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Discussion Starter #1
Hello all,
First off, many thanks to all the posts on this forum that have helped immensely with troubleshooting, but I seemed to be stumped.

My 1987 Saab 900 Turbo started stalling on the way into work last week. The stall occurred with a pulsing like behavior, somewhat stalling worse under load. I popped the hood while running on the shoulder and wiggled the connector at the distributor (hall sensor plug) and the car died. I could clearly see heavy oxidation on the ground wire. After doing a road side repair (twisting wires) I was able to get the car back home, but it ran poorly.

For the repair I cut back the wire loom, removed the wires from the plastic connector and repaired the three wires (12V-green, Ground-black, and hall signal- brown).

Since the repair I now have no spark (which on this engine means no fuel either). I am currently working through the Bentley ignition troubleshooting and have hit a snag. With the ignition on(but not cranking) and the hall sensor unplugged I am getting ground on the black, 12V on the green, and surprisingly 6V on the brown. My understanding is the brown wire should only have voltage when cranking. SO, I figured there was a problem with that wire or the ignition module.

Any insights? Thanks
 

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There is power and ground and the pulse from the Hall sensor. It's conceivable that with the hall sensor in just the right place you'd see voltage. Mark the position of the distributor, remove the clamp, and then disengage it from the cam a bit so you can spin it around by hand.
 

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Discussion Starter #3
It's conceivable that with the hall sensor in just the right place you'd see voltage.
Would that be possible given that the 3 pin connector is disconnected at the distributor? IE the voltage regulator itself (the module on the drivers side firewall) appears to be outputting 6Volts with ign on.

thanks
 

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Ohhhh… I misunderstood.


I'd say the answer is maybe, but it's just for lack of information. There could be a pull up voltage on the brown wire supplied by the ignition control module. I've never actually checked.


I think my approach would be to disconnect the connector at the ignition control module and check continuity/resistance across the 3-wire shielded cable to the Hall sensor. If that checks out, it's one side or the other.


It is unusual IME to have an intermittent ICM or Hall sensor. Usually they are just dead, or *maybe* dead with heat and ok when cool. To have one work intermittently is weird.
 

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Discussion Starter #5
There could be a pull up voltage on the brown wire supplied by the ignition control module. I've never actually checked.
That's interesting. Whats the purpose of a "pull up voltage"? I assumed that brown wire (pin 6 on the ICM) was only supposed to receive voltage when the hall sensor passed over the magnet.


I think my approach would be to disconnect the connector at the ignition control module and check continuity/resistance across the 3-wire shielded cable to the Hall sensor. If that checks out, it's one side or the other.
Good idea. I'm actually working on it currently. I got slightly more aggressive with trying to find out why I had 6 volts on that brown wire and removed that wire from the ICM connector. This allowed me to probe for pin 6 independent of anything to do with the hall sensor. It was still 6 volts. I ended up buying a new ICM and testing it, still 6 volts, and still no start with everything connected. So my conclusion is either like you stated it may be normal to have 6 volts coming out of pin 6 (brown wire) of the ICM, or something feeding the ICM on one of the other 6 wires is messed up causing this voltage.


It is unusual IME to have an intermittent ICM or Hall sensor. Usually they are just dead, or *maybe* dead with heat and ok when cool. To have one work intermittently is weird.
My worry is that during that limping drive home something got cooked. maybe the hall sensor, maybe this 6v on the brown wire coming from ICM is a red herring. Is there a way to test the hall sensor independent of the ICM? Resistance between pins etc.
Thanks!
 

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That's interesting. Whats the purpose of a "pull up voltage"? I assumed that brown wire (pin 6 on the ICM) was only supposed to receive voltage when the hall sensor passed over the magnet.

That's how it works, but often times in these types of circuits employ a pull up resistor to prevent a floating ground. This ensures that the signal at the pin is in a fixed state until the device at the other end causes it to assume another state. I don't know if the ICM does this, but it might.


or something feeding the ICM on one of the other 6 wires is messed up causing this voltage.

