3.2 Speed & Reference Sensors...both bad at the same time...
 

After solving a no crank issue (poor wiring crimp by yours truly) I'm now chasing a no start after clutch job, starter, fuel lines, AC conversion, etc. Previous thread is here for reference

We problem solved a lot today on the no start:
- Fuel pump fuse has continuity
- DME is not the issue (swapped the sold state with previously functioning DME)
- Terminal 86 on the DME has 12v
- No fuel smell at the tail pipe
- Fuel pump primes and can feel it vibrate through a screwdriver
- Tach doesn't move on the crank
- Starts with carb cleaner sprayed in the intake
- 36psi at the fuel rail
- CHT: 1.03 ohms at ambient 85 degrees F

Drumroll...

Reference Sensor AND Speed Sensor:
1&2 read 1031ohms
1&3 nothing
2&3 nothing

It looks like BOTH are not properly functioning. Before I go and order these, is this something that happens? We speculated that maybe we knocked them when mating the engine to tranny but don't specifically recall this.

Any thoughts before I go and order new parts?

Appreciate all the help so far.

Isn't 3 the shielding braid?

Did you connect them up to the harness correctly? They have the same style connectors going to the engine harness and it's easy to mix them up. Switch the middle and lower connectors and see if it starts. These are the three vertical connectors near the #3 intake runner.

They are connected in the right order.. The middle connection has a silver paint pen stripe on the plug and receptacle to avoid mis-connection.

The page from the Bentley is below. This is what I was referring to.

Still seems unusual that both would prove faulty at the same time.

http://forums.pelicanparts.com/uploa...1691453202.jpg

1&2 Should be around 1000, so yer probably in the ball park.

What about the other readings returning nothing?

The Bentley makes it sound like the other terminals should return a reading.

In the Bentley manual, the > symbol means GREATER than 100,000 ohms. Infinite resistance is GREATER than 100,000 ohms. You're good to go.

Like I said, terminal 3 is the shielding braid and it should not be touching either of the two other wires.

Also, the sensors should generate an AC voltage across terminals 1 and 2 with the engine cranking.

Helpful, thank you. I was looking for the meter to return a value.

Given that…what are thoughts on the next test? Running a little low on options.

Swap the two lower harness connectors and see if it starts.

No sign of life shown on tach means either the Motronic is not powered or it is powered but does not receive correct signals for speed and ref when cranking.

Also, are you sure of the ohm reading for CHT? If so it looks like the CHT is shorted. Certainly doesn't help startup as this simulates a very hot engine. But since ambient temp is 85F it should eventually start. So this goes back to the sensors. Have you tried to swap them as suggested?

If it will run on starting fluid then the speed and ref sensor must be working. If either one id dead you will have no ignition spark at the plugs, so it wouldn't make any difference if you added starting fluid or not.

Swapped them. Same result http://forums.pelicanparts.com/suppo...s/a_frusty.gif

SO for clarification, per the original post, "- Starts with carb cleaner sprayed in the intake". this indicates that you are getting spark if the engine will run on carb cleaner. when started by spraying carb cleaner into the intake, does the tack show that the engine has started, or is it still dead? will the engine stay running if you continue to feed it with cab cleaner?


You state that "- Fuel pump primes and can feel it vibrate through a screwdriver". There is no prime feature of the original fuel injection system. there are aftermarket DME relays that incorporate a prime feature. what DME relay are you currently running?

My last pair of flywheel sensors would ohm test just fine when they were cold. Just after warmup and driving for a while, they acted like a demon possessed them. The idle would go up to 2,500 on it's own, then die. I had a heck of a time getting in 1/2 a mile back home to the garage. After I replaced them, I heated them with my heat gun, and they started acting weird. I was happy to send them to the landfill.

I replaced the sensors, and "while I was in there" another head temp sensor and the car run like it should for the last 6 or 7 years and I hope for many more years.

So yea, test the sensors, but when it doubt, replace them unless they are very new. If they are the original ones, replace them just because they will not last much longer. I considered them a consumable item. And they are not that expensive or hard to replace considering how much pain and suffering they can cause. They are like having a flat tire, and a dead battery at the same time. Annoying as can be.

