Continuing MD22 ground fuse problem, keeps on blowing

[Peter]

Still trying to find an answer why my ground fuse keeps on blowing. Think I've got part of the answer but need some confirmation if I'm on the right path. With ref to the neutral relay, bottom relay 5 on Volvo wiring diagram, my understanding is that this relay should be energised when the key is in "glowplug" position and in the engine stop position other than this it should be de-energised. For my engine when the key is the engine start position this relay stays energised. This means there is neutral path from the starter motor neutral to ground (engine block) via the ground fuse so when the starter motor is starts it will blow the fuse. When the engine was running with the original Lucas starter, nor problem, maybe a lower run current then the new starter I'm using, a Presolite 20550101 starter motor which is a insulated return, this has been checked OK.
Am I on the right path? or any comments from any MD22 experts out there.

Thanks

Peter

 

[VicS]

Not an MD22 expert,I know nothing about the engine as such, but quite happy to try to unravel the mysteries of the wiring schematic


You are almost right.
The lower relay is energised from the glow plug circuit and the engine stop circuit in order to provide a negative path from the engine block during these two operations.

However what I think you have missed is that the glow plug circuit remains energised via the "ignition" switch during the starting phase .

The result is that the lower relay is energised during preheating and during cranking, connecting the engine block to battery negative via the fuse. That is what you observe.

If the starter motor is of the insulated negative type there should be no path from it to the engine block. The negative return should be directly back to the battery negative via the negative isolating switch.


You need to double check the isolation of the starter motor negative from the engine block
There must be a connection there somewhere that should not be there for the cranking current to flow back to the battery via the fuse.


However its a nice summer's day out there.. I'll have another look at it later and post again if I have any flashes of inspiration But as it stands I think the finger of suspicion points at the stater motor or its installation.

 

[VicS]

Bump!

Gosh folks nobody got any ideas. Peter has PM'd me because I'm the only one to reply to this thread.

Please though no guesses. Ideas thought through in conjunction with the wiring diagram in THE MANUAL

 

[pvb]

I think you summed it up already, Vic, when you posted "You need to double check the isolation of the starter motor negative from the engine block. There must be a connection there somewhere that should not be there for the cranking current to flow back to the battery via the fuse."

 

[mjf107]

Possible Test

I think you summed it up already, Vic, when you posted "You need to double check the isolation of the starter motor negative from the engine block. There must be a connection there somewhere that should not be there for the cranking current to flow back to the battery via the fuse."

+1

This test should confirm if current is flowing through the ground fuse when cranking
Firstly the ground fuse needs to be sound, secondly if the engine starts you will have stop it manually. Disconnect the following wires 6 preheat, 8 ground relay, 2 stop solenoid. Connect a test lamp across terminals 3 and 4 on the ground relay. Press start, with the starter motor cranking the test lamp should not light. If it lights current is flowing through the ground fuse, most likely from a fault on the starter motor or its wiring or the circuit is not wired as drawing. I say most likely as there is always other possibilities. The reason for disconnecting the preheat relay and the stop solenoid is that the return current from these items would normally flow through the ground fuse. If the lamp does not light the fault could lie with the pre heaters as these are on at the same time as cranking. BTW what size is the ground fuse........Good luck michael

 

[mjf107]

+1

This test should confirm if current is flowing through the ground fuse when cranking
Firstly the ground fuse needs to be sound, secondly if the engine starts you will have stop it manually. Disconnect the following wires 6 preheat, 8 ground relay, 2 stop solenoid. Connect a test lamp across terminals 3 and 4 on the ground relay. Press start, with the starter motor cranking the test lamp should not light. If it lights current is flowing through the ground fuse, most likely from a fault on the starter motor or its wiring or the circuit is not wired as drawing. I say most likely as there is always other possibilities. The reason for disconnecting the preheat relay and the stop solenoid is that the return current from these items would normally flow through the ground fuse. If the lamp does not light the fault could lie with the pre heaters as these are on at the same time as cranking. BTW what size is the ground fuse........Good luck michael

I have now looked at the Prestolite site to check if the the 20550101 is indeed an isolated return starter, it doesn’t say it is or if it does I cant find it! But the amount of connection and type look right for it to be used as isolated. Confusingly what I assume is the isolated return terminal is labelled “Earth” in your case this does not want to be connected to the engine block, but to the cable that goes to the battery negative via the isolator. I have not found a description of the terminal designation or a wiring diagram, but terminal 30 and 50 look to have the same function as the Volvo ones Terminal 15 which I assume is the other side of the solenoid coil will have to be linked to the terminal marked as “Earth” along with the isolated return cable as previously mentioned You may have already connected the starter like this? Its also important that these cable lugs are not making fortuitous contact with the starter case or any other metalwork If you have any connection info supplied with the starter let me know before connecting as suggested. I hope all this is not overstating the obvious!

http://www.prestolite.com/productinfo/startermotors/20550101/20550101_dim_s.jpg

 

[VicS]

I did find , I cannot remember where now, that the Prestolite starter in question is in fact an isolated return one but that earth terminal in the diagram worries me now

Peter has said ( it may be in the PM ) that he has checked that there is no contiuity to the block with all the wiring disconnected

I've suggested a couple of other unlikely causes for the fuse blowing but said that I think the most likely problem is something to do with the negative wiring of the motor esp as the problem seems to date from the time of the starter motor replacement.

 

[pvb]

I did find , I cannot remember where now, that the Prestolite starter in question is in fact an isolated return one but that earth terminal in the diagram worries me now.

The Prestolite model mentioned is cross-referenced on various sites to the correct Volvo Penta part number for the MD22 alternator, so it should be an insulated return model. Looking at the Prestolite photos of it, I'd say it is an insulated return.