Fabrik8

Sorry.. Continuity just means you have an unbroken (continuous) path from one point to another. It's just a resistance check but you don't care what the resistance is; you care that there is a connection. If you look across a blown light bulb, you'll have no continuity because there isn't a continuous path for electricity to flow.

So if you check for continuity, you're just checking whether or not you have an electrical connection or not. Good for rough troubleshooting and to eliminate possibilities quickly without having to do to much thinking or interpreting.

marlinspike

And so I have continuity if I have 0 ohms, O.L., or like 5 ohms?

Fabrik8

OL means Open Loop (or is it Over Limit?), which is basically saying that there is more resistance than the meter can measure. That means there is no electrical connection. If you're just checking for continuity, you're generally testing for something that should have a low resistance reading. If you don't, you need to think about what you're seeing and why it might be like that. For example, if you remove that relay and do a check across the coil terminals, you'll see a resistance that isn't very low. That's the resistance of the coil itself. If you see nothing, the coil is broken and is therefore bad. If you see something very low (a few ohms or less maybe) the coil is most likely shorted internally and is also bad.

Along the same lines, if you're checking continuity between two terminals on either end of a wire, you'd expect to see a very low resistance because you're looking at two ends of the same piece of wire. If you have no continuity, you can assume that the wire is broken. Often that's all the information you need if you're just looking for broken connections.

marlinspike

So wait...the coil terminals...are these on the car side or on the relay side?

I need an older car....

Of course the 3 local 80s benz parts cars would all be the few models that don't have this relay anywhere in them.

Fabrik8

The relay has the coil inside it. The coil is the main part of an electromagnet that magnetically pulls the switch contacts open (or closed) when the coil is energized.

If you're trying to figure out whether the relay SHOULD be turning on and off, and are looking at the coil terminals to see if the pressure switches, etc., are working properly, you would want to look at the terminals on the car side of things (the socket the relay plugs into). If you're trying to figure out whether the coil itself is broken, you'd want to look at the terminals on the relay itself.



That's a good point though. I usually don't realize whether something is obvious or not until someone says something like that and makes me realize that it isn't obvious. Familiarity makes me completely overlook a lot of important stuff that doesn't occur to me. Kinda like when you want to read a book so you automatically rotate it the right direction when you pick it up, and also start from the front cover instead of the back. If you hadn't read a book before, none of that is necessarily obvious.

marlinspike

Ok, I think it just clicked for me how this all works. The goal is to apply 12V to the coild side so it closes the drawbridge on the 30 and 87 side in the circuit diagram, and to check that I can measure the other side's resistance. FWIW there is plenty of resistance between the coil side.

Fabrik8

Yes.. LOL. Check the resistance between 30 and 87 while applying power and ground to the coil terminals.

marlinspike

Ok. I don't have 12V to 85 and 86. I have 8.275V there no matter what I do with the push button unit (and 0V with the car off of course). So...push button unit replacement time?

Oh, and the relay clicks when connected to the battery but not when plugged into its socket.

Fabrik8

If you have 12V on the input of that switch assembly, and 8.275 on the output, and the switch isn't even supposed to be on, that says there is a problem obviously. You should obviously have 0V on the output when the switch is off, and 12V when the switch is on. If you unplug the switch assembly, and jumper across the input and output connector positions on the harness plug, the A/C should turn on if you've correctly tracked the problem down.

Make sure that you actually have 12V at the input to the switch assembly, and if you do then get out the credit card.

Sounds like you've got it pretty well solved though.

BAD4DR

He would also see that with a partial short to ground on the relay feed circuit from the control panel. Basically, needs to check the wires going through that switch, as you said, on both the input and output sides.