3.2 DME - Speed & CHT Sensors...
 

Hi Gang,

Can someone PLEASE tell me how best to test that these sensors are operating correctly (and where to test from)?

- Cylinder Head Temp Sensor
- Flywheel Speed Sensor

Is there a simple Ohm/Volts test that will confirm if these are OK?

What point(s) do I test from?

Some guidance appreciated :confused:

The Bentley Carrera manual is a good place to start to test these as it has some values, etc.

From memory, I think you can test the CHT sensor with a meter, but to actually fully verify the speed and reference sensors, I think an oscilloscope would be needed as their output is a pulsed voltage type of thing. You can verify their resistances, but their correct operation is a function of their installation (distance from the flywheel teeth), so you need to verify the output also. You might be able to at least see whether they are creating output or not (but not the magnitude of the signal) by using the AC voltage setting on your meter and then cranking the car. It should register "something"...

I would suggest testing them at their connector pins at the DME. That way, you're not olny testing the function, but also the continuity from the sensor all the way to the DME.

What kind of problem are you experiencing?

Mike

Re: 3.2 DME - Speed & CHT Sensors...
 

Roughly 3K ohms cold to 300 ohms hot.

you need an ocilliscope to check the operation of these sensors, you need the o scope to check and see the sine wave given off from the sensor when engine cranking or running


check from the sensor itself to see if the sensor is working then from the dme to verefy no loss through the harness

The CHT can be tested but since it's failure tends to be intermittent, testing the sensor doesn't always work. I "fake out"/spoof the DME by replacing the CHT sensor with a resistor shoved into the harness (white connector) held in with a piece of tape.

If your speed and reference sensors are the original ones, I'd just replace them. The heat cooks the wires and that's what killed the 3 failed ones that I know of.

-Chris

OK thanks guys.

My problem is hesitation under heavy throttle. I've pretty much replaced the whole igntion system as well as replaced a crap alternator.

I'll replace the sensors and see how that goes. This is P!SSIN ME OFF!!!

Had same problem once, removed speed and reference sensors. Cleaned them and reset the gap to 0.8mm

Thanks. I've been told for my high-HP Turbo setup, I should not use the factory setting of 0.8mm. I'm told I should go as close to the flywheel as possible, without hitting it of course.

Not sure if I should replace the reference sensor. I'm not having any startup issues. It's only high load issues. I guess worth removing it and cleaning.

Does the engine have to be dropped to R&R these flywheel sensors, or can it be done with the motor still in the car?

Does the Reference sensor also need to be aligned to a specific specification of 0.8mm, or is that only for the speed sensor.

Sorry for the dumb newbie questions :confused:

The 'reference' and 'speed' sensors can be tested while still mounted.

1. detach sensor electrical connectors located on the left side of the engine

2. check the resistance when the engine is at room temperature (~25C or ~75F)

3. assume the following notation:
terminal 3 is the top pin in each respective connector (terminal 2 is center pin)

The resistance between terminal 1 and 2 should be ~ 960 ohms

The resistance between terminal 1 and 3 should be >100,000 ohms
The resistance between terminal 2 and 3 should be > 100,000 ohms

Hope it helps (?)

Hi Merv,
I was talking to a Porsche mechanic (and Bosch Master Tech) this summer about flywheel sensors - I mention how was surprised at how small the signal could be and still work. He mentioned that if the sensor was a further away than spec it would still work but that it was possible to get the sensor too close and then things wouldn't work. I guess I remembered it because it was so conterintuitive.

You can change the sensors with the motor in the car. It looks like it would be a pain but it's possible. Save the old sensor and glue a .8mm washer to the tip to use as a gap setting tool.

The speed and reference sensors are held in a common bracket. When you adjust one of the sensors (I forget which) the other sensor goes along for the ride.

Just thinking out loud, I wonder if the speed sensor wire insulation was compromised and the ignition was "leaking" a lot of radio energy (think "better than stock" aftermarket ignition, plug wires, etc) if it could interfere with the speed sensor signal and cause your problem?

-Chris

Indeed, the sensors can be changed by taking the left rear wheel off (cramped quarters but quite doable)

The insulation is of little consequence as far as the RF interference is concerned the wires are shielded beneath the insulation.

