Pls help tracing voltage drop (1 Viewer)

[jvoelcker]

Hi,

I am working on a 1979 HJ45 from Aus and am trying to trace a voltage drop when the ignition is turned on.

With the ignition off i get the full 12.x volts to the ignition switch, the top 4 fuses, basically to everything that would normally get it.

As soon as the ignition is turned on the lower fuses get powered, but the power to all of the fuses and ancillary circuits drops to 9-10 volts.

Apart from the power direct to the starter I have a white wire and a black/red wire that appear to go in to the loom and head off in to the cab and across to the glow plug relay and starter relay under the glove box.

The power for the fuses appears to be coming from some white/blue wires and I am assuming we have a joint issue further up the wiring, but I can't tell where these white/blue wires get their power from.

Before I start stripping back the whole loom, does anyone here have any suggestions where to start?

Thanks.

 

[Coolerman]

The large 10ga White/Blue wire goes to the alternator B+, the key switch, the Amp Meter - terminal, and the fuse panel and is the main connection between the alternator and the battery (through the Amp Meter and fusible link). That wire is always hot.

Just guessing, as I have never owned a diesel engine, but don't the glow plugs come on when yo turn the IG on? Those pull massive amounts of power and will cause the system voltage to drop. I would give your battery a full charge then repeat your test. If the battery is weak it would do exactly what you describe.

 

[jvoelcker]

Thanks for getting back to me.

The large 10ga White/Blue wire goes to the alternator B+, the key switch, the Amp Meter - terminal, and the fuse panel and is the main connection between the alternator and the battery (through the Amp Meter and fusible link). That wire is always hot.

OK, I have 12.4 volts on that circuit at rest, but as soon as I turn the ignition on it drops to 9-10volts and that is testing it at the alternator, fuse panel, ignition switch etc, so I am assuming that there is an issue between that circuit and the actual battery, but I cannot see where it connects to the battery.

From the battery I have one large cable going direct to the starter, one pure white one that appears to go inside the cab to the starter relay and then a black/red one that goes to the glow plug relay. I don't appear to get any voltage drop across these circuits when the ignition is turned on.

I'm assuming the white/blue wire probably tees off the white wire (since the black red in all wiring diagrams is flagged as dedicated for the glow plug) but I don't know where this connection is occuring.

Just guessing, as I have never owned a diesel engine, but don't the glow plugs come on when yo turn the IG on? Those pull massive amounts of power and will cause the system voltage to drop. I would give your battery a full charge then repeat your test. If the battery is weak it would do exactly what you describe.

No, the glow plugs are either triggered by reversing the ignition key before starting, similar to old tractors, or via a press button on the dash - this one has a press button on the dash and works fine.

I don't feel this is a current draw issue since the voltages at the battery and at the starter and glow plug relays don't get affected when the ignition is turned on.


Oh, and the battery is fully charged.

Turning the ignition on shouldn't be drawing much power, but it seems to be enough to drop the power on the circuit.

Thanks.

 

[terrx]

...
OK, I have 12.4 volts on that circuit at rest, but as soon as I turn the ignition on it drops to 9-10volts and that is testing it at the alternator, fuse panel, ignition switch etc, so I am assuming that there is an issue between that circuit and the actual battery, but I cannot see where it connects to the battery.

...

I don't feel this is a current draw issue since the voltages at the battery and at the starter and glow plug relays don't get affected when the ignition is turned on.

Oh, and the battery is fully charged.

Turning the ignition on shouldn't be drawing much power, but it seems to be enough to drop the power on the circuit.
...

If the battery is fully charged it would be 12.7 volts at rest not 12.4 -- 12.4 would be around 70% state of charge for a good battery. If it ever goes to 9-10 volts it's completely dead. Further, if it was a good battery, the current needed to pull it so low would more than likely smoke a 10 gauge wire. For those reasons I'd swap in a known good battery before digging into the wire harness. If still not satisfied, then perhaps pull all downstream fuses and begin adding one at a time to find out which has the greatest effect. My 2 cents.

