1985 3.2 just died
 

I was on the highway with my 1985 3.2 911 and it just died.
It started backfiring heavily and died not sure what happened first. Towed it home and checked the fuel pump and it is running. Tried to start it while jumping the pump and it did not fire. Replaced the relay under the seat and did not help. Now I am narrowing in on the sensors (speed and Crank). I tried to check resistance and my meter and it was not working right.
So with that said which one should I be looking at the crank or speed? And anymore Ideas of what to look at?

Suggest changing both while your in there, as its a bit of pain to do either, also; if you can stretch to the cost of a new head temp switch' do that as well while your in there, as all three are routed through the same grommet and tin wear etc.
Good luck with it.
Ant.

Good advise. If one of the sensors is on its way out the other is usually not far behind. Testing them can be done by disconnecting the 35-pin connector from the DME under the drivers seat and measure inside the connector:

Pin 8 - 27 for the speed sensor and pin 25 - 26 for the reference sensor. For each check the resistance and then measure AC voltage while cranking. The resistance should be near 1000 Ohm and the AC voltage for the speed sensor is somewhere around 2V - 3V. The reference sensor voltage is much smaller because its only one blip per crank rotation. Depending on your specific multimeter you should see some change of several 100 mV.

For changing the sensors take the rear wheel off for better access. Undo the screws holding the sensors in the bracket but leave the bracket attached to the transmission. And one last piece of advise: Measure the new sensors on the bench on resistance mode before installing them. I helped a guy out recently and we found one of his new sensors was bad out of the box causing his no-start.

Thank You for the response, For the problem that I described which one would you figure went bad? Speed or crank? I have one sensor on the way and have another one coming. But trying to narrow it down first?

The head temp sensor is right there. Replace it for sure with the others. The hardest part is getting the grommet back in place properly.

The speed sensor and crank sensor are both vital, as is the head sensor.

I did the head sensor 2 months ago. The speed and crank I did 4 yrs ago.

There is no reliable way to identify which sensor is bad when you have a no-start unless you measure them as described.

If a resistance test fails with an open or you measure see any resistance from shield to the signal wires the sensor is bad. But there instances where a sensor ohms out fine and still isn't working properly.

If you have access to an oscilloscope you can see the waveforms of the sensor signals. Even a cheap 30$ version suffices here.

That leaves cases where old insulation can draw moisture into the COAX degrading signals and sensors that fail under temperature due to internal issues.

With how cheap a little handheld oscilloscope/multimeter can be had online I really can't see a reason to start throwing parts at a problem in the hope that the Parts Cannon™ will fix it without testing the sensors first.
The spec on the sensors is they should give you a 2.5V pulse.

A search on Amazon for "oscilloscope multimeter" brings up a number of cheap ones for $50.

Yes, ^ is a good point. And these gadgets are less than 30$ on evil bay….

Yeah, don't throw parts at the problem (yet).

Simple stuff first:
First question is whether it's lacking fuel or spark? Shoot some starting spray in the intake and see if it fires. If it does, you're lacking fuel. If it doesn't, you're lacking spark.

It ended up being the ECM. I switched it with my other car and it started right up!
Thanks for the input.

Always good to have a running car to swap parts with!

Glad you found the root cause. Feel free to reach out if you need help getting your bad ECU repaired.

Good to hear that, In my original post I had assumed [my bad] that you had already ruled out the ECU, hence my suggestion of testing replacing the sensors. :)
Ant.