I drove with one of them for some time without any insulation (old one crumbled and pealed off))

OK, thanks guys.

Latest update is, I've turned off the Boost Controller and caned the living ***** out of the car (checked for undercover detectives this time) and whoala, problem disappears at 0.5-0.6bar of boost. I think I have a fuel pressure issue, or a damn vacuum leak at higher boost pressures.

A faulty/dodgey speed sensor would cause problems at any speed as it's not really load dependant, so that rules out the speed & reference sensor.

Now I have to air & fuel pressure test the whole damn system :(

try closing up the spark plug gapp alittle to see if the stumble goes away, you can be misfireing from the boost blowing out the spark causing the misfire, ive seen it quite a few times..

also if it was a fp prob it would probably get worse ie not just a stumble

as far as a vacuum leak it would be a boost leak and unless it was a major leak should not affect it that bad,

since your running the dreaded fmu setup with motronics
check the boost/vacuum signal line to the fmu ive seen these lines cut and the car would boost and then would hit a wall it seemed like<no fuel>from having a tear at the vac line to the fmu

thanks ssetek, however, I'm using a proper Protomotive Fuel Press Regulator with their Stage3 CHIP. Should all be matched, however, it must be a vacuum leak or fuel pres issue.

I have an aftermarket Crane HI-6 high perf ignition system on the car now, and plug gap is 0.6mm (W3DPO's). Should be OK.

I think what I have to do next is pressurise the intake system at 1.0bar and see if there are any vacuum leaks. if not, then check fuel pressure under load. Should be at least 6.5bar under load. if not, then definately fuel pressure.

First thing is to check for vacuum leaks tho :mad:

no im not saying your not using the proto fpr, the proto fpr is a fmu
not the best way to manage fuel your fp will be 94.25psi:eek:
fp should never creep past 75 imho, thats what i mean by the dreaded fmu that kind of fuel pressure
proto chips and fmu is a good bolt on solution but when pushing large boost real eng management is the only way to go.

also what injector is in the engine because at 94 psi fp a stock carrera injector is only a 35 lb injector and at 3.2l at 6400 rpm at 1 bar that motor will need 40 lbs a min air flow which at a 35 lb injector will be at a 97% duty cycle which might as well be static

hope this helps

Thanks ssetek. I'm using 944T injectors with twin BOSCH 044 pumps in series. The system works on high rail & fuel pressures. The injectors flow slightly over 60lb/min at the higher fuel pressures at close to 100% duty, 0.5BSFC.

ok just making sure a 60 lb injector will be fine, and as far as rail pressure goes i just never like seeing 80+ psi on full boost but thats just how i design my efi systems.

on a side note the carrara engines are nowhere near .50 bsfc they are quite ineffiecient and they are more close to .70 ish unless twin plugged.

hope you figure out your prob:)

Thanks. It's not really a "Carrera engine" anymore. It's now a 3.3L, GT2 EVO CAMs, ARP fasteners, sodium filled turbo valves blah blah blah :)

A setup very similar to Juan's, except for the flamed ring heads. I have twin plugged heads too, however, 2nd set of plugs haven't been hooked up yet. WIll wait until I go aftermarket EMS.

Todd of Proto tells me fuel pressure should be around 2.5bar idle, 6.5bar at 1.0bar boost :eek:

sorry what i mean is the air cooled porsche engine in general

yes the proto reg pumps the fp up 3.89:1 fmu
3.89psi of extra fp for 1 pound of boost

OK, thanks for that.

See, the setup was working fine for a while, so something has definately gone bad. Either a vacuum leak at higher boost, OR :-

- checkvalve on FPR
- ripped diaphragm
- blocked bleed hole on FPR

Gotta check all these out.

At around 0.6-0.7bar, I can drive the thing to its limits and I get absolutely no hesitation at all. Once I go ~0.8bar+, that's when the problem arises :mad:

I saw a vacuum line at the back of the throttle body hanging there, which runs along to where the oil filler neck is. It goes into some sort of diaphragm device?!?! Any idea what that's for? (See next page for pic).