snipes Reader
12/21/10 8:02 p.m.

Okay the Porsche project is making me see that I have to get over my fear of wiring diagrams. Up to this point every time I get in over my head with wiring I have to call a friend for back-up. Well now that all the fab work is done its time for me to dig in. All I have now is a $4 analog multimeter. The porsche is OBDI but we have two OBDII cars in the family. Should I buy an OBDII reader with built-in multimeter (I think they make them) or something simple like this. Keep in mind that I over this type of stuff fast (my friends maybe over helping me).

BTW I really enjoyed the article GRM did on finding electrical faults.

Toyman01 SuperDork
12/21/10 9:05 p.m.

Not knowing exactly what you are doing, here is some generic advice.

The analog meter will do almost everything you need. Fancy meters have some nice functions, but a $4 multimeter will have the same basic functions a $200 multimeter will. Testing sensors you may well need a digital meter to get accurate ohm and voltage readings.

Are you wiring the car from scratch or building off a factory loom. Personally I hate using a factory loom unless the entire thing has been unwrapped. I have seen more than once a wire go into the loom one color and come out another.

Wiring diagrams are just a bunch of lines kind of like a road map. You follow them the same way. Instead of towns and cities you have sensors, fuse blocks and computers. Get good diagrams and copy them. Trying to crawl under a dash with a book sucks. Make notes as you go along if you deviate from the factory diagrams. You won't remember how you wired it three years from now. Trust me on this.

Automotive wiring problems can usually be traced to grounds. Those are the most important connections. You can hook everything up perfectly and forget one ground and the whole car will do strange things. Finding bad grounds will drive you crazy. Do them right the first time. Just as a hint, you can't ground through paint.

Don't forget the fuses. There is nothing worse than doing a really nice wiring job and watching it go up in smoke because something shorted and you didn't fuse it properly.

Ranger50 HalfDork
12/21/10 10:10 p.m.

Right now, I'm working with the $2.99 coupon'd multimeter from HF instead of my nice Fluke. The Fluke is MUCH nicer then the other, but it doesn't do anything really better then the cheapie.

Agreed on the copies of the best diagrams you can find. Also agreed on the marking up the copies for harness modifications.

One more thing, MOST diagrams run from a positive voltage source, at the top of the page, to ground at the bottom of the diagram. Now I have seen some Ford diagrams run left to right and back with no obvious markings denoting chit. Just have to trace the wire around.


curtis73 HalfDork
12/21/10 10:33 p.m.

The only reason for buying a digital meter is for computer stuff. Analog meters use much higher juice to test resistance since they need the increased amperage for accuracy. This can damage delicate circuits on some computers. Digital meters use tiny amounts of juice to test resistance and won't damage computers.

Otherwise (like testing everything except computer circuits) an analog meter is fine.

car39 Reader
12/22/10 7:37 a.m.

In some situations, analog is better. When you're hanging upside down under a dash, it's still pretty easy to watch the needle swing.

iceracer Dork
12/22/10 10:13 a.m.

The digital is nicer when working on the computer controlled items since voltages are 5 and below. Other wise the analog is fine.

Dr. Hess
Dr. Hess SuperDork
12/22/10 10:32 a.m.

For most automotive uses, an analog meter is fine. A digital meter, however, is VERY nice to have when diagnosing charging issues, because the difference between 12.7 and 12.9 volts can be difficult to read on an analog meter, but can mean the difference between a functioning and non-functioning charging system.

Curtis, the critical spec on a meter is the Ohms per Volt rating. 20K Ohm/V is about the minimum that is acceptable. Sometimes the ultra cheapo meters have a rating much less than that, and an ultra cheapo analog meter is more likely to have a lower rating than an ultra cheapo digital meter. This rating is the amount of "load" placed on a circuit when the meter reads the voltage. As for the voltage supplied, most analog meters that I've seen run on one battery, so that's about 1.5V. The digital meters usually have a separate setting for testing diodes, which will supply over the ~.7 V needed to overcome the junction of diodes and transistors and a setting below ~.7V for checking when you don't want to overcome the junction (handy when testing things without pulling them from the circuit). No decent meter is going to supply enough current to damage anything but some really exotic circuits, and I doubt that happens in all but extreme cases, and certainly not with anything we would be checking, including ECU's.

A $20 meter today is about all any of use will ever need, and that's what I use.

There's been a lot of threads lately on chasing electrons, and they have demonstrated a distinct lack of knowledge out here in GRM land. Perhaps a nice article covering the basics of electricity would be a good idea for an upcoming issue. In the mean time, google up the ARRL and see if you can DL a beginning electricity course.

Osterkraut Dork
12/22/10 11:50 a.m.

Recently bought this. It's got a lot of neat little options (temp, tach, etc.) It's way more than I need, but I'm a big fan of overkill!

kb58 Reader
12/22/10 1:17 p.m.

For some reason this thread reminds me of something our college solid state electronics prof said, talking about "Erectrons and Hores." Maybe something was lost in translation, but we found it hysterical.

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