35 replies to this topic

[fuzzypumper]

Hi,

AS the original torana tacho is a 2 wire ( ground & signal) its a current driven meter unlike modern tachos which are 3 wire (12v, Gnd & signal) and pulse driven.
However if you upgrade to High energy ignition ( Blue motor type) the your tacho wont work properly.
So does anyone know of a circuit or module that converts pulse to current?

Cheers

[fuzzypumper]

Thanks chopper

If you can find that info i would be good to compare.
I have also been looking at some circuits involing either either a conversion module to sit infront of original tacho or another that guts the internals and only uses the original meter.
Yep been to Jaycar and got some parts including a LM2917 chip.

I called ringwood speedos and they want $165 to put a whole new 3 wire PCB setup using the original meter & tacho face that will do the job.
At that price an off the shelf tacho will have to considered.

[Toranamat69]

I wonder if one of these would do the job for you??

http://store.summitr...15&autoview=sku

http://store.summitr...15&autoview=sku

Probably only worth it if you need to place a larger order with summit though.

I am not up on how the original torana tacho's worked as I have always been to scummy to buy a Torana that came fitted with a Tacho )

Which model Torana are you referring to? I may be able to design something for you and tell you which parts you need from dick smith to make it work if I understand the problem a little more .

I have always had pretty good results with my own electronics projects - For my LT5 I had to make a module to trick the VATS system in the control module to make it work to save buying another chip at huge expense - I think that cost me $10 of parts from dick smith - $6 being for the little black case.

I also had to make a module to make the VDO tacho work from the tacho output from the Delco control module - I think that one cost me about $1.20 of parts.
It was just a tiny transistor and 3 or 4 resistors.

Brett_32i, I think someone mentioned on here a couple of months ago that there is a special output from the MSD which provides a normal tacho signal. I can't find it at the moment though.

M@

[Dangerous]

Fuzzy, early tachos aren'e current driven - they are simply frequency to voltage convertors, and the tacho is really just a voltmeter. Stock LC and LX series tachos work reasonably well with the Commodore style of electronic ignition - at least as accurate as the stock tachos (which aren't really accurate anyway).

My advice is to fit the ignition, and see if you can pick the inaccuracy in the tacho. Not sure you'll be able to.

[Dangerous]

Yeah, the msd has a tacho output (green) but that still didnt work.

the tacho does actually work, at idle to about 4K rpm by memory. the problem being when you nail it, and then you lose your tacho, needle just drops to zero. so now the almighty wanker tacho is in place i no longer have that issue.
the issue now is i have a wanker tacho...

Brett, older tachos won't work well with MSD ignitions, as you have discovered. As you are no doubt aware, MSD stands for multi spark discharge, ie many sparks per combustion. The older tachos misinterpret this, and still think that it's one spark per combustion, or thereabouts. At higher revs, the older tachos lose the plot entirely.

[antelopeslr5000]

My MSD 6AL works fine with my stock LX tacho.

[Dangerous]

My MSD 6AL works fine with my stock LX tacho.

Is that off the tacho signal output on the 6AL - all early tachos should work off this, but not off the coil -ve.

[antelopeslr5000]

No, I'm using a Mallory Unilite distributor so I've connect the green wire from the distributor to the white trigger wire of the MSD. I've also connected, in parallel, this wire to the brown wire from the tacho (which is no longer connected to the -ve side of the coil).

Edited by antelopeslr5000, 14 February 2008 - 10:36 AM.

[fuzzypumper]

Fuzzy, early tachos aren'e current driven - they are simply frequency to voltage convertors, and the tacho is really just a voltmeter. Stock LC and LX series tachos work reasonably well with the Commodore style of electronic ignition - at least as accurate as the stock tachos (which aren't really accurate anyway).

My advice is to fit the ignition, and see if you can pick the inaccuracy in the tacho. Not sure you'll be able to.

I have been busy and designed a circuit to gut the old tacho board and replace with a 3 wire electronic circuit and keeping the original face & needle as well a replacing the mechanical regulator with a 3 terminal reg. so the dashes fuel, temp & oil meters dont change readings when battery voltage changes i.e 11- 14volts between idle and high revs.
I will hopefully post some pics tomorrow.