Nah, probably not this. The ICM gets power and ground from the chassis, supplies power and ground to the sensor, gets a signal from the sensor, triggers the coil with a ground, and supplies a cleaned up signal to the car. There's nothing external that could cause 6v to appear at the ICM unless something REALLY WEIRD happened with the tach circuit.


Speaking of, does your car have the "ignition amplifier relay" in the fuse panel? If so, bypass that.

My worry is that during that limping drive home something got cooked. maybe the hall sensor, maybe this 6v on the brown wire coming from ICM is a red herring. Is there a way to test the hall sensor independent of the ICM? Resistance between pins etc.
Thanks!

The Bentley manual has a procedure for testing the Hall sensor, but I don't know it offhand. Something like measuring AC voltage between the brown and black wires when the distributor spins. You're looking for the rise and fall of voltage as the sensor passes the window. The proper way to observe that is an oscilloscope.
 

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Discussion Starter #7
Speaking of, does your car have the "ignition amplifier relay" in the fuse panel? If so, bypass that.
Yes I bypassed that a some time ago.

The Bentley manual has a procedure for testing the Hall sensor, but I don't know it offhand. Something like measuring AC voltage between the brown and black wires when the distributor spins. You're looking for the rise and fall of voltage as the sensor passes the window. The proper way to observe that is an oscilloscope.
I removed the distributor (FYI PN# 0237507007) from the car so I could work with it on the bench. I found several online resources that outline exactly what you said regarding testing voltage output from the brown wire. I do not get any AC or DC voltage when I rotate (this was done with a volt meter as I do not have access to an oscilloscope). Furthermore, I found a youtube video that outlines the typical resistance between the ground and signal pins (~5K ohms) on 3 wire hall sensors, I have no resistance.

This leads me to believe that there is a problem with the hall sensor. I removed the rotor and a few other pieces to get a good look at the hall sensor (photo attached). there is a PN HKZ101, this appears to be sold by various online vendors although it looks tricky to remove (held in by 2 rivets). Alternatively Rockauto offers a distributor rebuild service for around 400 CAD including shipping. Lastly there is a seller on Ebay with used distributors for $75 CAD shipped.

What do you think?
 

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Failed Hall sensor is definitely a thing. The factory (like, the Siemens chip) sensor went out of production years ago, but in recent times there have been Chinese approximations of it. If you're good with small work, replacing it is probably the cheapest option. I tend to shy away from 30 year old used parts, but have no firsthand experience with on-demand rebuilds. Maybe someone else has some recent/relevant experience.
 

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Update. I finally received the used ebay distributor today (the new hall sensor is coming from China so it could be a while). I cleaned it up, installed it and plugged that 3 pin connector in and.... It started!!! I set the timing and went out for a ride and it drives great! When I receive the hall sensor I will try to install it in my old distributor.

My next concern is what to do about the 3 pin plug (male end) that goes into the dizzy. Its in really rough shape (half the pins are exposed from the plastic breaking down). I've searched the web high and low trying to find that oval connector but no luck. I am considering a few options.
1)Fill the dizzy side connector with epoxy then shove my busted plug into it.
2)soldering 3 wires directly onto the dizzy's three contacts then implementing a universal 3 wire connector into the factory harness.

thoughts?
 

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You're in the same situation countless c900 owners have been in. The irony is that Saab did update to a more reliable connector around 1988, but Volvo kept making the old oval one much longer than Saab made either, so you generally have better like finding the oval one than the square one. womp womp


I have seen all manner of repairs - RTV'd wires hanging out, plastic epoxy'd connectors, JB Weld'd distributors. I would probably use a piece of sheet metal to cover the hole in the distributor body, put a grommet in there to protect the wires, put a dab of silicone to keep dirt out, and then use some sort of automotive connector (TE Mini Power Timer, etc.) secured to the distributor body.


No joke, disappearing distributors was my primary motivation to switch to Trionic. (Now I get to fear disappearing DI cassettes instead!)
 
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