Toyota Supra MK III digital gauge swap


Toyota Supra MK III

Digital Instrument Cluster Disassembly

Standard Disclaimer:
    This is an account of what I did with my vehicle and my own comments. Any use of this information is at the user's risk.
This exercise was performed on a 1G-GTE MK III Supra but advice to hand is that 7M digital dashes as they're basically identical and that the 1JZ dash is only different in omitting the oil pressure gauge.
The instrument cluster of the MK III is a little more than a collection of instruments to inform the driver. As part of the vehicle electronics, if it falls into disrepair, it can be the source of operational failures. This is particularly so as the No. 1 speed sensor signal (used in shift control of the automatic transmission) comes via dash instrument electronics. The desire to rid the gremlins from my recently rebuilt transmission (along with annoyance at the infamous unreliability of the fuel guage) prompted me to embark on removing and inspecting my instrument cluster. Mine is a right hand drive vehicle.
Removal of the instrument cluster itself is not difficult, although requires a little patience (that's what cars are for). From memory, I removed
  • the centre console facia and heater controls
  • the small panel to the right of the steering wheel which houses the fog light switch
  • steering column trim pieces (4 panels) and covered the column with a soft cloth to protect the instrument cluster face on removal. The steering column was dropped to its lowest adjustment and extended to be farthest from the dash.
  • the "under trim" of the dash pad over the cluster. The screws were easily accessible in their recesses. The under trim piece was fragile and required careful removal. I will have to replace some of the screw recess bases to properly secure it again.
The instrument cluster was thus revealed with six fixing screws securing it to the dash. Having removed the screws, I could move the cluster assembly outwards and get my hand behind to remove the 4 or 5 connectors from the back (actually, to push painful little tags, wriggle and cajole the larger connectors with expressions of frustration). With the harness disconnected, the assembly could be carefully maneouvred out from its "cave" and past the steering wheel toward the passenger side of the cabin.
Here you can see the neat white plastic speed sensor housing fed by the top speedometer cable (not visible here) and secured to the front cabin brace member behind the instrument assembly (removed in the photo, of course). The sensor itself is a simple Hall effect chopper assembly.The sensor separate from the instrument assembly made speedometer cable maintenance easy. I lubricated the cable and the sensor 'bearing' with TruFlo (a teflon lubricant advertised as suitable for applications such as firearms) as it had developed a scraping noise ... we will see how long that lasts before the cable needs replacing.
To the left of the speed sensor I found a metal box with two wiring harness leads disconnected. Having reconnected them, I subsequently found out that this box was a 105 km/h speed warning chime. One of the more pleasant vehicle alerts I have heard but it still ended up with an isolation switch on the console (for use on 110 km/h freeways here of course).
Once the instrument assembly is removed from the vehicle, I placed it face down on a SOFT SURFACE (I did not want the ugliest scratched instrument facia when I put it back) where the vent registers could be carefully removed from it. These were held by a couple of screws and plastic clips. If anyone can preserve the plastic clips, he / she deserves a medal.I found that, of all the exposed screws on the back of the assembly, the ones in the centre hold the instrument internals in place. I did not want them released until the front was removed. So to remove the front. I undid the screws around the outer edge of the back of the cluster.
However, a small modification was required to complete the removal. A single small screw INSIDE the casing also secures the front panels. The picture on the right shows a slot made at the lower part of the passenger side end of the rear of the assembly. This slot was carefully cut with a drill bit to access the single internal fixing screw. The casing is already extensively vented, so another slot would not cause any extra problems. The location of the slot was determined by removing the other perimiter fixing screws and carefully separating the shell and front panels just enough to sight the internal screw.
Once the final screw is removed and the front panels carefully separated from the back shell, the front was revealed to consist of three layers (all smoke and mirrors to make it look flash - well hopefully not smoke.). Particularly as the casing is not sealed, these layers accumulated a layer of grime. Carefully cleaning each with a SOFT cloth and a plastic cleaner (I used one called Plexus I use on my motorcycle and helmet) made a HUGE difference to the dash appearance.At the bottom of the image to the left is the naked instrument assembly with its custom glass fronted IC package components. I cleaned these glass components with methylated spirits.
On the right hand side is the engine temperature bar graph indicator unit. It is a separate circuit board which may be removed. Note though that it is on an angle to match the fuel guage on the other side of the speedometer but its fixing screws do not fix its position i.e. when re-installing it it needs to be adjusted to match the fuel guage angle and then the fixing screw tightened to secure it. The fuel guage is fixed to the same circuit board as the speedometer, so it could not be adjusted.A couple of dark patches are just visible near the bottom left and top right corners of the indicator unit. I would figure these to be due to heat from to power connections for the display.
I found it interesting that the temperature guage has a couple of elements missing in the region immediately above normal operating temperature. We don't want the driver panicking about temperature fluctuations now do we?
With neither illumination nor masking, the speedometer unit showed all possible displays. Sorry, anyone who wants to go over 399 kmh will need another instrument (or convert to MPH but that just shifts it to a 399 MPH limit). The odometer is a mechanical counter mechanism driven by an electric motor on the lower right of the speedometer. .
The screws exposed at the rear of the cluster shell were removed to lift out the speedometer subassembly for inspection as the major wiring harness connectors attach directly to this module. As this subassembly comprises three stacked daughter boards with interconnecting ribbon cables, I was not eager to disassemble it. However, if suspected electrical connection problems continue, I would separate these boards to examine the connector solder joints on the back of the board shown on top.
At the left hand end of the cluster is the third instrument component which comprises the boost and oil pressure guages as well as numerous vehicle subsystem status indicators (TEMS, engine and doors). Note the earthing pad on the far left of the component circuit board. This is the location of the internal facia fixing screw. (Beats me how they assembled this part ... )
One variation from the overall design is the seat belt warning light which is separately mounted on the back shell and accommodated by a hole through the boost etc instrument component. I guess this was only required in some countries. All the same, I would have thought it cheaper to include it in the component and simply not activate it ...
After cleaning surfaces and connectors, I reassembled the cluster (twice of course having found something left out the first time). Installation back into the vehicle revealed that I may need to go back and disassemble the speedometer unit ... If the cluster assembly did not sit just sweetly against the mounting points, i.e. if tightening the fixing screws applied any bending pressure across the unit at all, it exhibited total operational failure (including undrivable transmission shifting - limp home mode?). I found I had to do the final mounting with the ignition switched on.I now have a brilliant instrument display and sound transmission behaviour for the past six weeks. From previous experience with the climate control panel, I suspect however that I will need to repeat this job to ensure sound connector joints to the speedometer unit.
Post Script: 
After a few months, I have noticed the speedometer cable again occasionally make the 'tsk, tsk' noise. Looks like a new top cable will be required. The O/D light has also shown itself on occasion although transmission behaviour has been OK. It looks like I may have to revisit the speedometer assembly and I expect to have to resolder those harness connectors ...