In the process of trouble-shooting my electrical system (another story), I came across this power diode in the engine compartment fuse box. It appears to be in the circuit that activates the PCM power relay (in the same fuse box). I've always tested diodes by checking the continuity twice, reversing the leads each time. If it works one way but not the other, diode's good. My diode, however, shows overload both ways (that would be infinity on an analog meter).

But, my meter only measures up to a thousand ohms, so anything over that would probably also show up as "OL". How can I tell if this diode is any good?

 

Don't know an easy way unless you have another meter. That cheaper meter should be able to check the diode, unless it has only one 1.5v battery in it, and the battery was low. Most diodes need about .7 volts to start conducting.

 

If you turn your key on and PCM Power Relay clicks (energizes) the Diode is good. Its to stop the power to PCM if you hook up jumper cables wrong, at least that is what I think it is for.

 

If you turn your key on and if the PCM Power Relay clicks (energizes) the Diode is good. Its to stop the power to PCM if you hook up jumper cables wrong, at least that is what I think it is for.

 

The meter must have a diode test function if it's a digital in order to test diodes and transistors.

The cheapies aren't able to source enough voltage at constant current to forward bias the PN junction.

The diode appears to be there to keep the PCM POWER relay coil from experiencing back EMF from the ignition coil which might cause it to chatter. It will also keep the PCM from receiving power in the event that battery power is reversed.

Steve

 

Thanks. Diode is good, and my journey continues. It really bugs me to run across something like that and not know what it's there for or what it does, so I wanted to put my mind to rest.

 

I was working on a 1992 E-350 Clubvan, it would run for months with no problem and once in a while it would be driven some where, usually not very far like to the store. When the van was being restarted to go home, it failed to start. It would have to be towed to the repair shop. After being dropped at the shop it would start and run just fine again for months before it did the same thing.
I found that when it wouldn’t run it had fuel and no spark.
To make a long story short, the pcm power relay diode was not allowing full battery voltage from the ignition switch. It was working like a resistor, it was dropping .6 volts. So when the battery voltage was low and the engine was being cranked by the starter, the voltage to the relay was too low to energize the relay so the pcm didn’t get power to it. The engine would crank over till the battery was dead and not ever start. Once battery voltage got 14.5 v the engine would start. A new diode fixed the issue.
One way to know if the diode is bad is to check it for a voltage drop.

 

Silicon diodes are supposed to drop .6 to .7 volts when forward biased. It's their natural behavior.
A normally charged battery should measure around 12-13 volts. The ONLY way it would ever get to 14.5 volts is if it has an external charger attached or the engine is running and the alternator was providing power.
I'd suspect something else....
See above.

 

Your'e right. But the pcm relay wasn't getting the voltage it needed until the system voltage got to about 12v, there maybe some resistance elsewhere, but it seems the relay won't close the contacts for the pcm power until voltage from the ignition switch reaches at the diode is least 12v. Apparently the relay needs at least 11.5v or more to power up and close its contacts, with any voltage at the relay less than that it was not always able to power up the pcm when the engine was being cranked over. Now that's not completely the problem because the vehicle does have long periods of non use which doesn't do the battery's state of charge any favors.But the replacement of the relay and the resistor fixed the problem.