I do not know how familiar readers are with the inside of the pre facelift XJS instrument cluster, in particular the minor guages? The guages themselves are pretty reliable (being early examples of the air-cored type of guage, modern versions of which are used by Speedhut, according to their website). The problem lies in the way these guages are electrically connected to the flexible circuit board that is behind the nacelle.

My volt meter started playing up, and then the oil pressure guage, which is why I was and am, considering aftrmarket guages. As I have a brand new flexible board in stock, I decided to give the cluster a last chance; so I had the cluster apart last week and fitted the new flexible that I had in stock. After a bit of a struggle I had everything working except the oil pressure and volts guage; and these work when directly connected to my 12v source and respond properly to voltage changes.

However once mounted into the nacelle they do not, and the only reason I can see why not is the very iffy way they are connected to the flexible. Which is by tiny screws the threads of which need to bite into the copper strip of the instrument:


Flexible circuit board, showing the holes through which the minor guage screws go, they secure the instrument and make the electrical contact


This shows the very tenuous way the circuit board makes electrical contact with the instrument


The screw, inserted through the flexible board has a very short contact length with the instrument board


Instrument inserted showing the very small contact point for the threads to make contact

I therefore decided to see if I could do better. My idea is to solder 3mm threaded rod to the instrument board and to poke that through the nacelle plastic so that a nut can secure the guage to the white plastic cluster chassis, and thus make a far more reliable connection. I tried using stainless rod but I found it is damn-near impossible to solder to stainless!

I therefore ordered brass threaded rod to try that. I have jigs all set up (unbelieveably of the four minor guages there are THREE different spacings!) so fingers crossed.


Jig for oil pressure guage board with clamping block to hold it in position during soldering, in place


Three different fixing screw spacings


Jig closeup


New flexible but with the minor guages still with their screw contact to the guage board


Mockup showing how the threaded rod soldered to the guage board will contact the flexible more reliably, I hope.

So yesterday and today I got to it. I have a spare cluster so trial fitting the instrument boards was easy, as was making the jigs, as the spare cluster could be used as a template. I found that the brass rod needed to be 26mm long, and have the soldered end flattened off so it resisted turning in the solder:


26mm long for the outer fixings, the middle contact used in three of the guages needs to be shorter, once soldered it can foul the plastic protrusions so these were removed, see pic further below


End flattened off to fix into the solder better

Soldered instrument boards. These are fixed to the actual guages by 8mm nuts and are easily removed for soldering:


Board attached to guage


Board removed for soldering


Trial fitted to chassis, note shorter central rod


Ready to solder

An advantage of the brass rod is that it is easy to bend slightly so as to line up the rods with the chassis holes. Even so, I opened up the chassis holes to 3.5mm from 3mm to give a bit more play. You need 10mm of rod showing through the chassis to get the nut on easily. Additionally, because the threaded rod holds the guages securely, and to make the soldered parts easier to get properly home in their slots, using pliers I pinched off the plastic sticking up from the chassis:


Plastic protrusions pinched off for easier fitting of the soldered board

The soldered boards were trial fitted in the spare chassis and are ready to go into the car nacelle:


All ready to be fitted, dreadful soldering hidden!

The guages in the car nacelle were removed and their boards swapped for the soldered ones. Then refitted as in this photo:


Threaded rod and nuts now holding the guages.


Just in case, tape over the fixings.

All went back in the car and the oil pressure and volts guages started to work properly and the other two continued to work. Poor connections to the guages are a known weak spot that the Great Palm discusses in The Book. I think that this modification is reasonably permanent, electrically more reliable, and what is more, in the event of any further trouble, it is easy to troubleshoot, disassemble and fix.

 

Excellent work Greg! I might start to look at embodying some of those principles on my 4.0 facelift car.

Paul