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Old 01-19-2009, 12:41 PM
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94 caprice speedo modify

I have been building my project for a while now and all is perfect except for a speedo problem
first I took my 54 Chrysler new yorker coupe and installed every thing from the frame up from my 94 caprice including 4 wheel ABS brakes LT1 motor 4l60 trans complete wire loom and temporarily plugged in the caprice dash. ALL IS PERFECT every function, every warning lite, but the caprice has a digital speedo so I want a sweep needle type in the stock dash location.
I located a 94 pontiac grand prix dash assy with the speedo and gauges all in one panel about 17'' wide mounted on a single circiut board also 17'' wide. with the unit in tact I traced the power, ground, and VSS signal wires and pluged them into my dash loom drove the car and it worked fine odometer and speedo were correct. so I band sawed the speedo section out of the panel and mounted it in the chrysler round speedo housing, pluged it back on the 17'' circiut board and tested again still worked fine.
Problem is I cant fit the 17'' circiut board behind dash its just too big, so I found a 95 chevy van with a very small circiut board that exactly plugs on to the cut down speedo right down to a notch that lets it screw down with a factory screw. I traced the path of my three wires and it appears the components are the same as in the diodes, resistors, capacitors.
THe problem is now the odometer is right on but the speed reads EXACTLY double, Im pretty sharp but IM not an electrical engineer I think its in the micro chip piece but dont know any help would be greatly appreciated
thanks for your time

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Old 01-21-2009, 10:20 AM
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I would send the speedo to North hollywood speedometer (no affiliation) they do this sort of stuff for us all the time. I am intersted how this turns out. There has to be a way to cut the signal down internally. Do you see any potentionmeters in there? (small round things with a screw top?)
Mark
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Old 01-21-2009, 10:52 AM
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hey gauge guy thanks for the reply there is no adj potentiometer Ive ever seen in a factory dash just square or rectangle CHIPS with many connectors soldered to the board. I went back to my local wrecking yard yestrday and checked MANY dash assy from same era cars vans and trucks to find all have same layout as in diodes, resistors, capacitors, and now familliar CHIPS many have same number chip for the odometer but different chips for speedo. I did NOT take into account that some speedo are 0to 85 some 0to 120 and one I found 0to140 all sweep style round face all the gauge pieces seem to be the same electro magnet unit. I was able to do some testing by connecting to the VSS signal at the cruise controll plug and try different combinations with my dash in tact on the lift running up to a constant 50mph inside and checking speed on test unit. result all the chips are different I studyed my 17'' board very closley traced power, ground, vss signal, soldered wires to the closest spot i could to speedo and started cutting testing the thing after each cut was able to get it down to about 5 in long 21/2 wide pulled the dash out switched boards and reinstalled its not as pretty as it was but it works perfect checked it with my GPS for speed last nite and is very close dash looks great Ill try and get a picture today and post it just curious about north hollywood speedo think they could swap chips in therse boards? where are they in no hollywood I grew up there and may have used them in my early days building cars where I graduated from no holly high thanks again for your reply watch for photo
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Old 01-21-2009, 11:22 AM
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Oops... never mind.
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Old 01-21-2009, 04:01 PM
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OK here is whats left of the grand prix dash after the band saw and the installed unit in the chrysler with the gauges from the caprice dash thanks for your help oh yes the small circiut board in the forground is what I was trying to use just looked cleaner its behind the dash I just like tidy and clean
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Last edited by hemivolvo; 01-21-2009 at 04:07 PM.
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Old 01-22-2009, 03:58 PM
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Just FYI I learned this when I was trying to recalibrate the speedo in my '95 Caprice after a rear gear change...
The ECM receives the signal from the VSS as a frequency, not a voltage, and then sends the signal to the speedo. I tried putting a pot (which varies voltage) in line between the VSS and ECM and it did nothing until voltage was cut entirely and it went into "limp-home" mode when it lost the VSS signal.
Pretty cool what you did there hemivolvo, I would have never had the patience to trim away at that board and constantly test to make sure you still had signal to the speedo...
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Old 01-22-2009, 05:00 PM
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hey 20z28 how did you finnally calibrate the speedo? Ithink it would be done by reflashing the main ECM through the diognostic port, a buddy had his done on a 4x4 truck after a tire size change but Ive been curious as if that truly is how its done I plan a tire change in the future and will need to know thanks for your input
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Old 01-24-2009, 01:09 PM
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Well I sold the car before I fixed it

But yes as far as I know you have to change the calibration in the ECM to correct for gear ratio or tire size changes with a handheld or tuner software. And I wasn't about to drop 300 bucks to do that...
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Old 01-24-2009, 08:13 PM
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Some manufacturers have a limited number of tire size and gear ratio choices for a standard reprogram of the ECM by a dealership, so it can't handle really oversized tires or unusual axle ratios. You could probably get a custom program for the ECM that could correct for any tire size and gear ratio, but that would definitely cost more than a standard reprogram.

Bruce
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