why are the tach wires sometimes not identified directly. some diagrams identifiy them as "NOT" this or that color. i'm confused. please clarify. thanks
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RAMON
There are people here who can answer this more lucidly than me but here goes. You are looking for a signal which is the + side of an AC sine curve. This is directly related to the ENGINE speed AKA RPM or TACH signal. Not road speed or VSS. In a petrol engine the two traditional places to take this feed were the coil (distributor, Contact Breaker or- side) or the rev counter (tacho fed) if you had one. In othe words a point on the ign system that varies its (it will be measured using AC on a DMM) voltage as engine speed increases. More modern vehicles dispense with the coil using separate coil packs and injector ports, also instrument panel feeds may be data,e.g.VW, euro Fords. We have to look somewhere else. Two places are 1) engine management pin outs, 2) Injector ports or plug power packs with the first being the better choice normally for access reasons. The normal rule of thumb (test of course) is that each injector port has 2 wires; one will be the same colour on all ports this is the common and the one you DON'T use, it's the other. Hpefully the last sentence answers your question because if we dn't know, it's easier to tell you what not to use. Unless you know which pin-out avoid engine management, usually for accessibility reasons. Certain exceptions to the above include various Honda where its a single lead going to a blue plug by one of the suspension struts, late BMW and Toyota/Lexus where it's on the OBD II plug. Also if any vehicle has a single plug pack then one of the 3/4 wires going there is the one. With regards to testing I don't use a DMM, I use either a Snap-On or Mack tools LED tester with red and blue leds,(the Mack is more robust). When you grab the right wire, the LED wil flash; I've even used a Snap-On test light but don't do this at home!
Howie is on the right track.
I believe the reason they do this (and I could be wrong) is because they derive their information from written documentation and not actual cars. If we always knew the installer was going to use injector number four because it is the only one accessible they could give you the wire. But, if they look at a wiring diagram of the car and see there are 8 injectors and all 8 injectors have a hot wire that is red, and each individual injector has a difference color (-) trigger from the ECU it is much easier to just say "hey, don't use the red wire at any of the fuel injectors". The same thing for coil packs, but most coil packs have three wires.
If they told you the exact color they would have to tell you the exact coil pack or fuel injector as well. And, since most installers arn't mechanical masters not all installers would be able to find the "#4 injector".
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Kevin Pierson