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C900 Bulletin Board
1979-1993 & 94 Conv [Subscribe to Daily Digest]
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Found the differences...(long)
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Posted by JT [Email] (more from JT) on Sun, 20 Feb 2005 14:23:48 Share Post by Email
In Reply to: ignition amplifier question, JT, Sun, 20 Feb 2005 08:11:42
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I think I figured out the difference between pre-88 and 88+ amplifiers. The 85 appears to pick up the ignition pulse directly off of the negative coil wire while the 86 and up pick this signal up off of pin number 7 in the amplifier. It looks like the old style amplifier does not have circuitry to boost the output signal to a voltage that the tachometer in the instrument cluster can read. This is why the 86 and 87 have an external "pulse amplifier" in the underhood relay block. Th 88' and up got rid of the pulse amplifier bacause apparently this feature was integrated into the circuit board of the ignition module OR integrated into the instrument cluster. It looks like one can easily use an old module on a newer vhicle by simply tying the instrument cluster tach into the negative coil wire directly. If the instrument cluster remained the same between 85 and 88+, then this is an easy mod. If the cluster incorporated an integrated amplifier in its own circuitry in 88+, then an 85 ignition amplifier looks like it will work identically. All of the pinouts are common with the exception of pin 7. Pin 3 looks like some sort of inductive circuit, but it looks like it is tied to a distributor ground in the later models, so no big deal. If my theory is correct, the worst case is that everything works fine, except my tach reads zero. Then I'll know that a pulse amplifier was not included in the printed circuit board of the instrument cluster. I doubt it would b since the circuit is probably very high impedence and the distance it needs to travel to get to the cluster would probabl be too far without being "boosted". Being a EE has its merits.

posted by 69.14.19...

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