I went to a electronics store today but couldn't get any help finding these parts because it doesn't give the full details of what each part is.
Since the power source is 12 volts they said go 16 or 20 volts and exactly what diode.
So if someone knows can you list the full specs for each part.
Thanks
http://www.gmtuners.com/swap/speedoMOD.jpg

a 1/2 watt diode should be fine.

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87 GT
series 1 3800sc (7.597 @88.53 1.579 60ft)
(series II swap in progress)
85GT Northstar
86GT 3800 n/a

my local radioshack had all the parts.

quote Originally posted by Fiero_Guy: I went to a electronics store today but couldn't get any help finding these parts because it doesn't give the full details of what each part is.

You need to find a better and more knowledgeable "electronics store." This is a signal-level circuit, so it doesn't handle any power at all, and none of the component values are critical. The capacitor should be rated at 16 volts or (anything) higher. The resistors should be rated at 1/4 watt or higher; even +/-20% tolerance would be fine. The diode can be any cheap switching diode; something like a 1N914 or 1N4148 would be ideal. (Even a rectifier diode like the 1N4001 would probably work here, but a switching diode is a better choice.)

I hope this helps.

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Tell the truth I don't see what the diode does for you in that circuit anyway. It's ploarity puts 12 volts to the PCM, the PCM output could never go higher than 12 volts so it cannot be a blocking diode and the capacitor blocks any DC to or from the spedo. The 1K resistor is just a pullup that gets a 12 volt pulse across it and the 22K resistor must just be there to drop the pulse level.

quote Originally posted by trotterlg: Tell the truth I don't see what the diode does for you in that circuit anyway.

I agree, but I've never seen the PCM schematic so I don't know what the output driver looks like. The driver is probably an open collector pull-down, but that's not guaranteed.

quote The 1K resistor is just a pullup that gets a 12 volt pulse across it and the 22K resistor must just be there to drop the pulse level.

I also question the 1K value for the pullup resistor, which would make about a 14 ma load ... or about 10 standard TTL loads. The 22K may be there to broaden the output pulse ... i.e. to tune the time constant of the series R/C pair. Again, I have no idea what the speedo input circuit looks like.

[This message has been edited by Marvin McInnis (edited 06-14-2013).]

The 22K ends up in series with the internal resistance of the spedo input circuit, it does not matter which side of the cap it it on. The 1K resistor was probably just something someone had and used, it worked so it became the design. Probably a 10K would put less load on the output circuit and work just as good but you may have to then lower the value of the 22K. Who knows, guess you cannot argue with success. Larry

i just posted (2) GM yellow buffer boxes in the Mall if it will help. i have one of these on my 3800SC to feed 2000 ppm to the ECM, 4000 ppm to the aftermarket speedo and 4000 ppm to the electronic (digital) cruise control - works like a charm.

https://www.fiero.nl/forum/Forum4/HTML/066320.html