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Fri Apr 16, 2010 9:56 pm

Alright Gurus! I have "very" little experience with the 2.1 engine. I'm trying to help a friend with his 88 Westy (rust bucket).

The engine will not start..... no fire to the plugs. Cranks over good, fuel pump works... just no fire from the coil. I've checked and re-checked "grounds" due to all the rust! Good voltage at the coil hot side.. ground checks okay!

So, in reading about the ignition system, the scan below from the Bentley gives confusing instructions (my opinion) on how to test the Hall Sender unit. (How can a two lead tester test both sides simultaneously with one of the leads on the center?)... that's what's confusing!

I need a simple explanation from a Guru! When I followed the procedure (one side at a time), one side the light stays on, the other side nothing.... never a "flicker."

Sat Apr 17, 2010 4:47 am

The connector has three wires. You are testing for voltage between the inner wire and one of the two outer wires. Have you peeled back the rubber boot yet? Once you do it might clear up your confusion.

It's not just you, by the way. That Bentley is a confusing book! The Protraining book is a bit easier. Halfway down this thread there is a link to download them for free!

http://www.thesamba.com/vw/forum/viewtopic.php?t=407500&highlight=protraining

Sat Apr 17, 2010 9:00 am

Is the two lead tester your using a light bulb style or LED?

Sat Apr 17, 2010 9:54 am

To answer a few of the questions:

Yes, I did pull back the rubber boot.

I used an LED (Light Emmitting Diode) for the test procedure. I also used stiff copper wires for probes.

With engine cranking.

Results: One side gave no response (nothing)
On the other side, the light come on and stayed on.

So, assuming the test procedure I conducted is the proper method, (test one side at a time), the sender must be bad!

I still say the Bentley manual instructions are confusing!

Also, the link above to another manual has confusing info too!

Mon Aug 21, 2023 5:43 pm

So, both sides should make the LED flash?

Mon Aug 21, 2023 5:59 pm

Testing and real world dependability are two different animals!

The hall sender fails under extreme heat situations.
It will test just fine..... heat it up under real world operating conditions and you
WILL be parked along the shoulder of the road until it cools off.

Mon Aug 21, 2023 5:59 pm

maybe this thread will help

https://www.thesamba.com/vw/forum/viewtopic.php?t=701391

Mon Aug 21, 2023 7:14 pm

bobhill8 wrote: maybe this thread will help

https://www.thesamba.com/vw/forum/viewtopic.php?t=701391

Again.
I will state that you can test all you want to.
But.
These units are finicky and will happily leave you stranded once hot.

You cannot test for this. Even when it happens, within mere seconds the unit will work again...... only to fail again within a few more seconds!

Mon Aug 21, 2023 7:25 pm

This thread is talking about a no spark/no start condition. A fail in this test condemns the Hall sender since the OPs van is never making it out on the road for an intermittent failure to come into play. We don't know the conditions of failure for today's poster so it may be a no spark/no start or it may be an intermittent on the road failure.

Tue Aug 22, 2023 1:49 pm

Not sure if this test is still valid. Carefully unplug the wire connector at the distributor. Unplug the center coil wire from the distributor cap and hold close to ground. Take your trusty 12 volt testlight, key in run position. With the test light grounded, touch the one of the outer wires with the probe, then the center wire, then the opposite outer wire and then the center. I recall the coil will fire each time you probe an outer wire in this sequence.

If the coil fires, and you get no spark, you've narrowed it down to the sensor or the sensor wiring. Memory is fading.

Tue Aug 22, 2023 6:32 pm

If you look at the Hall sensor plug on the distributor you will see three symbols. They are: - 0 + . Left to right they are B- or ground, signal, and B+ or power in. The Hall switch opens and closes the gate that bleeds the power to ground. Most people with some mechanical chops can test for power and ground. The hard part is the signal terminal. If it is working you could see it with an LED test light with the key on and cranking the engine. Back in the day we would ground the center wire and see if it made one spark. That test worked but now I know it might give false negative results in some circumstances.

As Dave has learned from hard won experience the failure is often sporadic and intermittent. So in practice the only practical test was substitution of a known good distributor with a known good Hall sensor.

A long time ago I started using Digital Storage Oscilloscopes to diagnose electrical circuits where you cannot see the electricity with your eyes. At first you look for Yes/No circumstances. But with practice you start to see subtle details. The Hall switch works by allowing the voltage to pass through it and then not. The sensor generates a square wave. Input voltage at the top of the wave and ground at the bottom of the wave.

The control unit does not see the entire wave. It only looks at when the voltage crosses a certain threshold on the up-slope or down-slope. As long as the voltage changes according to the criteria the control unit knows what to do.

As Hall sensors fail they no longer have a clean square wave. The up-slopes or more likely the down-slopes are no longer square but rounded or jagged. The full voltage might not pass through the sensor. The sensor might not bring the signal voltage all the way to ground.

The sensor can be quite unhealthy and still provide the voltage change threshold the the controller needs to act correctly. So you do not really need to catch the sensor in the act of failing profoundly. If you know someone with a DSO or a graphing multimeter you can look at the signal and see if is clean and strong or not.

When I first started using a DSO I was only looking for yes or no. Now we use it for all kinds of things. We actually test batteries with a DSO now. Pico Scope has software as a canned test to check the battery and starter together. You see the voltage drop that occurs for micro-seconds. It still is not always conclusive but it is a lot better than a VAT 40 and an analog ammeter.

You can still do swap-tronics if you have a spare distributor. But the most accurate, least intrusive test is to look at the signal. And it takes less time.

Tue Aug 22, 2023 7:08 pm

deprivation wrote: The connector has three wires. You are testing for voltage between the inner wire and one of the two outer wires. Have you peeled back the rubber boot yet? Once you do it might clear up your confusion.

It's not just you, by the way. That Bentley is a confusing book! The Protraining book is a bit easier. Halfway down this thread there is a link to download them for free!

http://www.thesamba.com/vw/forum/viewtopic.php?t=407500&highlight=protraining

Thanks for the download