Volvo Md22 Wiring issue, glow plugs wont turn off

[Mastermuffin]

Hey, recently purchased a rebuilt Volvo md22 to replace my Perkins m50 that had bad compression. While testing it before it goes in the boat i noticed that the glow plugs wouldn't turn off after i engaged them with the ignition switch. If i unplug them the relay they pass through seems to work fine, but if they are plugged into the relay it stays engaged until i remove power from the battery.



Another issue is the rpm gauge likes to peg itself at 5000 revs when the engine is at idle.



Here's a video demonstrating the problem with the glow plugs.

https://www.youtube.com/watch?v=dGqH...ature=youtu.be

anyone have any ideas on how to diagnose this?

[DougR]

The MD22 engine is equipped with an isolated ground (two pole) electrical system. This means that there is no current flowing thru the engine block, and that all the senders, tach, alternator, etc. return current back to the battery thru ground wires.

There are two exceptions to this isolated system, the glow plugs and the stop solenoid, and there are three relays that are used to control these functions. One relay is a grounding relay, and it is used to connect the engine block to the ground wiring, and then there is a glow plug relay to connect current to the glow plugs, and the stop relay to feed current to the stop solenoid.

When you turn the key to the glow position current runs from the key switch to both the glow relay and also to the ground relay. The glow relay feeds current to the glow plugs and the current returns from the block to the ground wiring thru the grounding relay. The same scenario happens when you turn the key to the stop position, the stop relay energizes and so does the grounding relay. The stop relay works the same for both functions , and there are two diodes to prevent feedback from one function to the other function.

I doubt that a sticking glow relay would be causing your problem because both the glow relay and the grounding relay would have to be sticking to cause the problem. I would be more inclined to think the key switch is defective, as it can control both the glow and ground relays at the same time. Pull the connector off the back of the ignition switch and use a jumper wire to connect the B+ red wire to the orange glow wire and see if the glow plugs energize and stay energized.

[Mastermuffin]

Thanks for the response, i already jumped the ignition and it showed the ignition wasn't the problem, luckily i think its a much cheaper fix. I found someone had jumped the red/green wire with the orange glow plug wire.





They also added a one way diode to the red/green wire that i don't see any need for in the wiring diagram.



Found a broken diode as well.



I'm almost certain this is the problem because when i unplug the red/green wire the glow plugs work. I'm just wondering why they would jumped the red/green to the orange wire. Would you happen to know what the red/green wire does? It doesn't seem to effect anything when unplugged



pictures below.

[DougR]

The red/green wire is supposed to be connected to the orange glow plug feed wire though normally it is connected at the relay. THe purpose of the red/green is to illuminate the glow light in the instrument panel when the glow plugs are activated.

[Mastermuffin]

Originally Posted by DougR

The red/green wire is supposed to be connected to the orange glow plug feed wire though normally it is connected at the relay. THe purpose of the red/green is to illuminate the glow light in the instrument panel when the glow plugs are activated.

It is connected to the relay. That's what was causing it to stick, as soon as the relay was energized the red wire would get current from the relay and hold it open because it was attached to the orange wire further down.


No idea why someone did this to a perfectly good wiring harness.

[Uncle Bob]

Originally Posted by Mastermuffin

It is connected to the relay. That's what was causing it to stick, as soon as the relay was energized the red wire would get current from the relay and hold it open because it was attached to the orange wire further down.


No idea why someone did this to a perfectly good wiring harness.

There are plenty of people out there that are inclined to believe that they are the fonts of all knowledge, unfortunately we get to clean up after them.