tach wire ac what does ac mean?

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.

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!