I did not look at the tach when we sprayed carb cleaner in there. The tach does not move on the crank though.

We did a second carb cleaner spray/test and it started but when we sprayed more in to see if it would wake up something fuel-related it backfired so we stopped. I think it would run if it were fed an appropriate amount of fuel, whether that be carb cleaner or gasoline.

DME is a Focus 9 Solid State DME with the pump prime feature.

CHT was replaced in 2012 at 136k miles (car now has 151k miles).

I don't have a record showing Speed or Reference Sensor replacement and they appear to be original.

CHT reading is in spec with this post’s parameters.

Then you have a fuel problem the result of;
1. a bad DME ECM (fuel injection section)
2. a bad DME relay - Focus 9s go bad too like the OEM relay
3. a bad fuel pump/fuel regulator
4. stuck fuel injectors
5. no 12V power on fuel injectors

It’s not the DME relay. We swapped in my old, functioning, relay.

Pump and Regulator are getting 36psi to the rail.

Could be injectors. What are the odds of ALL of the injectors being stuck? Would assume it’s power delivery to the injectors rather than all of them being stuck.

And then there’s the DME ECM…is there a way to test this without swapping in a new one?

If your sensors are indeed original, replace them even if it ain't broke. They will die someday soon, at a very bad time. 36 years is amazing that they lasted that long.

And the head temp sensor s just a while you are right there replacement item. The wire goes through the grommet, and getting the grommet back into place is frustrating enough to do that alone is a good reason to replace the three sensors as a unit. And do them one at a time, and mark the positions of the connectors top and on the flywheel positions BEFORE you do removal.

If your fuel pump is original, replace that as well.

Read here; http://forums.pelicanparts.com/porsche-911-technical-forum/621262-911-3-2-no-start-troubleshooting.html

The injectors need a good earth - were any disturbed during the engine removal?

The injectors should be supplied a constant 12V from the DME relay. the ground path is controlled by the Motronic DME to manage open time for mixture control.

If you have a noid light, you could connect it to one of the injector leads and see if you are getting an injector signal.

you could also check to verify that you have 12V at the injector. the injectors should receive 12V form the 87 pinon the DME relay with the ignition in the run position. IF you test for 12V with only 1 injector disconnected you will actually show 12V on both pins of the plug as the injectors are wired in parallels and you will have voltage passing through the coils of the remaining connoted injectors to the DME controlled side, which is ungrounded when the engine is not running.

Post #21 provided the above info, did you miss it?

I don’t currently have a noid but may grab one tomorrow. However…

Injector plug #1 resistance tested within spec at 3ohm

But it DID NOT light the test light with the ignition switched to “on” or on the start crank.

I believe that fuel is getting to the rail from the pump (36psi) and the injectors are not getting power.

All the injectors should have 12V with the key-on.

Problem Solved!

I was reading up on injector threads and came across this one, which referred to a six-prong connector near the firewall. I didn’t remember disconnecting or re-connecting this switch, so I went out to find it…unplugged.

I’ll have to double check, but I don’t recall the Bentley listing this one as a step.

This forum has been great. I’ve learned a ton through all the help here and troubleshooting some fairly easy, but seemingly complex, issues. Thank you.

Tomorrow…we drive.
http://forums.pelicanparts.com/suppo...s/beerchug.gif

Ethanol sucks
Fuel filter replacement
Fuel pump pressure check valve replacement

Just did this
Car would crank for 5-10 minutes on and off
Finally started.

After 40 plus years, ethanol has ruined the fuel pressure check valve….cheap
Fuel filter was cheap,

You state it starts when you spray fuel cleaner into the intake.
Spark is not the issue.

Congrats.

I say there's a good chance the engine "unplugged" the connector for you during the drop.

That thread has some oscilloscope traces of the sensor and injector waveforms.

https://forums.pelicanparts.com/porsche-911-technical-forum/748619-injectors-not-spraying-4.html#post11792513 has som

I think you’re right. Pretty well hidden on the triple check that all the plugs had a home.

Now I have to figure out why it’s running rough and the cat was red hot after a short drive tonight…