Saab: Following up..., Instrument Cluster Intermittency

Question
Mr. Schwell,

On 3/12/2008 you answered an inquiry regarding a '97 Saab 900 SE Turbo with instrument gauge problems.  ( http://en.allexperts.com/q/Saab-796/2008/3/Saab-900-SE-Turbo.htm# )  I have a '98 900 SE Turbo with a similar story.  All the dash gauges (tach, speed, temp, turbo, and gas as well as the panel's backlighting) spike "off" as described in the earlier post and then "reset" except for the gas gauge, particularly when the fuel level is below 1/4 remaining, which takes a bit to recover leaving the "nearly out of gas" indicator on for several minutes.  Also, when the panel is "out" the odometers (regular and trip) does not increment.  The SPORT light however, works perfectly fine (different circuit or power source I suppose).  The frequency and duration of the outages appears to increase with ambient temperature (this problem occurred more frequently and for longer "outage" durations over the summer months).

I'm working to try to narrow down the underlying cause, but it's not clear how to go about testing this since it's somewhat unpredictable as to when it will occur.  The working theory at the moment is that the Main Instrument Unit (MIU) (the assembly with all the gauges in it) has an intermittent fault/short/etc. causing it to seemingly reboot/power up and down, sometimes staying down for extended periods.  (I recently replaced the ignition switch as the contacts seemed of concern, but to no avail.)  Like the earlier post, I too had a fuel pump replaced about a year ago and these symptoms started to manifest a few months after that.  Could merely be a coincidence, but with such a similar storyline to the previous post, I felt compelled to post for your opinion.  -- Did you ever hear from the earlier poster how his/her situation was finally resolved?  (This bit of context seems to me to be potentially quite useful given the NG900 model year proximity.)  Any other thoughts short of replacing a rather expensive MIU?

Thanks in advance!


Answer
Chuck:  It certainly sounds like an intermittent power problem as you've suggested. Also, the possibility that the problem is related to your ignition switch or fuel pump replacements seems remote. Since I had never heard back regarding the earlier problem that I'd addressed, I do not know the ultimate outcome. A few suggestions though, short of getting a new control unit come to mind:
  a. A very tedious check of all applicable wiring (I have used a homemmade tone generator to  facilitate one-person wiring integrity checks such as this#
  b. A simple line filter, i. e. a large filter capacitor across the +12 volt input power lines. This would only help for short #transient) faults.

 If it comes to getting another MIU, I would consider a recycling yard source such as located by querying <www.partshotlines.com>.
Also, I would consider subscribing to website <www.alldatadiy.com> [~$25/vehicle/year] as the very best, most complete source of technical information that I know of.

      Hope this helps,
                     Jerry