MY original tacho hasnt shown a correct reading since i placed a electronic bosch unit it 10 years ago. Even after I adjusted the internal trim pot it gave a different reading everyday. If i set it on the high side the needle waves across the dial, if i set it lower its stable but is out by a 1000rpm at 6000 mark.
This reading changed again when I simply replaced the old coil with a new epoxy bosch.
If a WB or VC commodore dont run an updated 3 wire tacho in their dash then I would bet they have a update circuit design with extra dampening.

Dangerous, You may have misuderstood, In electronic terms the meter element is driven by current in these old 2 wire tachos by design.
I'm not saying its current from the coil that directly drives it because what comes off that is always a voltage pulse.
Unfortunately its the size of the pulse that throws the old tacho out, not to mention the length of the pulse as eletronic ignitions change dwell angles as well sometimes.
I called Ringwood Speedos and guy on the phone told me exactly that.

[fuzzypumper]

Well I did it and it works but the problem is the old Torana meter needs recalibration in the form of weighting. While I successfuly adjusted the electronic side to show correctly between 1000 to 3500rpm, after that the needle progresively began showing higher.
Ah Well!




BTW: WHat voltage is the mechanical bimetallic regulator on the back of dash cluster suppose to put out?

[Dangerous]

Chop, the LH/LX regulators are supposed to be 10 volts.

Fuzzy, all moving meter coils are moved by current through the winding, which sets up an opposing magnetic field to either a permanent magnet filed or a static coil. What I meant by referring to the older style tachos is that they looked directly at the average voltage on the coil primary, and (loosely) converted that into a DC voltage which gave a known meter deflection. A far better design is to count the pulses an work that back to exact revs, as later model tachos do.

Sounds like your clever mod (well done on that one!) might be more or less linear in the rev range, but the moving coil part of the meter is non linear.

I've built a few regulator replacements using 78H12 regulators, and I found that you have to heatsink them pretty well, or they can suffer from inaccuracy as well, in this application.

By the electronic bosch unit, do you mean the VC onwards dizzy with the module in the dizzy, or the one with the separate control box with the TO3 transistor on the top. I've never had stability or variability problems with the module in dizzy type, although they do tend to read a bit inaccurately.

[fuzzypumper]

Firstly, I apologize for my long winded, pedantic & possibly boring forum posts

Its definately isnt 10 volts. I made a 10 volt reg and it sent all the gauges to full immediately. I suspect the 10 volt ones are what they use on 3 wire powered gauges like Commodores have using a TCA700Y 10 volt reg.

I did find a google automotive book that made the general statement, Automotive mechanical bimetalic regs are 5volts which sort of corresponds to to numbers on the casing i.e. 50/5/1 which I suspect could mean 50% duty cycle, 5 volts max, 1 volt min.

I did measure a couple of the old can ones on an analogue meter and they came up at an average of 2 and 2.8volts each. On the digital meter they swayed between about 1.3v and 4.4volts.
So they have obviously deterioted over the years giving lower needle readings.

Chopper , good point, yes I'm not looking for accuracy in these gauges since they dont have numbers but I am hoping for consistency. I had the same idea using the fuel gauge as its only one that doesnt vary as the engine warms or revs.
Interesting to note you had to set yours to 6.2v. I suppose the gauges also being of bimetallic heated strip construction have suffered the same degredation as the old can regulators, thus giving everyone that false sence of security.

Dangerous, I did mean the Bosch VC comodore type with module screwed to it & Hi volt coil. My tacho had issues from day one with this setup.
Youre right, the meter element probably isnt as linear as we like it.
Yes , I have noticed that th3 78xx series 3 terminal regs can tend to heat up more than Ide like in some applications. Whether it be too much input voltage or load not sure. But not sure why you have been using 12volts regs as they have 1-1.5 volt dropout voltage which means you need at least 13volts input for them to regulate 12volts out. Since at idle a cars battery voltage can be 12volts, the best maximum regulated output you can hope to achieve in all conditions is 11volts.
Which is why I suspect most current dashes use the TCA700Y - 10 volt regs. I would check to see what current the reg is pulling because if you are putting 12 volts into your gauges im not suprised the reg is overheating and /or dropping out considering they are supposed to 5 volts. Maybe your gauges have suffered burnout and need 12 volts to get them going????

