Cranks no start story so far

[amehl]

I have been getting support from a local group throughout the say, but I would just like to get as many thoughts on my problem as possible.

Car ran fine on wednesday, and this morning; no start. Cranks, will not fire, no tach bounce. All advice from locals and search have pointed me towards a bad reference sensor.

Steps i have taken:

1: checked fuse, it is in good working order (replaced it for good measure)
2: jumped the DME/fuel pump relay with a paper clip. Could audibly hear pump running.
3: Removed reference senor and cleaned it (didnt use dielectric grease, but its in good shape)

What are the odds of it being a faulty DME?

I ordered a new sensor so hopefully that fixes it. Anything else im missing?

[guru944]

amehl:

The 2 Biggest Issued with the car starting or stalling intermittently.

1.a You should check your engine Ground (impedance) with a digital volt meter set on Ohms. Read Impedance between Batt (-) terminal/ post and Valve cover bolt and or Throttle body. It should read ~0.01 Ohms Thats Normal. Anything higher is NG. Thats some type of open circuit.
1.b With your DVOM set to 20v dc, read across the same places. Voltage should be ~0.001 Volts. anything higher is NG. That is an indication of current flow in a Ground loop (97.8%) or short (rare). Run a 8-10 ga. wire from a upper waterneck Bolt back to the Batt(-) terminal clamp.

2. Big problem, Harness to either Reference sensor or Timing sync Sensor are Damaged, Open or shorted. Best way to verify this is to have someone crank engine with you safely under the hood and wiggle the wires at the neck of connector on these sensors. The Tach needle may jump or the vehicle may start or sound like it wants to start. That's It. You will have to replace these sensor harnesses all the back to the ecu. "Big Job". I will post pics asap. Lindsey Racing and Ian Porsche (Ebay), a few others carry the Pig tail wire Kit for around 100. Which i recommended.

Porsche 944 Turbo 86 to 91 Reference Sensor Crank Position Sensor Harness New | eBay

I found and just did both, last week on an 1987. Both problems. i took my time(4hrs) but it worked great. Car starts like a champ and idle is at 840rpm after cold start (warm).
I will publish pics later tonight. Whew! Sensors were fine....
PM Me if needed. Later...

[guru944]

Amehl,
Pics Update for part 1 Ground loops or Floating Ground on engine.
Using a Harbor Freight $6.99 DVM and Taking DV Voltage Readings, Between the Batt (-) Terminal and the Cylinder water neck Bolt. or TB
With engine off, Digital Volt Meter "DVM" reading 0.01 volts.
with engine running and Fans "Off", DVM reading 0.05 Volts.
with engine running and fans "On", DVM reading 0.09 Volts see pix #5.

NG, Bannana's Ground is Open or Floating...current is flowing in a ground loop. See attached pics... PM Me if needed. Hope pictures help. Later...

[DannoXYZ]

Here's a coupon for that HF multimeter. Free with no purchase necessary.



As for speed/ref-sensors, TEST and DIAGNOSE as bad BEFORE replacing. I've seen people replace speed/ref-sensors, spark-plugs, plug-wires, distributor/rotor, coil, injectors, fuel-pump, FPR, fuel-rail, DMEs & KLRs, DME relay, pretty much everything on the engine except for the engine itself. Only to find out at the very end it was the sensor-bracket that needed adjusting. You end up with a tonne of spares you won't need for decades by just swapping parts without testing & diagnosing.

This is the very 1st test that should be done. Hook up an oscilloscope to see the sensor signals. First, check for those signals at the sensor-plugs themselves in the engine-compartment. Then check those signals again at the DME-plug under the glove-box. This 2nd test will check the wiring-harness in between the sensors & DME. If the signals at the DME-plug doesn't match what you got at the sensor-plugs, then harness is bad. The 2nd test also verifies that you don't have the sensors swapped.



EVERYTHING starts with these signals. If they are not up to spec, nothing else will work. And most of the time, it's not even the sensors, it's just their height-adjustment above the flywheel that needs to be adjusted. Engine-vibrations can move the sensor-bracket over time.

Here's a Speed/Ref sensor test & adjustment procedure at Clark's garage
Here's some related threads:
Suspected bad DME
Home on a flatbed

[JonA]

Try this to test the sensors.

https://www.youtube.com/watch?v=zeFOOcLSk60&feature=youtu.be

Jon

[amehl]

Originally Posted by JonA

Try this to test the sensors.

https://www.youtube.com/watch?v=zeFOOcLSk60&feature=youtu.be

Jon

Quick update: Tried the procedure in this video. Speed sensor was normal as per the video. I was getting weird readings with the reference sensor. ACV testing didnt have enough decimal points to register according to the video, but the ohms reading returned nothing. So hopefully im on the right track identifying the bad sensor.

I also followed the first few steps that guru pointed out and my readings looked normal.

This thread could be in chinese for all I know. I have never dove into electrical stuff like this so bear with me.

ANOTHER thing i checked with was the DME. Replaced it with a different one, no diference so that can be ruled out again.

What further tests can I do to check the reference sensor?

[jeffrsmith]

Originally Posted by amehl

What further tests can I do to check the reference sensor?

Disconnect the sensor from the harness and check the resistance readings at the sensor itself; this will tell you whether there is an issue with sensor or the harness wiring.

Use the resistance readings from the Clark's Garage link at the bottom of Danno's post.

[mattdavis11]

Originally Posted by DannoXYZ

EVERYTHING starts with these signals. If they are not up to spec, nothing else will work. And most of the time, it's not even the sensors, it's just their height-adjustment above the flywheel that needs to be adjusted. Engine-vibrations can move the sensor-bracket over time.

It all starts with the little wires at the positive terminal. If I remember correctly, there are 3 small wires on the early cars, one of them feeds power to the DME. I've had a no tach/ no start, it was one of those wires. Cut out the bad section, spliced in new, and it fired right up.

[amehl]

Great thank you. I will test these today.

[amehl]

Just a quick update: I after trouble shooting i ruled out everything i could. I didn't have an oscilloscope so i stopped there. Frustrated, and after jiggling every possible cable, I was about to give up. Threw in the new sensor just for kicks, and what would you know it fired right up. I am glad it was an easy fix and I even learned something about my car. Thanks for all the help!

[Backyardbodyman]

That's awesome