Barrel_Ball Posted November 8, 2009 Share Posted November 8, 2009 Today I went to my local Pick n' Pull, and found a '77 Jaguar XJ12 L sedan, so I took the gauges. These would be the electrical Smiths units, dunno the exact measurements, but they look like they would fit into a Z dash with some custom mounting of sorts. Sure, I could just go and use aftermarket gauges, but I don't feel like it - I like the look of these guys better. I know the speedo would probably work simply with the right gearing. As for the other gauges, I think I'd need to nix my clock, and then make another mount for the fourth, since they're not combo units. However, as far as the tach's concerned, I'm not sure how I'd get it to work. It's made for the V12 engine, but I'm running an L28ET in my car. Is there some sort of way for the tach to receive half the pulses as it was intended to and still read somewhat accurately? Quote Link to comment Share on other sites More sharing options...
hoov100 Posted November 8, 2009 Share Posted November 8, 2009 You've got alot of reading to do. The reason you would replace factory gauges, is to be more accurate and precise. If you like that style of gauge, I would check VDO and stewart warner. Quote Link to comment Share on other sites More sharing options...
Avernier Posted November 8, 2009 Share Posted November 8, 2009 Nes ftw!!!!!!!!!! Quote Link to comment Share on other sites More sharing options...
Barrel_Ball Posted November 8, 2009 Author Share Posted November 8, 2009 Well, I'm just doing this as something different. Dead-on accuracy is great, but those aftermarket units get really expensive for some reason. I would like to use aftermarket innards with these faces and housings if possible, but I just want to see if I can get the stock Jag hardware to work on my car - My Z is moreless an experimenting testbed that's also my prideful toy - This is why I'm asking if there's a way to make a V12 tach work properly on an L6 engine's ignition system. I know there was an old aftermarket tach I had with a pile of parts that had a switch on the back for 4, 6, and 8 cylinder applications, but I gave it to a friend to use on his project. I know there's a source of reliable info out there, it's just a matter of where to look for it. And yes, NES for the win. Quote Link to comment Share on other sites More sharing options...
m4xwellmurd3r Posted November 8, 2009 Share Posted November 8, 2009 Well, aftermarket gauges tend to be expensive because of quality. Take a look at the speed hut gauges for instance. THOSE are some nice, high quality, SLIM gauges. Well worth the near 800 dollars for a 5 gauge set. Anyways, I think the only way you would be able to use that V12 gauge on a 6 cylinder, is to make some sort of special circuitry that can take the original tach signal, and boost it to 12 signals instead of just 6. I have no idea how it would be done, or if it would be possible however. Quote Link to comment Share on other sites More sharing options...
hoov100 Posted November 8, 2009 Share Posted November 8, 2009 heres some info, but the senders for them are fairly expensive. http://www.gaugeguys.com/Smiths/classic.htm Quote Link to comment Share on other sites More sharing options...
Barrel_Ball Posted November 9, 2009 Author Share Posted November 9, 2009 Does anybody know the sweep on the factory Z tach? If it's 270 degrees, then I can just take the innards from that and retrofit than into the Smiths housing. Like I said, dead-on accuracy isn't my prime goal at the moment, as I can just get some new gauges in the future when I get the cash for it, I'm just after cosmetics in that department for the time being. I'm sure I can find another 280Z tach kicking around somewhere (I knew I should've taken a few more things from those parts cars) and see what I can do with that. Just minor uniqueness, is all. Quote Link to comment Share on other sites More sharing options...
hoov100 Posted November 9, 2009 Share Posted November 9, 2009 Quote Link to comment Share on other sites More sharing options...
waddiejohn Posted November 10, 2009 Share Posted November 10, 2009 A long long time ago I read an artical or instuction sheet for setting up a tach for a V8 to read correctly on a four banger and it involved placing a resister in line with something. I think the wire that ran from the distributor. I'd be interested in knowing for certain myself and will see if I can find it. I've got an old Sun brand tack that has a selector switch on the back for 4,6 and 8 cylinder engines, but I'm not sure I'd want to tear it apart to see how they do it. Quote Link to comment Share on other sites More sharing options...
Barrel_Ball Posted November 11, 2009 Author Share Posted November 11, 2009 Okay, after a couple hours of research, I came to conclude that in order to get this tach to work with my engine, I pretty much need to replace the resistor with one of double the value in order to doulbe the pulse width, and time-constant (or so in reverse to what the source said as they were decreasing resistance to get a 4 cyl tach to work with a V8) to make each of the 3 pulses per rotation (I think) be there long enough to amount to two pulses (of I believe 6 per rotation on the V12), turning the 12 cylinder tach into a 6 cylinder tach. This is the only real way I can think of doing this, as it doesn't have a calibration pot like most others seem to have. I just gotta find out which one's the resistor - It's a tiny 70's circuit board with five components soldered to it, so It shouldn't be that hard. Thanks for the tips, everybody. It helped. Quote Link to comment Share on other sites More sharing options...
