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 Post Posted: Tue Feb 24, 2009 1:36 am 
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 Post Posted: Wed Feb 25, 2009 3:00 am 
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I didn't have a pot. in the range I needed (highest was 10k-ohms), so I dove into my treasure trove of electronic bits & pieces, came up with a couple 100k-ohm resistors. I'll wire these in temporarily (with the white/red wiring) and try to find myself a signal generator...
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 Post Posted: Wed Feb 25, 2009 4:07 am 
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Huzzah! I found one :lol:

I just need to figure out how loud it needs to be for the correct voltage (I'm guessing it doesn't need much) to test it for the "rpm" it's supposed to read.
http://heliso.tripod.com/download/generator/dsg.htm

To do this you'll need a 12v power supply (car battery will work, however cumbersome... a used computer PSU will work as well, just be sure to be using the +12v rail) to power the gauge, and a way to tie into the audio output of your computer... easiest way will be by cutting a headphone cord (don't do this with your Sennheisers! :lol:).

Working on this right now, will update with pics.

*UPDATE*
I couldn't get the voltage output I needed from the computer (could only get maybe .3v max), so you will just need to pipe the sound through an amplifier to get the voltage you need for the gauge to register the signal (be sure to disconnect all the speakers, the volume you will need will be very loud if any speakers are connected). Best way to find the required voltage (I used 5v, pretty common sensor-wise) is to hook an ac voltmeter to the speaker outputs, play a frequency in the range you will be using (50-400Hz), and adjust the volume accordingly. You will also only need the positive lead from the speaker connection of the amp (no, it won't hurt the amp).

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Last edited by Doober on Wed Feb 25, 2009 8:05 am, edited 1 time in total.

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 Post Posted: Wed Feb 25, 2009 8:00 am 
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After doing a bit of research as far as the tach signal goes, I did the math (based upon the crankshaft rotates at ½ the speed of the cam, or the other way around... I'm a little tired, but the math I did works out) and it comes out to 62.5Hz (1,000/8=125, 125/2=62.5) = 1,000rpm on the tach. From that you can create a chart of what frequencies you will use to verify gauge accuracy.
  • 1,000rpm = 62.5Hz
  • 2,000rpm = 125Hz
  • 3,000rpm = 187.5Hz
  • 4,000rpm = 250Hz
  • 5,000rpm = 312.5Hz
  • 6,000rpm = 375Hz

I had already disconnected the original 'resistor' from the resistor pack on the gauge, and started with the 199.8k-ohms showed in the previous post. I found out that this was a little too low... posts in other threads mentioned capacitor leakage (mainly on a couple threads on thirdgen.org), but I'm not too concerned about that currently. I found a 10k resistor and put that in series with the 200k in resistors I already had, which cut the error in about half. I wound up using a total of 220k-ohms to get an accurate reading on the tach. I won't know for sure if this is 100% accurate until I can check it on an actual running engine, but it should get me in the ballpark.

Ignition waveform on an oscilloscope I stumbled on:
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200k-ohms
1,000
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2,000
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3,000
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4,000
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5,000
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6,000
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220k-ohms
1,000
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2,000
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3,000
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4,000
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5,000
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6,000
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You can see at 200k-ohms that it's about 100rpm off per 1,000rpm (pretty clear by the time it reaches the frequency for 6k on the tach). My assumption so far would be that 165k-ohms could be used for a v6, and 110k-ohms for a 4-cylinder.

Here's a little clip of the tach running after I found the correct resistor.
http://www.youtube.com/watch?v=lvup_fKo6Qs

So as a rule of thumb here, the higher the resistance, the higher the tach will read. If your tach pegs immediately when you start the engine (but sits at 0rpm when it's off), then chances are the resistor replaced here is 'open', aka the resistor isn't making contact. Either the solder could have had a bad connection and broken (in which case it can be re-soldered), or the resistor is bad and needs to be replaced.

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 Post Posted: Tue Mar 31, 2009 6:03 pm 
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awesome troubleshooting! I thought I'd add that police package Bu's (at least the four door 79 I seen for sale locally) did have a 120 mph speedo and according to the paperwork with the car, you could order them with the 85 mph instead for cars used for undercover work (so as not to give them away) and a tach was also an option you could order for them also. This one featured a smog except 350 engine, 4-barrel rochester carb, HD cooling, larger rear drums, F41 suspension, and this car has a 3.08 limited slip rear too. All the original police paperwork on this demo car was pretty cool to read through. The local police dept. passed on this bodt style in favor of the Caprice due to needing more room. They were noted as being very impressed with it's relative quickness though.


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 Post Posted: Tue Oct 25, 2016 3:11 am 
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Doober wrote:
Well, after cutting the trace from the #4 pin, I measured the resistance, came out to about 305kΩ... Everybody on the F-Body forums and in the MonteSS.com article have been saying it should be somewhere around 190-200kΩ. I'll probably go the route of using a small potentiometer to get the desired resistance.


Hello , I have a pegging tach and I saw your post and tried it with 2 100k ohms. It know doesn't peg, but hovers around 3pm (clock position). When I accelerate it pegs completely to max. Can someone shed some light please..[img][img][/img][/img]


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 Post Posted: Wed Oct 26, 2016 9:29 pm 
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It's a combination of the resistor "chip" and a capacitor. I never really fiddled with this after putting the LeMans dash in mine.

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 Post Posted: Thu Oct 27, 2016 2:44 am 
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Does it Matter the size of the resisten if its v6 or v8 ? I purchase e the tach on eBay?


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 Post Posted: Thu Oct 27, 2016 2:07 pm 
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Yes.

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