Dashboard instrument cluster information

[Casey]

Saw this for lot sale on fleabay for £35 delivered. Seller claims it's a working cluster and a set of "spares", so I thought it was worth a punt as I enjoy playing with old electronic stuff - seeing how they work and repairing where I can.

I have ordered some "Molex" female pins, which look to be perfect to make up the two round, multi-pin connectors, unless anyone has a harness they are willing to donate to the cause of understanding the dash operation?

I'd love to work out how to install a slightly louder and better sounding redline alert for a start!

It's amazing to see how crude the gauges are, compared to the equivalents I have been playing with in an FD cluster.

RX-7 FB dash cluster PCB 4.jpg (169.59 KiB) Viewed 27694 times

[Casey]

Having some fun reverse engineering the dash cluster to understand it's operation. Pretty simple stuff, as it is based on early 80's tech!

The fuel, temp and oil pressure gauges are all low impedance (~50ohm). Very crude, with resistance wire wrapped around bimetallic strips which, when heated by current flow, physically drive the gauge needle.

I managed to drive the tacho using a simple, low frequency (~40Hz- 220Hz) square wave source and discovered the over-rev buzzer wasn't operating. The buzzer is driven directly from a PCB on the tacho. Measurements showed the buzzer output from the tacho was at near zero volts until 7000rpm was registered, then it changed to 12 volts. The other supply to the buzzer is a constant 12 volts, so the change to zero volts should activate the buzzer! Faulty buzzer? No, much simpler! The output form the tach is via a steel screw and washer to the copper PCB. 40 years of sitting in variable, often humid, conditions, had caused corrosion deposits between the copper track and the steel washer, leading to a 3 megohm resistance. The buzzer then worked fine when the faulty contact was bridged.

The washer corrosion:

20230516_120251.jpg (113.39 KiB) Viewed 27665 times

Corrosion on the copper PCB track:

20230516_120422.jpg (140.91 KiB) Viewed 27665 times

Now working by bridging the faulty contact:

[Casey]

If anyone has an issue with their fuel sender not working at all, then, apart from the obvious wiring break, a faulty sender (I'm just replacing mine!) or faulty gauge, there is one more component that is unique to the fuel level measurement. There is a small PCB, hidden inside the dash assembly which generates a stabilised supply of 7 volts to the gauge. I guess it's there to give better gauge accuracy, given the battery may go as low as 11 volts and when the alternator is charging, the voltage can be around 14.5 volts. That would cause the gauge to give wildly different readings when the gauge is checked with the ign key on "ACC", or with the alternator running.

For the record - maybe not of interest to many - here is the 7 volt PCB (removed from a small heatsink) and the circuit. As someone who grew up with 1970's electronics, it's a very neat circuit which, today, would simply be a 3 pin integrated voltage regulator. It would be easy to repair one of these, or replace with a modern equivalent. For those "into" electronics, D2 is a 5 volt zener diode, which give a clue to how the circuit works

20230515_114627.jpg (82.99 KiB) Viewed 27665 times

[Hobbawobba]

This is awesome bro. I had looked at my cluster a few times since I've taken it out of the car aye. I was debating swapping all of my lights on it for LED ones. But also I had wondered about if its possible to convert the speedo to VSS like the FD uses. Then we could use and FD speedo gear in the tranny and get rid of the horribly intrusive speedo cable that wraps around the engine bay

Otherwise it would be a case of making a box with a little motor in, like a couple of companies do. But without the ridiculous £500 pricetag hahaha

[gt_james]

You're much more capable than I! I shied away from figuring out what was what on the stock cluster when I wired my race car, let alone understanding it's inner workings!

my solution was to put new gauges in. To be fair the stock cluster didn't fit after installing the rollcage, as the dash bar goes right through the space the stock cluster takes up, luckily the new gauges I bought just sneak in as they're more compact.