PS. If any one wants the circuit design & pics for the tacho mod, just let me know.

Edited by fuzzypumper, 16 February 2008 - 08:35 PM.

[Dangerous]

Hmm, interesting, I'd better retract my comment about the regulators being 10 volts until I can check it. Maybe I'm getting the Commodore instrument regulators confused with the Torana ones - damned memory recall....

Fuzzy, re using the 12 volt regs, I just built a few with variable outputs, and used one of them in an LX dash, and adjusted it accordingly, so nowhwere near a 12 volt output - that would fry all the gauges within seconds. It was a while ago, and from memory, I just put dummy loads on all the gauges for half needle deflection and adjusted the regulator so that the gauges read accurately, so it may well have been less than 10 volts.

The heat dissipation of the regulator depends on the input voltage, output voltage and the load current. The reg after all just acts as a a smart resistor, so power dissipation is voltage drop x load current. What I noticed when I didn't bother heatsinking the regulator too well was that the gauge readings dropped, as the reg was probably going into current limiting protection, as it was getting too hot.

If you're going to reference off the fuel gauge empty reading, make sure the fuel tank is very well earthed, which they are not from factory.

[fuzzypumper]

Dangerous, I hear you on the memory recall issue. Ive been pulling apart the dash and sitting there dumbfounded at why I put that wire there 10 years ago.

Thanks for the tip on the fuel tank issue but i have a LPG only setup and the guage on the tank is 10-73ohm which has never made the dash fuel gauge hit full when filled but that could be due to the mechanical regulators low output. I'll tell you what though I never ran out of LPG accidentally.With the needle on empty, there was always plenty in reserve. I would like to know what the ohm range on the original petrol tank sender is supposed to be?

I also did some load testing on the guages using a 5 volt 7805 reg with a small heatsink. Basically each of the 3 gauges( fuel, temp, oil) that are powerd by the reg. pulled up to 190-200mA when needles at full, so 600mA at the most. I also have to take into account the 8 or so volts that has to be dissapated to drop to 5volts.
Which adjustable regulator did you use?

[Dangerous]

I would like to know what the ohm range on the original petrol tank sender is supposed to be?

I also did some load testing on the guages using a 5 volt 7805 reg with a small heatsink. Basically each of the 3 gauges( fuel, temp, oil) that are powerd by the reg. pulled up to 190-200mA when needles at full, so 600mA at the most. I also have to take into account the 8 or so volts that has to be dissapated to drop to 5volts.
Which adjustable regulator did you use?

LH/LX gauges use a 73 to 10 ohm range.

Re the regulator, can't remember. Maybe it was an LM317 in a T0220 package, rather than a fixed reg. If I was building any more, it would probably be a T03 package LM317/350, and mount it remote to the dash, with some better airflow.

[fuzzypumper]

Thanks.

I did some load measurements on my gauges using the new 7805 5volt reg and got the following:
Fuel gauge empty line=68ohms
Fuel gauge full line = 11.9ohms
Temp gauge empty line=55ohms
Temp gauge Full line =9.7ohms
Oil press gauge low line=55ohms
Oil press gauge high line=9.7ohms

So the 73 -10ohm range you mentioned sound right.

I also loaded the temp gauge to get a half way reading on the meter using the 7805 5v reg. Then I plugged up the original mechanical reg to compare and the temp gauge went exactly to the half position again. I even increased the input voltage to 14.5 volts and the needle didnt move.

So conclusion, Im starting to see the beauty and simplicity of the old mechanical regulator. It still good and steady and with its 50% duty cycle i can see the meter elements lasting longer than having 5 volts constantly on them like the 7805 reg would. I have more confidence in my gauges now.

[Dangerous]

That's good info to have posted on this forum.

As for the old regulators, yes, they're simple and work well for the bimetallic style of gauges that the LXs use. It's usually earthing problems that screw the gauges up more than failed regulators. If the regualtor loses its own earth, or the earth path is high resistance, I would think that it would output a much higher voltage, which would not be good.