[chengny]

Schematic and brief explanation of the "Combined Instrument" (i.e. the fuel tank & coolant temperature gauges):



The combined instrument (fuel gauge/engine temperature gauge) are both two pole instruments where one coil in each instrument is fed with about 12 V from control module pin 22. Both gauges are grounded to grounding point G8.

Fuel level gauge
The control module feeds 1 - 9 V on pin 24 to the second coil in the fuel gauge and in this way controls the gauge reading.

Engine temperature gauge
The control module feeds 1 - 9 V on pin 30 to the second coil in the engine temperature gauge and in this way controls the gauge reading.


The way I explain gas gauge operation on other (GM truck) forums - edited for a Saab 9000:

The fuel tank sender is connected to pins 33 & 26 of the EDU (via the WHT & BLK wires). A variable ohmic value - which is a function of tank level – is generated by a float that drives the sender’s integral wire wound resistor. The EDU receives the resistance value from the sender and then uses that input to generate a variable voltage (1-9 VDC) on pin 24 – that voltage is applied to the gas gauge at pin 1.

The variable voltage output from pin 24 of the EDU - when connected by the GRN wire to pin 1 of the printed circuit - forms the "sensing leg" of the gas gauge. Pin 1 on the printed circuit is connected to the positive side of the sensing leg’s coil. The negative side of the coil is connected to ground on pin 5.


The higher the voltage that the EDU applies to pin 1 ,the greater the electro-magnetic force created by the coil. Consequently the force that it (the measuring coil) exerts on the gauge's magnet increases. The magnet is attached to the lower end of the indicating needle.

In addition to the sensing leg, there is another - opposing - circuit called the "control leg". It is identical to the sensing leg in every way except that the positive side of its coil always receives a full 12 VDC signal from the EDU on pin 2 (as opposed to the variable voltage the sensing leg receives). The control leg’s coil is located on the opposite side of the magnet from the sensing leg’s coil. It's negative side is also grounded via pin 5.

So what you have is; two coils located on either side of an indicating needle - AKA the "ray". The ray is pivoted somewhere in the middle and has an integral magnet mounted at the bottom.

These coils create their own magnetic fields - the strength of which is a function of the electron flow (i.e. current) passing through them. When these circuits are energized - by turning the ignition switch to the RUN position - the opposing magnetic fields begin to "fight it out". Each coil tries to pull the magnet - mounted on the needle - towards itself.

Due to the fact that it always sees full battery voltage, the current flow through the control coil is essentially constant (as is it's magnetic force).

On the other hand, due to the variable voltage signal generated by the EDU (as a result of changes in the fuel tank level), the current flow/magnetic strength of the sensing coil can range higher or lower.

The location of the two coils - to the magnet - is such that:

1. The control leg is always trying to peg the indicating ray to the FULL end.

2. The sensing leg is always trying to peg the ray to the EMPTY end.

 

[scantar]

I think I'm going to have to take apart one of my spare clusters and look at this common ground.



Going through this...the loss of reading must be either from the constant 12v input or the common ground point failing. I doubt that it's from the variable inputs because the loss of reading is total and it affects the entire cluster. Ground failures in general are much more common throughout the whole electrical buggiverse so that's where I'm leaning.



I think this is getting close...

 

[chengny]

I think I'm going to have to take apart one of my spare clusters and look at this common ground.



Going through this...the loss of reading must be either from the constant 12v input or the common ground point failing. I doubt that it's from the variable inputs because the loss of reading is total and it affects the entire cluster. Ground failures in general are much more common throughout the whole electrical buggiverse so that's where I'm leaning.



I think this is getting close...

I was trying not to influence your diagnostic process - I only intended to supply the background information - that would help figure it out. But did you catch this:

The temperature gauge 47c, fuel gauge 47a, instrument lighting lamps 18, and indicator lamp 47i are connected to ground via pin 5. As noted above, the low fuel level lamp is grounded independently of the other 4 components.

What would be a big help would be if one of these incidents occurred at night. With the head lights/parking lights on, the dash would be illuminated by the back lighting. The "combined instrument" and the back lighting lamps are supplied with power from different sources.

So, if in addition to the gauges dropping out, the backlighting also went black...that would suggest a loss of that common ground on pin 5.

On the other hand, if the lighting stayed on - and only the gauges stopped functioning...that would seem to point to a loss of the common power supply to the gauges. That is, the feed from pin 22 (EDU) to pin 2 (on the CI).

