strotter Posted March 18, 2002 Share Posted March 18, 2002 I'm swapping the EFI and tranny from an '89 Firebird into my '72 Z. The donor car used an electric speedometer, which I'm going to use in place of the original unit (with a whole bunch of super clever surgery and a laserprinter for the faceplate - I called JTR but there's apparently no straightforward way to adapt the mechanical speedo on my T5). Because the Z will use "shorter" tires, I'll need to recalibrate the thing, but after taking it apart I'll be darned if I can figure out how. There seems to be no trimming pots anywhere in the instrument itself, or the VSS buffer. So here's the idea: because the tires will be shorter, and therefore turning faster at a given speed, I just need to lower the readout of the speedo throughout its range. I believe I can put a variable resistor in the hot lead running to the instrument head (after the digital/analog converter) to lower the voltage a bit and cause the needle to read a bit lower. Is this a workable idea? I'm thinking I can run the pot someplace under the dash, and adjust it by, like, following school busses, or comparing the numbers on the tickets to the speedo readings, or something. Suggestions/comments please. Quote Link to comment Share on other sites More sharing options...
strotter Posted March 18, 2002 Author Share Posted March 18, 2002 Just to clarify, there is only the VSS signal generator in the '89's - no mechanical drive of any kind. JTR has what *looks* like a combination VSS/mechanical drive, as was used on police cars to drive the speedo and radar-gun computer, but they tell me it won't fit. Yes, I was going to cut into the analog portion of the speedo. They're set up so a digital signal is sent from the ECM to the instrument cluster, where there's a digital-to-analog converter which in turn outputs just two wires (hot and ground) to the actual armature of the speedo. So, more current equals more "throw". Those are the wires I'm talking about. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted March 18, 2002 Share Posted March 18, 2002 Aha, that is much clearer. In such case, you just need to shunt the meter with the appropriate resistor. You can wire a lower resistance pot across the speedo winding, say 10K or 5K (wire the wiper to one of the main legs). Quote Link to comment Share on other sites More sharing options...
Guest greimann Posted March 18, 2002 Share Posted March 18, 2002 Dude, I think we have solved your problem, check this out: http://www.dakotadigital.com/Detail.cfm?Category=122&PartNumber=SGI-5 Quote Link to comment Share on other sites More sharing options...
strotter Posted March 18, 2002 Author Share Posted March 18, 2002 Guys, thanks for the replies. Greimann, I appreciate the link, but man, that thing cost money! Cheaper is better, assuming it works, so I'll go with the Radio Shack solution if I can. (BTW, the Z in your sig is *gorgeous*. The engine bay picture just makes me want to weep - really sanitary, very professional. I'm going with the factory TBI setup on a 327 w/ Vortecs, 10.0:1, T5WC. The 4Di is a ways down the road, I was thinking about the holley 670 TBI after I get this thing moving. Digital Dyno tells me I could be in the 400 hp range with the 670, hoping I can maintain fair mileage while I'm at it. It's my first EFI project. Any words of wisdom?) SleeperZ, you'll have to lead me through here by the hand, I'm more digital than analog in my dealings with electricity. I was thinking of a two-pole variable resistor, cut the red wire leading from the D/A converter to the armature, splice it in. You're suggesting I, what, put the resistor *between* the wires? I'm not really clear. I understand that electrically it's different, but I'm not sure what the advantage is. Also, what do you man by "wiper"? Clarify? Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted March 18, 2002 Share Posted March 18, 2002 What I believe may get you from here to there is to put a variable resistance "across" the meter. What across means is one end of the resistor at your red wire, the other end to the other wire of your meter (you need two wires to complete a circuit). You don't cut, or interrupt the connection, you are effectively lowering the resistance of the circuit by putting in a variable resistor in parallel with the resistance of the meter. Since the meter is current driven, you are bypassing some of the current required to drive the meter through your pot instead. The "wiper" is the variable leg of the pot (the middle connection). Connect this leg to one of the outside legs and the resistor becomes variable. If I'm not adequately describing this, email me, and tonight I can draw up something on the computer and email it to you. Quote Link to comment Share on other sites More sharing options...
strotter Posted March 19, 2002 Author Share Posted March 19, 2002 I get it now! Thanks a bunch. I'll pick up a precision 10k pot at Rad Shack and wire it up. I won't know if it works until the thing is rolling, but if I make it accessible under the dash I can adjust or replace it easily if there's a problem. Again, thanks a bunch. Quote Link to comment Share on other sites More sharing options...
