Still wont start new gpr new gp's

[00ryanstroker]

i put a new gpr in it and the back small terminal wont lite up when i use a test lite so i went out and got another gpr and it this damn thingdoes the same damn thing wat could this be. i am gettin mad at it.

[NoRalPh]

With the key ON, you have no power at either of the sm terminals on the relay with the wires on them?

[00ryanstroker]

Originally Posted by NoRalPh

With the key ON, you have no power at either of the sm terminals on the relay with the wires on them?

With the key on just the small one in the back towards the firewall dont lite up.

[NoRalPh]

Inside the Relay, those two terminal are actually connected together though. There's just a coil of wire, that makes-up an electromagnet, they go to. One wire has 12v in it the other goes to the PCM and the PCM activates the relay by supplying Ground (so the current flows, magnet starts, pulls the contacts together)

So, if you disconnect both sm wires, one will have power (but again, connect it and that power travels to the other term) and if you put the power wire back you can test the relay by manually grounding the other terminal.

[kaya@bernhausen]

it shouldn't light up though. there should be no voltage there since it's after the load. ground side. voltage drops across the load

[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.

[NoRalPh]

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

[kaya@bernhausen]

i'm not sure i understand how the test light would damage the circuit. power for the control side of the relay comes straight from the battery. constant. the pcm ground switches the relay, so after the coil in the relay, there will be no voltage (ground side). that's the idea behind ground switching. it's used because it's safe.
i can't completely agree with you on your opinion of a test light. the power probe and other meters put no load on the circuit. a test light will show that the circuit is capable of flowing current.
believe me, i love my power probe. great tool. all i'm saying is that a test light, while extremely low tech, isn't a complete waste of time. and i only mentioned test light because i'd assumed that was what the op was using. myself, i use a microvat or other ammeter

[NoRalPh]

As I said, I'd think the semiconductor in the PCM for this is probably pretty robust and you could do what you're doing without damaging it. But, if you connected those two wires together, and asked the PCM to carry the full load those wires could carry to ground, you'd "pop" it - right?

So, somewhere between the resistance offered by the coil windings, and a True dead-short, there's a point where the PCMs circuit is going to get overloaded. And you can see where the filament in the Test Light is adding to the normal load. Sometime this can be a significant addition and you're gonna find yourself damaging modern electronics (like CanBus and other serial lines) if you go around probing stuff with bulb in it...

Ever see those videos of Stereo Shop Installers, getting the surprise of their life, when they accidentally deploy an an Air Bag (that's usually with a Test Light)

[kaya@bernhausen]

but the pcm switches on the ground side. there's no voltage there. for circuits that are high side switched, yes, i can see how it would be of concern. but not a low side switch