It feels like it’s been a huge uphill battle getting a working tacho in this car, but I think I have finally mastered it.
Over the past few months, I have somehow ended up with a few different clusters, including two with the elusive tachometer module, and the one with the non-functioning clock I previously made a post about.
The first three dial with tacho I bought from Trademe.
It was the correct Aus cluster, with the lower 5500RPM redline, and a couple of other small differences as I would later find out.
Eager to get the tacho in and working I started to dig around the wiring diagrams to work out what I needed to do. I had heard rumours that the wiring was already in the dash, behind the cluster, and sure enough, after some poking around, I noticed a white wire tucked away with a joiner in the middle of it. This tracked with what I was expecting to find based on the wiring diagrams.
It was a big loop in the harness. I fished it out from behind the duct and unplugged it. A couple of quick checks and I was sure it was the coil trigger for the tacho. On the cars without tachos, this is just looped in the dash but needs to be connected or it cuts power to the coil. I guess it made it cheaper and easier, to just use the one harness.
The back of the tacho unit has corresponding bullet terminals
So I plugged it into the dash, and bam, nothing happened. Well, the alt/oil lights worked as they should, but the tacho was dead. I was a bit miffed
Nothing more to do then but to get to disassembly and find out what’s wrong. The tacho unit is held into its housing by a couple of screws through the back, and the fascia/glass in the front.
This is what the module looks like removed.
Having already googled “why does my Smiths RVI tacho not work” I could already see something was wrong.
Those four holes in the foreground, under that red wire? Yeah, according to google there should be a transistor there, and it’s crucial to the operation of the unit.
The solder pads had clearly been messed with too. It looked like someone had desoldered the component with a blowtorch.
I hit the seller up and asked if he knew why it’s been molested, and he just pleaded ignorance and fobbed me off with a “oh well, it’s an old part”.
I suspect the capacitor or transistor had failed at some point (which is common) and someone tried to fix it. Failing to have the right parts on hand or something, they just chucked it back together and set it aside.
As fate would have it, a fellow classic car sufferer on a forum I’m on knew I’m suffering the Marina affliction and mentioned that he had come across some Marina bits as part of a garage clearout, and would I be interested in a cluster he found? Heck yeah I would! The legend donated this to the cause, so a huge Thank You.
So, this was the second tacho cluster I have. It came in a tidy surround, but it was brown, not black. No issue, They are easy to swap, and I only really needed the guts.
Differences to observe. The silver rings around the dials, instead of the black the Aus cars have, the 6000RPM redline on the tacho face (ignore the askew tacho, I had already started to disassemble it), and the different markings on the fuel and temp scales (0 instead of E, and adding the N to temp). One final difference I didn’t notice initially, is the warning lights are different, with some either doing different functions, or in the case of the indicator telltale lamps, not there at all. The UK cars seem to use one single green light to show the indicator is on, whilst the Aus cars use the two spaces above the center dial as left and right signals.
*removes anorak* Right, so this cluster. Excitedly I plugged it into the car, and we had some success. The tacho moved!
But it barely exceeded 1000rpm when revving the engine. I think this one may have been suffering from the known issues Smiths RVI tachos suffer from (bad capacitors), but instead of messing around with the old inductive RVI style guts, I spent a hefty whack of cash on the Spiyda RVI-RVC conversion board.
The original RVI tacho is current sensing, so it intercepts the power feed to the ignition coil, and by some wizardry senses the pulses and creates a signal for the tacho to output. The issue with this is apparently the tacho only works with points, and in the future I want the option of upgrading to electronic ignition without having to replace the tacho again, so it had to go.
The Spiyda board removes all the existing guts from the tacho, and replaces it with a new board that reads the signal from the negative terminal on the coil (like 90% of tachometers). Heck, it can even be fed a signal from an ECU. It’s pretty swish stuff.
I stripped the second tacho out of the housing, and you can see the missing component of tacho one here; the silver can is a special transistor.
I started with the guts of the first tacho since that one was dead in the water anyway. Spiyda has extensive instructions on its site, here, so follow those, but this is how I went about it.
The first step is to remove the needle. You don’t need to mark where the needle sits, just make sure the mechanism is against its stop when you refit the needle. To remove it I used an old business card with a notch cut in it, and a sturdy fork. The business card is to reduce the risk of damage to the face.
A swift lever upwards popped the needle off.
Two little screws secure the face. The kit comes with a tiny screwdriver to remove these.
Now cut the power feed wire, and the two thin wires to the mechanism and remove the two screws holding the circuit to the frame. It should pull off the front.
The new board then gets screwed in place
Now solder the wires in their respective places (in my case the black and red wires had to be swapped). and you’re ready for testing and calibrating.
Now, I did make a mistake in the above photo. It turns out that the video I was following, made by Spiyda, was out of date, so you no longer calibrate the unit by putting the red clip onto the tacho feed (which I made from the old RVI tacho feed, by cutting a section off and soldering it to the board). Instead, you need to clip the red clip to the solder pad on the far right, closest to the big chip (on the other side of the board), or solder pad number 4.
With 12v connected to the spade terminal on the back, the red clip on the solder pad and the black clip on the metal frame, it’s time to calibrate.
Now, this was a real ballache for me. In the end, I don’t know why it suddenly started to work properly, or what I did differently, but it was chaos.
The instruction and files to download for calibration are on the Spiyda site, here.
Basically, you play a square wave audio file at a certain frequency through the cable at full volume. That frequency should correspond with a certain RPM reading on the tacho. Since I was using a low revving 4 cylinder engine, I used the 100 and 200hz files as this should read 3000rpm and 6000rpm on the tacho.
I had various levels of success depending on what device I was using. My Macbook, no good. My phone, initially average. My windows based Dell tablet, good but with some issues.
I could get it to read 3000rpm, but doubling the frequency would either make the needle drop or only increase by a small amount. There was also a big jump in the needle at the start and it kinda crept down. This Gif shows the issue I was having. This is the 100-200 sweep, where it starts at 100hz and then shifts to 200hz. You can see the 100hz once the needle settles down, but then when it changes to 200hz the needle drops to just under 2500RPM.
I had been in contact with Spiyda support for a while, since even before calibration issues I had issues getting the unit to respond in the first place, and they had been very helpful in getting to where I was, with prompt replies, but then the support suddenly went cold and I heard nothing further from them.
It even got to the point of stripping down the second tacho and seeing if the issues were limited to the first one; they weren’t.
I persisted with various things, and eventually I had great success using my phone. I don’t know what changed, or why it worked now but didn’t earlier, but suddenly I had 3000RPM at 100hz and 6000RPM at 200hz. It needed a small tweak of the calibration pot on the back, but it was rock solid.
With much excitement, I rushed to the garage and reassembled it into its housing.
And with a temporary wire run outside the car from the coil negative to the cluster, I fired the engine up. Nothing. *Sigh*
And then I remembered there were options for the input. There was a high voltage and low voltage signal option. In hindsight, if I had paid attention initially I wouldn’t have wired the input to the high voltage “sports coil” option, and should’ve used the “normal coil” option. Oops.
It was easy to fix once I removed the guts from the cluster and moved the white wire one pad to the right.
I reassembled the cluster, and plugged it in.
Success! I’ve run the car a couple of times since, and the tacho seems to be fairly accurate. It doesn’t need to be 100% accurate, just within the ballpark is good enough, and better than Leyland would’ve done. It responds quickly and is very stable.
I still need to run the tacho wire inside the car, but that’s easy enough to do. I should probably fit the surround back on the gauges too since they are more or less done now.