your no-start problem:
85 will not start

jlex.

[Cal44]

Rick-I and jlex, The Fluke reads OL when I touch any of the pins on the reference or speed senors that are connected to the holder on the engine. The Fluke is positioned on the ohm setting. I did remove the blower tube for better access.

One more thing, yesterday when I had the nieghbor crank the car over with the spark light connected to the plug wire the light barely went off once on the last rotation. So, the reference sensor issue may be sounding like an idea for now.

Considering the car is twenty five years of age I think I will just change out the sensors and
elimiDATE that part of the issue. The car has always been treated very well by the previous owners and with 75K on the clock it doen't have excessive mileage. When that doesn't work then on to the next. I will report back when something goes right or wrong. Mike

[jlex]

Mike: Make sure your Fluke multimeter is working okay. Touch the probes together to make sure. It will register 0 or no resistance if working. When the probes are apart, the 0.L reading shows there's an open circuit (break in the circuit).
I find it weird that you should be getting a bad reading on BOTH sensor connections... how could they both go bad at the same time?? Some trauma to the bracket holding them at the flywheel?? Maybe you should try to get a look at them.
Also, are you sure you're taking the measurement at the bracket, not at the wires? When I checked mine, I'm pretty sure the measurement was taken at the prongs attached to the brackets, not on the removed wires.
These things are too expensive to replace unless you're sure they're bad.

regards,

jlex.

[Liam O]

Both these sensors have the same part no. and as Rick says whats the odds in both being faulty though the chances are you will need one anyway.This happened on my car last year at 84,000 miles clock and though the mileage may not be relevant I have heard its a common fault.

[Cal44]

I got out the senors and will go shopping tomorrow. Someone forgot to mention that one needs the hands of a ten year old and the patience of a monk to to this. It gives me an excuse to clean up underneath. Funny when you look up from under the car and see how small the engine and transmission is. Will report later

Liam, I was in Scotland a year ago. Saw alot and darn fine people, everyone was so helpful. Bout' froze my arse off on the Isle of Sky.

[jlex]

So BOTH sensors are reading bad? Wouldn't have thought that was possible... perhaps the speed sensor had been bad in the past, but it wasn't a problem but now that the reference sensor has gone out, it's a no start situation. I can't recall what the symptom is if the speed sensor goes bad, but I don't think it's a no start. Let us know how it ends up & how much those things cost you.

regards,


jlex.

[Liam O]

Well if you were on Sky at this time of year Im sure thats the case, unlike me Im currently freezing my arse of on the south side of Glasgow.Nice blue skies but cold.

[Cal44]

Mornin' all, got to thinking. In the Bentley book it states getting the correct distance on the reference sensor from the flywheel. How is that possible if one does not remove the engine? I can barely see in there to get the darn sensor out in the first place.
I know I should not assume, but.....the sensor bracket looks to be in fine shape and if it hasn't moved I "assume" the position will be good. I am in the habit of using the small mirrors with the extension handle so more investigation and learning is on the horizon.

The sensor is attached with a screw into the bracket, that is straight forward. To answer the question about both sensors at the same time? I have no clue. But the speed sensor may have been bad as you said. If this doesn't do it then back to square one. Some adventure
Mike

[jlex]

I haven't done the replacement myself, so I can't empathize with you... but from what I hear, as long as you don't move the holding bracket, the sensors should line up okay. If the bracket is moved, yes, it can be a real problem to get the correct gap. Don't mess with the bracket!

regards,

jlex.

[ruf-porsche]

Glue a small washer the same thickness as the gap, install the sensor, note the position of the sensor with respect to the bracket or mark the braket, remove the sensor, remove the washer, reinstall the sensor making sure it line up with the previous mark you made.