Analog gauge cluster tech

[Ivan141]

I've been busy tampering with old analog gauge clusters and figured it would be prudent to put my findings in a thread of their own.

My first area of interest was in recalibrating the tachometer to suit different engine configurations and/or different gauge faces (10k redline for example).

I dumped most of the info I found in the digital dash thread by accident, so here's a little rehash of that:

Quote:http://www.toymods.org.au/forums/tech-co...utput.html
http://www.autosoftsys.com/supra/tachcompare/




Basically you have an R/C circuit where 3 resistors and a capacitor are, the resistors vary for different car models (R/C circuits have an exact discharge time that I suspect is important for the RPM calibration).

Then there is a resistor inline with the input signal. Something tells me this one is important for the input level of the rpm signal. If you take a look at the last link, you'll notice the 1JZ&7MGTE use 15k whilst a 7MGE uses 30k.
The turbo engines both have coil packs and probably don't use the coil- signal for input, whereas the 7MGE is a distributor driven car like the AE86, and has a higher resistor value for more attenuation.

The last resistor that's different between the above mentioned engines is R8, which is on a voltage supply input. No idea what it does, but I suspect it is related to the input detection as well. This one is lower for the 7MGE, so perhaps a higher voltage value is used for comparing the input signal? Without a datasheet it is hard to tell, but a bit of experimentation with some potentiometers in the correct value ranges could probably reveal all you need to know.

Comparison of tach components
# 7M-GE (1987) 7M-GTE (198 1JZ-GTE (1992)
R1 62K 130K 100K
R2 12K 12K 12K
R3 510 560 560
R4 30K 15K 15K
R5 55 55 55
R6 15 15 15
R7 560 560 560
R8 33K 39K 39K
R9 47 47 47
C1 ?? 0.1+ 35F 0.1+ 35F
C2 ?? 472 50K NCC 472 50K NCC
C3 47uF 10V 47uF 10V 47uF 10V
C4 ?? 33uF 25V 33uF 25V
IC1 ?? Toshiba 8806K T2685A Toshiba 9214K T2685A

Today I took apart a spare cluster and followed up on my earlier suspicions.
I put a potentiometer (adjustable resistor) in place of R4, and another one that effectively replaces R1,R2 & R3.

Tach signal input:
As mentioned in the quoted info, R4 is there to attenuate the coil negative input signal. If you just put a 12V rpm signal on an AE86 dash, it will not work without a tach booster of some sort. If you lower the resistor however (to about 15-16k) it will detect a 12V square wave just fine. With some more lowering it will probably also detect a 5V signal (around 4kOhm perhaps? Found that value on a Pontiac grand-am, which uses the same gauge driver IC)

Needle sweep adjustment:
If you search around you will find that resistors R1,2,3 have varying values for different applications, even in the same car with the same engine. Electrically speaking, these 3 can be seen as 1 (equivalent) resistor. The reason 3 are used is because it allows them to fine tune the value exactly, something that is otherwise impossible without a potentiometer (which is more prone to drift / sensitive to vibration, not to mention more expensive for production).
I noticed most 6 cylinder engines use an equivalent resistance of 8 to 10kOhm, whilst 4-cyl engines like the 4age use values around 15kOhm.
Practically speaking, the more pulses per revolution you get, the lower this resistance should be.
I found that adjusting these resistors to around the 12kOhm mark would be about right for a 10k RPM gauge overlay.

An alternative method exists in which a shunt resistor is placed across the output wires to the gauge, thereby lowering the power fed into the gauge motor. Personally I'm not a fan of this method, it only allows you to adjust the needle sensitivity in one direction, and it's probably not the most accurate way to do it.

That concludes the bit about the tachometer.. out there for everyone who might find it useful so they can get rid of fugly aftermarket tachometers.

In the meantime I'm researching options to have some alternative gauge faces produced (not just an overlay).

Drawing some inspiration from WWII aircraft cockpits:


I have some OEM style ones in the works.. and perhaps kouki-style pinstripes will also follow.

This might be an alternative solution for those with inaccurate speedometers due to gearbox / final drive swaps, I can create just about any scale you can think up.. just needs some careful measuring for solid reference points.

Now just to find out what production methods will suit.