Carpokes Porsche Forum

Another update. Was planning to go on a test drive today as I didn't yesterday and the car died while idling. Tested for spark again and there was 0 spark on two cylinders. Was planning to test the other 2 but my $5 spark tester ripped the wires to the rubber boot so now it's trash.. Tested voltage and resistance to the coil and they were reading the same before and after the engine stall. The resistance was teetering between 0.6 and 0.7 ohms so I want to believe it's within spec.

Are you checking for tach bounce when the car won't start?

Hi Tom, the tach does bounce when trying to crank and the boost goes to 1 BAR when switched to on so I don't suspect the sensors here.

Alright I have an update but this time I believe I found the issue.

I went to check the speed and reference sensors that I assumed were okay because I was getting tach bounce. Upon inspection, the housings were cracked and very frail. I’ll attach images but they crumbled when I disconnected them whoops.

Tested resistances on the reference sensor for pins 25 and 26 which didn’t provide a reading with “overload” condition. Spec is 600-1600 ohms. This was the same for pins 8 and 27 for the speed sensor.

Tested 8 and 23, 25 and 78 which spec is over 1 M ohm. 8-23 read 34 k ohm and 25-78 read 0.16 M ohm.

The normal failure mode is frayed wires under the rubber boosts. Never actually say a JPT connector break like that. Tach bounce is typically a good indicator that the sensors are working, but either way it looks like you'll need to fix those connectors....

I was convinced I didn't need to check the sensors but I followed NC944er's video and found that they are in dire need of replacing. Aside from the frail plastic, the resistance readings were.... less than ideal.

Okay so I have some updates. I've officially replaced many ignition components including: spark plugs, ignition coil, distributor cap and rotor.

I've also replaced both speed and reference sensors. The car starts up pretty well but it will usually stall as per my initial post here. So I'm back here now after new parts and I believe I can put the blame back on the speed/ref sensors so I have a couple questions.

Per Clark's Garage IGN-02, am I supposed to measure the resistances from the sensor pins or from the pigtails connections where the sensor pins plug into? I measured both anyways and I'm a bit confused. For both pigtail connectors, all measurements were 157kOhm. Which is severely out of spec (unless of course that's not what I'm supposed to be measuring). As for the sensor pins, 8-27 measured ~950ohm which is spec. 8-23 however didn't give a reading. Similar situation to the ref sensor where 25-26 measured about 900ohms before going to OL condition, and I didn't get a measurement for 25-78. Was I given bad "new" sensors?

The test in clark's only checks the sensors themselves, not the harness. Did you change just the sensors, or did you also change the connectors/wires on the engine harness side? The engine harness side is at at least if not more prone to frayed wires under the boots. Try testing the same signals, but at the 35-pin plug at the DME (in the footwell), and have someone wiggle the speed and reference wires under the hood while you do that. If that causes the readings to bounce around, it's a sure sign of bad connectors -- almost surely on the harness side if you just changed the sensors. Or just rattle the wires while idling to see if you can cause the motor to die. Do the same with the fuel injector wires.

That said, why are you focused on the speed/ref sensors?

Thanks for the reply Tom,

I’ll try giving them a wiggle next time I’m testing but I just find it odd that I’ve replaced a lot of ignition parts and it has sudden power loss. I smell fuel when trying to crank with no spark so I’m not entirely convinced it’s the fuel system but maybe the fuel filter is in need of replacement.

I haven’t tested the harness itself and was told I shouldn’t be reading resistances on the connector with the bulkhead plugged in because it can break some transistors.

But the sudden loss has me leaning towards the DME not getting a proper reading of the ref/speed sensors. The car drives great until it stalls

Sorry, forgot to mention I’ve only changed the sensors and installed a new 0.8mm spacer with it as well.