Mycarispurty Posted November 11, 2009 Share Posted November 11, 2009 Got a pic of the circuit board? Quote Link to comment Share on other sites More sharing options...
Barrel_Ball Posted November 12, 2009 Author Share Posted November 12, 2009 This is the circuit board of the tach. That big striped component is the resistor. I found out it's a 4.9k Ohm resistor, and that I have to either replace it with a 9.8k resistor, or find a second 4.8k, and attach it parallel to the first one to double the resistance (at least, I think that's how it works). Pretty simple, component wise. A diode, a resistor, a capacitor, a controller chip (I think), and what could be a signal converter. No potentiometer to adjust for calibration, which kind of makes adjustment for accuracy hit-or-miss, but I could just solder one in, I guess. Quote Link to comment Share on other sites More sharing options...
MAG58 Posted November 12, 2009 Share Posted November 12, 2009 You want it to be in series. A resistor in parallel cuts the resistance in half. Quote Link to comment Share on other sites More sharing options...
m4xwellmurd3r Posted November 12, 2009 Share Posted November 12, 2009 you could replace the resistor with a potentiometer instead. One that goes from 0-10k or something similar would work. That would make you able to calibrate it spot on with a multimeter or something similar. Quote Link to comment Share on other sites More sharing options...
JSM Posted November 13, 2009 Share Posted November 13, 2009 Why not look for a smith gauge from a standard XJ6? Quote Link to comment Share on other sites More sharing options...
Barrel_Ball Posted November 13, 2009 Author Share Posted November 13, 2009 Why not look for a smith gauge from a standard XJ6? And that's where the problem is - I found these at my local junkyard, and this is the first 70's XJ I've seen in almost 2 years. I can't see myslef finding an XJ6 tach laying around anytime soon, unless it's a late 80's XJ, in which case the gauges are different. I could try and use a pot to adjust it, but then I'd have to play by ear, or use a ECCS analyzer with its digital test tach (which somebody I know has). 0-10K was it? I guess I could look into that (I hope Active Electronics is open on Saturdays). Quote Link to comment Share on other sites More sharing options...
zcarnut Posted November 13, 2009 Share Posted November 13, 2009 I don’t think the brown striped component is a resistor. Sure looks like a film capacitor. I would not replace it with a resistor until you know what you are doing. You could possibly damage the circuit. I think your best bet is to make a schematic of the circuit and also identify the part number of the IC you saw on the circuit board. Hopefully the IC is the LM2907 or LM2917 charge pump which were commonly used in the 1970’s for tachometer circuits. The IC could also be a simple LM55 or 556 timer. All analog tachometers use either a charge pump or a monostable (“one shotâ€) circuit triggered by the input pulse to drive the meter coil. Once you get the circuit drawn you can Google “analog tachometer circuits†and hopefully determine the correct resistor to change/adjust. Quote Link to comment Share on other sites More sharing options...
Mycarispurty Posted November 20, 2009 Share Posted November 20, 2009 Yea that stripey brown thing looks like a cap to me too, as well as the blue one. The resistor appears to be that yellow thing. There are many resistors similar to that in guitar amps. But they aren't the areverage resistor you'll find in bins at Radio Shack for a nickel a piece. I'd suggest going to a Jaguar site and seeing if anyone on there rebuilds the gauges for those guys and get with him to see what he says. P.S. Don't know if anyone else has mentioned this, but you know you're going to probably need Jaguar sensors to run those gauges or build circuits to get them to work in the Z right? Probably WAY more of a pain than it's worth for fancy looking gauges, unless of course you have a relative that's an electrical engineer or something. Quote Link to comment Share on other sites More sharing options...
Pharaohabq Posted November 20, 2009 Share Posted November 20, 2009 Why Am I the only one thinking OMG! a XJ12??? and he only got the GAUGES?? Why didn't you get the V12??? save yourself the headache of modifying the gauges and PUT THE V12 IN!!! they fit, trust me Phar Quote Link to comment Share on other sites More sharing options...
garvice Posted November 21, 2009 Share Posted November 21, 2009 I can think of a reason. They started with 6 carbs (complete failure), then they started getting a bit smarter with the fuel delivery and managed a best of 13mpg. ARGH! Then when they brought out the "High Efficiency" model they got a best of 16mpg. HA, HA, HA. What a joke, maybe with a real computer you could make a car that can actually make it between service stations, but it wouldn't be by using the stock engine controls. Quote Link to comment Share on other sites More sharing options...
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