tach wire ac what does ac mean?

This is just to satisfy my curiosity... On the wiring diagram for my 2005 Toyota Tacoma next to the Tach Wire there's an "AC". What does the AC mean? All the other wires have either a "+" or a "-" showing polarity. After thinking about it for a few minutes I've come up with a hypothesis. Since it's the Tach wire, the amount of voltage running through it depends on what the vehicle's RPMs are. Is that right? Or did I just give you a pretty solid reason laugh?

It's a valid question.  And we are laughing with you. 

The question doesn't get asked directly very often, but does get answered frequently, after the new installer states that he can't locate a valid tach source and has been testing every wire in the vehicle for two solid days!

As you noticed most all vehicle wires in the wiring guides are listed as (-) or (+).  The vehicle has a 12 Volt battery and the basic wires will show either +12 Volts or chassis ground.   With the Tach signal, the remote start system is looking for a Frequency that would indicate the engine RPM.  When you perform a Tach Learn process on the remote start system, you are "teaching" it the correct idle Frequency. 

The testing procedure is different from the standard procedure used on the basic (+) or (-) wires.  Most people don't have an O'Scope or a Digital Multi Meter that can directly measure Frequency.  The indirect way is to use a DMM set to 20V AC.  The Black test probe goes to chassis ground and the Red test probe is connected to the suspect wire.  The reading you get is AC voltage and will vary slightly with engine RPM.  Expect to see a voltage level between 1 and 6 volts that will increase only by 1 volt ( or less ) with RPM's.  Newer vehicles, where a single F.I. injector or a single Spark Coil is used can read as low as .6 V AC while older vehicle with a single distributor coil will read around 4V AC. 

As a side note, this is an area where R/S system quality / design come into play.  Quality systems will have no problem learning the "weak" tach signals but some of the off brands need at least a full 2V AC to sync-up.  Additionally, using an actual Tach signal is the most accurate and reliable method of remote starting a vehicle.

Soldering is fun!

X 2 with Kregg. This harks back to a similar argument recently about how OTT you have to be with questions and answers. It's actually a good question though + - and AC should be a clue. To be completely anal it's not full AC, it's half of the sine wave, the POS side and doesn't go into the NEG side.

The letters "AC" stand for "alternating current". In reality a tach signal is more of a pulsing DC current and it always puzzled me as to why it is called so.Mike M2

Tech Manager

CS Dealer Services

Because Mike it "shows" on most DMMs as an AC signal since as Kregg stated, they won't read frequency.

howie ll wrote: X 2 with Kregg. This harks back to a similar argument recently about how OTT you have to be with questions and answers. It's actually a good question though + - and AC should be a clue. To be completely anal it's not full AC, it's half of the sine wave, the POS side and doesn't go into the NEG side.

Ok howie II There needs to be a basic fact for this topic that is a requied read for all posters like the one here! Testing Wires

Thanks for the detailed info Kreg. That will really help me once I get in there and start testing all my wires. And I'm glad I could give you a bit of a laugh. Lurch, that Testing Wires link is great. After reading that a few times I'm sure I'll be much more confident once I begin my install. I'm familiar with most of it, but I don't have a comprehensive knowledge of all of that stuff. This link will surely help me a lot. Thanks!

Your welcome.

Lurch just read the article, thanks. Here we go. The author is COMPLETELY wrong about using a test light. Disclaimer: In experienced hands it's much faster for lock and door triggers than anything else. Yes NEW car not sure, then a DMM. I think these stories about test lights frying ECUs are a load of rubbish, urban myths. I have a pal with $10,000 of diagnostic equipment who uses a power probe. That scares the life out of me. I've even found tach with a test light. I'm now going to sit back actually it's 1:35 am here so tomorrow I expect some flack so I'm readying round 2.

Test lights aside the article covers alot of basics that don't need to be answered.