 

[scantar]

So, if in addition to the gauges dropping out, the backlighting also went black...that would suggest a loss of that common ground on pin 5.

On the other hand, if the lighting stayed on - and only the gauges stopped functioning...that would seem to point to a loss of the common power supply to the gauges. That is, the feed from pin 22 (EDU) to pin 2 (on the CI).

This is a good point. Honestly can't think of a time when I noticed it at night where I paid attention to the backlighting.



That said, from looking at the pin 5 ground trace it should also happen that the right turn signal indicator light would stop lighting so that's something that could be confirmed day or night.

 

[scantar]

I have to re-think my re-thinking here. Despite the fact that the grounding point would be the likeliest of suspects, the fact does remain that changing the edu does solve the issue. And given that the turn signal indicator is on that same common ground you'd think that someone would have noticed and mentioned that before.


I'm back to this being a power-in to pin 2 issue from the edu

 

[Banman]

I have to re-think my re-thinking here. Despite the fact that the grounding point would be the likeliest of suspects, the fact does remain that changing the edu does solve the issue. And given that the turn signal indicator is on that same common ground you'd think that someone would have noticed and mentioned that before.


I'm back to this being a power-in to pin 2 issue from the edu

Since I've only had the temp/fuel needles peg down for brief amts of time I've never had an opportunity for testing...

When your needles pegged down for a longer period of time, and the low fuel & check radiator lights came on. Did the needles stay down or did they resume function after seeing the low fuel/check rad lights come on (and go out after ~4 sec?)

 

[scantar]

Since I've only had the temp/fuel needles peg down for brief amts of time I've never had an opportunity for testing...

When your needles pegged down for a longer period of time, and the low fuel & check radiator lights came on. Did the needles stay down or did they resume function after seeing the low fuel/check rad lights come on (and go out after ~4 sec?)

There are different...um...'degrees' of failure symptoms. Sometimes, the needles will drop for a second or seconds, other times they can drop out for as long as it takes to slap, kick, beat the dash cover until they come back. Obvi, it's the latter that is the real danger...


I should add that after changing through a couple edu's I seem to have found one that has everything working as it should. For now...

 

[Mimmi]

If you suspect that needles are positioned incorrect, you can simply pull needle out (use fork etc).
Then input 12V to pin 2, 4.5V to pin 7 (temp gauge) or pin 1 (fuel gauge) and gnd to pin 5.
Wait for 30 sec and insert needle to center position.

 

[scantar]

In thinking about this, and hypothesizing with the KISS principle coupled with Occam's Razor...it would seem that the simple solution to the issue would be to find a constant switched 12v source and tap it right into pin 2


Tell me why I'm wrong

 

[Banman]

In thinking about this, and hypothesizing with the KISS principle coupled with Occam's Razor...it would seem that the simple solution to the issue would be to find a constant switched 12v source and tap it right into pin 2


Tell me why I'm wrong

I'm thinking that would keep your temp/fuel gauge accurate BUT might not solve the (greater) issue of the EDM "thinking" the power is off and therefore NOT powering the cooling fans as needed...

I think the gauges losing power is a symptom of the EDM effing up...
The temp gauge does not control the fan --

Also -- if the EDM is not putting a voltage signal out at pin 2 and you put power to pin 2 from a different source -- be sure and isolate 'that' power source from the EDM -- there's no telling how the EDM might react to a 12v input where it's supposed to be delivering a 12v output...
(well, there IS knowing but w/out a full circuit board diagram of the inside of the EDM to look at there's no way for ME to know, so I can only urge caution.)

 

[scantar]

I'm thinking that would keep your temp/fuel gauge accurate BUT might not solve the (greater) issue of the EDM "thinking" the power is off and therefore NOT powering the cooling fans as needed...

I think the gauges losing power is a symptom of the EDM effing up...
The temp gauge does not control the fan --

Also -- if the EDM is not putting a voltage signal out at pin 2 and you put power to pin 2 from a different source -- be sure and isolate 'that' power source from the EDM -- there's no telling how the EDM might react to a 12v input where it's supposed to be delivering a 12v output...
(well, there IS knowing but w/out a full circuit board diagram of the inside of the EDM to look at there's no way for ME to know, so I can only urge caution.)

Well that's the thing. It's either that the edm is looking at 12v going to pin 2 OR it's the edm not getting some feedback from the gauge. If it's a straight one way signal, then I agree that the edm is only looking at power in. But I'm betting it's a binary thing and the edm is getting signal from the gauge and feeding that to the ecu.