Out of interest, can you check if a tacho is calibrated? I've heard of a few people claiming they can under or over read quite a bit, but I've never tried hooking up a signal generator and checking.

same goes with the water temp gauge, how feasable would it be to modify the stock gauge to actually respond quickly enough to be helpful, or even calibrate it somewhat. It would be nice to have the stock cluster but working "better". Also another outrageous idea, how feasible would it be to modify the gauge to read oil temp and water temp switchable between one or the other with a discrete switch somewhere? Presumably not very feasible and aftermarket gauges would be the go to?

[Casey]

Out of interest, can you check if a tacho is calibrated? I've heard of a few people claiming they can under or over read quite a bit, but I've never tried hooking up a signal generator and checking.

That should be possible. I was doing some maths on the gauge I'm using for testing and I need to post that up to get others to check it. I need to be sure on where to divide or multiply by factors of 2 and 3, to take account of firings per chamber and conversion to shaft rotation

same goes with the water temp gauge, how feasable would it be to modify the stock gauge to actually respond quickly enough to be helpful, or even calibrate it somewhat.

I doubt much can be done here, without changing the gauge. The fuel, oil pressure and water temp gauges are all extremely crude, "pass a current through a resistance wire wrapped around a bimetallic strip to make it bend and move a needle". The advantage is they are heavily damped, reliant as they are on heating and cooling. Theoretically, it should be possible to set up a warning LED/buzzer to operate immediately the sensor resistances for oil pressure and water temp passed a threshold.

It would be nice to have the stock cluster but working "better". Also another outrageous idea, how feasible would it be to modify the gauge to read oil temp and water temp switchable between one or the other with a discrete switch somewhere? Presumably not very feasible and aftermarket gauges would be the go to?

Theoretically possible, but I'd go aftermarket gauges!

[Casey]

This is awesome bro. I had looked at my cluster a few times since I've taken it out of the car aye. I was debating swapping all of my lights on it for LED ones.

I haven't looked into that, but something to explore.

But also I had wondered about if its possible to convert the speedo to VSS like the FD uses. Then we could use and FD speedo gear in the tranny and get rid of the horribly intrusive speedo cable that wraps around the engine bay

Otherwise it would be a case of making a box with a little motor in, like a couple of companies do. But without the ridiculous £500 pricetag hahaha

It would need to be a box of tricks driving a motor on the back of the speedo. Theoretically possible, but a lot design work required!

[Hobbawobba]

It would need to be a box of tricks driving a motor on the back of the speedo. Theoretically possible, but a lot design work required!

Yeah I had a feeling that might be the case with it aye haha

[rotarypower]

It would need to be a box of tricks driving a motor on the back of the speedo. Theoretically possible, but a lot design work required!

I've actually done some research into this one, due to my ongoing project, the new gearbox in my car can't drive a mechanical speedo.
There are boxes you can get that drive a motor to wind the speedo, but it all seemed a bit over the top.

I'm currently looking at swapping just the speedo, but keeping the clock faces for that OEM look.

The donor speedo needs:
- To be simple, no canbus - just speed signal input
- To have the same range (or very close as the RX7 Speedo)
- Have a MECHANICAL odometer that is driven by the speedo, not special signals from the car

As it turns out, after looking a lot, the Vaxhaul Corsa has one that has all those features. (Corsa B)
They're cheap and plentiful, and all it needs is power and speed signal input, I will post some pics of them side by side later.

The only hitch is that the push to reset is in a different position on the odometer, and it doesn't have a high beam light display, these could be fixable, but possibly may need a custom dial back printing off.

[rotarypower]

So here are some pics.

The 'potentials'

20231007_203719.jpg (149.17 KiB) Viewed 19220 times

This is an electric one from a Corsa B.

20231007_203855.jpg (153.62 KiB) Viewed 19220 times

As you can see the speeds are / almost / correct.
Most likely a new dial face needed to keep it looking OEM. (And yellow) I envisage moving the high beam light elsewhere & making a 3d printer adaptor to retain the OEM position for the odometer.

Pic of the mechanism:

20231007_203914.jpg (103.09 KiB) Viewed 19220 times

And just 3 connections, +ve, ground, VSS signal. Pretty easy!

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