Guest greimann Posted March 19, 2002 Share Posted March 19, 2002 Sounds like a lot of high tech "Junk Yard Wars" engineering to me. That T5 should have a conventional speedometer cable connection on the drivers side. Run a cable from that, right to the stock speedo. It should work great provided you have the correct speedo gear ratio to complement your Z rear. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted March 19, 2002 Share Posted March 19, 2002 I'm pretty sure dividing the voltage down with a resistor is not going to work. Most of those instruments have an internally regulated reference voltage (to deal with the varying battery voltage) - reduce the input and it will just cease to function. You may have to resort to what the Z folks have always done with speedo gears - swap a cog or housing to change the gear ratio. Err...I may have mis-understood what you are saying. You aren't trying to reduce the supply voltage, but the signal to the speedo display? Is this an analog meter driven by a VCO (voltage controlled oscillator)? It could possibly work if that is the case, but instead of a series resistor, wire the pot across the signal to ground, and wire the wiper to the instrument. That way you will not affect (by much) the load impedence driving the instrument. A high value would be appropriate (say 100K). Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 19, 2002 Share Posted March 19, 2002 One way you might try calibrating your speedo is with a handheld GPS unit. Most of those have speed readouts which will be more accurate than anything else out there. they are popular with hunters and boaters. Quote Link to comment Share on other sites More sharing options...
pparaska Posted March 22, 2002 Share Posted March 22, 2002 ...and if you want nutso speed accuracy (~.1 mph), get a differential GPS setup. Quote Link to comment Share on other sites More sharing options...
Guest Thurem Posted March 23, 2002 Share Posted March 23, 2002 (with a whole bunch of super clever surgery and a laserprinter for the faceplate - Hmmmm. If you are going to print a new faceplate for the speedo, you could just calibrate the faceplate to what the speedo is showing. If you know the final gear and the tiresize of the donor car you could figure the discrepancy and print a faceplate to match. Ie. the speedo shows 20mph, but your calculation shows you are doing 25, print 25 on your faceplate instead of 20. But your odometer is going to be off Later Thure Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted March 23, 2002 Share Posted March 23, 2002 Is the 10 K pot. you mentioned audio taper or linear taper? I would think linear but I have an audio taper 10 K and wondered if that would work or if it would make a difference since once it's set you wouldn't be changing it unless you changed tire size? Quote Link to comment Share on other sites More sharing options...
strotter Posted March 23, 2002 Author Share Posted March 23, 2002 Thurem, your idea of calibrating the gauge plate to the car speed was my first idea. The downside though would be that I'd have to disassemble the thing *twice* (once to install the new guts, a second time to install the calibrated face), and I'm a little nervous about that. And the PITA factor is high whenever dealing with gauges. I'm just one of those guys that prefers to do it once and then forget about it. And besides, if I use the potentiometer method, it can be adjusted on the fly for whatever tire/wheel combo I end up with. Besides, I found some really nice gauge face patterns at Steve Graber's website which are fine looking, readable, and free. They're designed to use the original Datsun faceplates, which appeals to the lazy guy in me. I'm thinking I'll print them out on white plastic (like an opaque version of an overhead transparency, $5.00 at Wal-Mart). Check out the rest of his Velo Rossa website. Awesome. Bbbowtie, I have no idea what you're talking about. Would an "audio" taper be logarithmic? If so, I would think a linear taper would be best. Actually, the thing I'm a most concerned about is having fine enough resolution in the potentiometer. It will need to be adjusted to within a mph or two, which is, what, less than one percent? And it would be nice to have big old knob to twist, rather than some tiny little screwdriver slot. Come to think of it, though, and audio pot might work well, if the "flat part" of the curve is at the "bottom" of the adjustment range. I'm not looking to apply a lot of resistance, and that might make it be easier to find the right point. No matter, if I run the wires under the dash I can try different parts until I get it right. Quote Link to comment Share on other sites More sharing options...
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