If that's the case then regardless of the edm fking up on the power in, as long as it's getting the signal back then all should be good. I'm willing to make my car the guinea pig on it and use a known bad edm for the test

 

[Banman]

Well that's the thing. It's either that the edm is looking at 12v going to pin 2 OR it's the edm not getting some feedback from the gauge. If it's a straight one way signal, then I agree that the edm is only looking at power in. But I'm betting it's a binary thing and the edm is getting signal from the gauge and feeding that to the ecu.



If that's the case then regardless of the edm fking up on the power in, as long as it's getting the signal back then all should be good. I'm willing to make my car the guinea pig on it and use a known bad edm for the test

I think the ntc thermistor sends a signal TO the EDM that is used to send a variable voltage to the gauge. I don't think the gauge sends any signal back to the EDM -- the gauge moves the needle as the only feedback of the gauge...

BUT I agree -- using a known 'failed' EDM is a safe way to add ancillary power (12v) to pin #2 and see what happens. (Safe in that you're not risking damage to a good EDM)

You have an EDM where the failure to power the temp/fuel gauge and fans is constant; not intermittant?

For all Chengny's pix & descriptions (maybe I missed it?!) I didn't see what the signal/triggering mechanism was for the EDM to switch the fans on. I could see where the EDM ties into the relays that power the fan but not what sends the signal/power TO the relay. But the temp gauge itself does not send a signal back to the EDM.

 

[scantar]

For all Chengny's pix & descriptions (maybe I missed it?!) I didn't see what the signal/triggering mechanism was for the EDM to switch the fans on. I could see where the EDM ties into the relays that power the fan but not what sends the signal/power TO the relay. But the temp gauge itself does not send a signal back to the EDM.

Well there are two possible explanations for what happens


Either the edm sees the gauge drop out and tells the ecu the data so that the ecu shuts off the fan


Or the two things are completely unrelated and coincidental



The latter seems unlikely so my assumption is something is reading the gauge and acting on it when it pegs out

 

[Banman]

Well there are two possible explanations for what happens


Either the edm sees the gauge drop out and tells the ecu the data so that the ecu shuts off the fan


Or the two things are completely unrelated and coincidental



The latter seems unlikely so my assumption is something is reading the gauge and acting on it when it pegs out

The ECU (T5.5) gets its eng temp signal from a different thermistor then the EDM and temp gauge thermistor.

Bizarrely (to me) the fans are controlled by the EDM and not the ECU...

I don't know that the EDM "talks" to the ECU at all.

 

[Mimmi]

Is EDM EDU (Electronic Display Unit)?
EDU reads sensor(s) and send voltage feed to gauge(s).

 

[Banman]

Is EDM EDU (Electronic Display Unit)?
EDU reads sensor(s) and send voltage feed to gauge(s).

Correct.
EDM electronic display module
EDU electronic display unit
They are the same thing

 

[scantar]

I'm more convinced than ever now that supplying 12v to the gauge cluster from a different source will be the permanent fix.


Off the paper and into the practical now...

 

[Timpa]

I'm more convinced than ever now that supplying 12v to the gauge cluster from a different source will be the permanent fix.


Off the paper and into the practical now...

Is there someone that have tried it yet ?


(My fix with changed temp/fuel gauges module didn't work the fault is back)

 

[My2290aer]

Can the ignition switch not cause these symptoms? I've got a 1997 2.3 Auto mis-behaving. I swapped out the ignition barrel and for a couple of weeks all seemed fine. Suddenly the gauges dropping out is real bad. Yes low fuel light comes on and it sometimes flashes. It would be brilliant if we could suss this out.

 

[secondsaab]

Can the ignition switch not cause these symptoms? I've got a 1997 2.3 Auto mis-behaving. I swapped out the ignition barrel and for a couple of weeks all seemed fine. Suddenly the gauges dropping out is real bad. Yes low fuel light comes on and it sometimes flashes. It would be brilliant if we could suss this out.

The ignition switch has nothing to do with it, or at least not in my 3 go arounds with 3 different 9000s. I will tell you what I know after a few years of experiencing this and my solution.

Simply clean all the ribbon connectors on the back of the cluster and the connector on the cluster. I learned about this recently, though I still have the dash cover off but haven't seen it happen since. I learned this from someone quite familiar with the problem.
OR
separately hardwire your cluster with separate 12v power. I haven't went down this wormhole but have with C900 and failed overhead dash illumination lights.