Originally Posted by kaya@bernhausen
the correct way to test the control side of the relay, using a test light, would be by placing your leads across the small terminals.
I'm having a hard time deciphering your last two posts....
Lets start with this, Test Lights, throw them away
or limit their use to testing the lights on your truck and trailer
They simply have No Business in today's electrical diagnosis and here's why. Let's take this very example of the GPR. The PCM is using a transistor, in this case it's like another relay, it uses a small amount of current to switch a larger amount. Only it's a Semiconductor, it has no moving parts, and very limited to the amount of current that can pass through it. You just can't go around adding the load of a filament bulb to these circuits (in this case you probably can
but it's just Bad Procedure...)
What's called for is either any kind of Meter (go to your local Harbor Freight, they Give them away for Free w/coupons we all see in the Mags & Paper) or a sophisticated "Testlight
" like a Power Probe (I Love
my Power Probe, Google it if you haven't seen one, everybody
should have one
I would never put a test light across the two sm terminals. I'm not sure what would happen if I did. Electricity flows in the path-of-least-resistance
and I guess I just assume the coil inside would be easier than overheating the filament in the test light bulb to the point it would glow. Plus, there's very little difference in potential between them, it's like putting both the probe & clip on the same post of the battery.
In circuit diagnostics it's always best to isolate the path. Take the relay out of the picture. Disconnect both sm wires, put the meters leads in series (one on each wire so anything flowing between them goes through the meter) to test if the PCM is doing it's job by supplying the path to ground for that coil. THAT will safely tell you if it's working as it should. Failure could be either the PCM itself or the EOT (Engine Oil Temp) sensor telling the PCM the engine is already up-to-temp so it doesn't have to.
Next, use the meter to determine which wire is which. The one with 12v in it with the key On is the supply. Connect it back on the realy and use a jumper wire to ground the other sm term on the relay (YOU are now doing the PCMs job, supplying the path to ground) and you should hear/feel the relay throw. THAT will tell you if the relay "works". Now to confirm that the contacts inside are capable of handling the load of all the Glow Plugs you can perform the "Voltage Drop" test (here again, what's a Test Light gonna do for you?) and compare the Supply voltage (should be battery voltage) of the Large terminal to the Load side (GPs) of the Other Large terminal. If the contacts are clean, there should be little difference.
THAT's how you test a relay and the PCMs control of it