another 84 Carrera will not start, help please

[SiberianDVM]

Yesterday I had my 1984 911 at Carolinas Motorsports Park for a DE, and it ran great for 2 sessions. Tried to start it for session 3 and it cranked well but would not start. Swapped out DME relay with a new one with no improvement. Pulled a spark plug wire, used a spark plug, then a screwdriver close to ground, no visible spark.

Checked voltage at coil: 12.5 volts
Checked coil fuse: good

Car just had new plugs, plug wires, coil, rotor, distributor cap installed 2 months ago.

So I winched her onto the trailer and came home. This morning I swapped out new silver Brazilian coil for old black German coil, still no spark. New coil passed continuity checks. Checked rotor, dist cap, coil to distributor wire, all OK.

I had Steve Wong install his chip in my DME about a year ago. After reading the other thread, I guess I should pull it out and look at it and maybe send it to him to be checked.

Reference and speed sensors? I'll have to order those. I have a CHT sensor that I have not yet installed as it looked like a PIA. Any other things I can check?

[Joe Bob]

Is the DME relay audibly clicking when turning on the ignition?

[zippy_gg]

Can you re-install the stock chip in the DME if you still have it? This would rule out a failed SW chip.

[SiberianDVM]

Originally Posted by Joe Bob

Is the DME relay audibly clicking when turning on the ignition?

That's the one under the seat between the DME and the altitude sensor, right?

Yes. It is not terribly loud, but I can feel it vibrate when the ignition is turned on and off, and using a stethoscope I can hear a distinct PING.

[SiberianDVM]

Originally Posted by zippy_gg

Can you re-install the stock chip in the DME if you still have it? This would rule out a failed SW chip.

I can. May save that for later, see next post.

[SiberianDVM]

Found the reference and speed sensor connections in the engine compartment and removed enough ducting to get to them and disconnect them. I did basic ohmage tests.

Speed sensor (DG): INFINITE on 1-2, 2-3, 1-3
Reference sensor (BG): 1064 ohms on 1-2, INFINITE on 2-3 and 1-3.

Since the engine won't start at all but ran fine on the track an hour before, this makes absolutely no sense to me.

Should I go ahead and order 2 sensors? I already have a new CHT sensor.

[Joe Bob]

$200 for a DME computer.....

86 Carrera computer with performance chip

And read this....

reference sensor

[scarceller]

Your ohm test shows:
Speed sensor is bad
Ref sensor is good

It's very common to have the sensor's windings go open circuit after heat and or vibration and cool down. Your track session simply caused the speed sensor to fail.

Originally Posted by SiberianDVM

Found the reference and speed sensor connections in the engine compartment and removed enough ducting to get to them and disconnect them. I did basic ohmage tests.

Speed sensor (DG): INFINITE on 1-2, 2-3, 1-3
Reference sensor (BG): 1064 ohms on 1-2, INFINITE on 2-3 and 1-3.

Since the engine won't start at all but ran fine on the track an hour before, this makes absolutely no sense to me.

Should I go ahead and order 2 sensors? I already have a new CHT sensor.

[yelcab1]

Originally Posted by SiberianDVM

..

Speed sensor (DG): INFINITE on 1-2, 2-3, 1-3
Reference sensor (BG): 1064 ohms on 1-2, INFINITE on 2-3 and 1-3.

...

Infinite resistance on speed sensor is ... not good.

[hayday]

I have an 84. I came home after a drive. It ran great and I put it away. The next day it would not start. It was the reference sensor......

[SiberianDVM]

Thanks everyone, esp scarceller. Your posts here and on Rennlist have been invaluable. 2 new sensors on order, hopefully will be here by Friday and I can spend the weekend skinning my knuckles. That is a PITA area in which to work.

[JTL]

This happened to me twice and both times it was a bad ref sensor.

[scarceller]

This pic helps understand howto remove these sensors:

[SiberianDVM]

Unfortunately, I had to remove the bracket in order to remove the sensors, as they were really stuck. So yesterday I retested the sensors, speed sensor is the bad one, and JB Welded a thin washer onto the bad one. Good one I will keep as a spare.

Also installed a new CHT sensor as the old one was a single wire and the plastic connector was broken. Still tested good, though.

Is there a torque setting for the bracket to bell housing bolts? Or use Loctite?

[SiberianDVM]

Got 2 new sensors installed last night. She fired up on the first crank.

Thanks everyone.