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.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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

howie ll wrote:

---

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.

---

I've used test lights on known +12v wires and the such as they have there place. As for the proposed argument it is just splitting hairs and not worth the time to over look the rest of the articles merit.

As far as the test light thing, in some cases there are no worries using one. It all depends on the circuit that you are testing as well as whether you have the right wire, hence the reason for testing as such. In many circuits it's easy to cause issues when you cause a slight draw to the circuit when you aren't on your target wire. Not typically, but as a installer it is hands down the best practice to just use a multimeter. The (quality) of a test light is a big factor. Test lights were a widely acceptable way to test circuits in older cars, yet newer cars use a different technology which can cause issues easier. I'm sorry for the rant, I just wanted to throw my two cents in there. Do I never use a test light? Occasionally yes, but 98% of the time no.

The best thing to tell a new installer is to just not use a test light. Yes, it may be safe on most wires in most cars but the whole idea of using one to begin with is you don't know what the wire is for that you are testing. Isn't the the entire reason for testing to begin with? If we knew for sure we wouldn't test.

After having an installer pop both dash airbags in a new 380Z a few years back because he used a testlight looking for a horn wire, i don't even own one....

-------------
Mike M2
Tech Manager
CS Dealer Services

C'mon Mike, a bright yellow loom and the guy probed it?
Who was training him and forget to explain about them?
Back in the day before the Viper invasion I had a top Clifford Techie from the UK with me on an audit job (you know him I believe) and we were looking for something. I offered him a DMM and he asked for a test light!
It's really down to the knowledge level and what you are used to and skilled in using, there's one car audio place near me, I used to do their alarms, I always told them about disconnected bits on VAG cars and leaving the ignition on, especially completely removing the glovebox and it's passenger air bag switch.
You know what comes next don't you.
I suppose if you were starting out now as a DIYer I would recommend a DMM.
Glad I started this, judging from the reaction.
PS I'm doing something on a Prius fleet for a customer, I wanted to find a single wire from the BCM so I pulled it from the car and ran continuity from the fuse input till I found it.
Much safer than than a test light ON AN UNKNOWN (to me) VEHICLE.
Please note I only recommend them on circuits where I've been myself.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Howie beleive it or not on a lot of American Nissans the horn wire is in with the airbag wiring. Most new cars here don't use yellow loom anymore to tell the difference, but Nissan used to stick it right in the loom with all the other clockspring wires. I remember a few that scared the bejesus out of me testing.

As for the installer, i didn't hire him. He was worthless and i caught him twice sleeping inside of customers cars on the lot. After the airbag incident my boss(who did hire him) let him go....

-------------
Mike M2
Tech Manager
CS Dealer Services

Quality of hired installers? That's another can of worms isn't it Mike?
I'm aware of the horn wire being inside the airbag loom, so look somewhere else, go to the relay find the right wire BEFORE probing.
BCU pins apart, general experience tells you that if say you're looking for lock triggers, go to a plug or wire in the kickwell that corresponds to the front door that has a key. Assuming we're not talking about those bloody cars with door modules!
And data control!

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.