 

[mattapo82]

Check grounding and if you have access to a thermal imager you can use that, look for circuits and grounds that heat up

 

[jvoelcker]

If the battery is fully charged it would be 12.7 volts at rest not 12.4 -- 12.4 would be around 70% state of charge for a good battery. If it ever goes to 9-10 volts it's completely dead. Further, if it was a good battery, the current needed to pull it so low would more than likely smoke a 10 gauge wire. For those reasons I'd swap in a known good battery before digging into the wire harness. If still not satisfied, then perhaps pull all downstream fuses and begin adding one at a time to find out which has the greatest effect. My 2 cents.

Thanks for the response.

Don't get distracted by the battery voltage, it was fully charged the other day and to be honest I can't remember whether it currently sits at exactly 12.4 or 12.8 - that isn't the issue here.

As I originally stated, we are only getting the voltage drop across parts of the loom which confirms it is a loom issue, not a battery issue.

I am trying to find where the white/blue wires are getting their power from.

All the wiring diagrams indicate fusible links by the battery, but they aren't there any more - this truck has had a tough life in the Australian outback.

I have the black/red and white wires running power in to the cab and across to the glow plug and starter relays below the glove box and I assume that the white/blue wires tee off one of those, but can't pin down where it occurs.

Without any other ideas from here I think I am just going to start stripping the loom from the engine bay back to see what I come across.

 

[jvoelcker]

Check grounding and if you have access to a thermal imager you can use that, look for circuits and grounds that heat up

Thanks for the response.

The earths are all fine.

As I said, it is a voltage supply issue on part of the wiring harness that I am trying to trace hence seeking a better understanding of how the loom is made up.

I guess if there is a poor/dry joint in the loom there may be some heat generation - I have a thermal spotter for hunting so might give that a go.

 

[Coolerman]

I do not have a schematic for a 1979 HJ45. Would you be willing to scan that and send to me or post a high resolution scan here so I can add that to my schematic collection? If I have a schematic I can better help you trace stuff out.

If the issue is not current draw then the only thing left is a poor connection somewhere. Never rule out the fuse panel as a source for grief. Pull every fuse and throw them away. Then clean that fuse panel as described below, before replacing them with new ones.

A fast easy way to clean a fuse panel:
Removing corrosion from brass terminals/fuse block.
Get the following: 1 gallon white vinegar, common table salt, box of baking soda
Mix up 1 pint of vinegar in a jar, then dissolve 1/4 cup of table salt into it. This is a mild acid solution.
Mix up one pint of water then dissolve 1/4 cup baking soda in it. This is an acid neutralizer.
Get another container of just plain water. This is the final rinse.
To clean the entire fuse panel. Remove the fuse panel from the harness. Scrub under hot water with an old tooth brush and some soap. Rinse in plain water.
Submerge the entire panel in the vinegar solution for about 10-15 minutes. Shake off the excess, then submerge it in the baking soda solution for 1 minute. This neutralizes the acid. Then submerge it in plain water, or just rinse it WELL under the tap to remove the baking soda solution. Blow dry.
Now use a Dremel tool with a wire wheel and go over all the brass /copper pieces to make them shine like new!
To clean individual connectors and terminals on the harness just dip each one in the vinegar solution for 1-2 minutes, then 30 seconds in the neutralizer and then 30 seconds in the plain water. This cuts the oxidization from the brass terminals in the connectors.

FYI
If someone could post some pics of what the fusible links on a 1979 HJ45 look like, I might be able to build new ones.

 

[jvoelcker]

I do not have a schematic for a 1979 HJ45. Would you be willing to scan that and send to me or post a high resolution scan here so I can add that to my schematic collection? If I have a schematic I can better help you trace stuff out.

If the issue is not current draw then the only thing left is a poor connection somewhere. Never rule out the fuse panel as a source for grief. Pull every fuse and throw them away. Then clean that fuse panel as described below, before replacing them with new ones.

A fast easy way to clean a fuse panel:
Removing corrosion from brass terminals/fuse block.
Get the following: 1 gallon white vinegar, common table salt, box of baking soda
Mix up 1 pint of vinegar in a jar, then dissolve 1/4 cup of table salt into it. This is a mild acid solution.
Mix up one pint of water then dissolve 1/4 cup baking soda in it. This is an acid neutralizer.
Get another container of just plain water. This is the final rinse.
To clean the entire fuse panel. Remove the fuse panel from the harness. Scrub under hot water with an old tooth brush and some soap. Rinse in plain water.
Submerge the entire panel in the vinegar solution for about 10-15 minutes. Shake off the excess, then submerge it in the baking soda solution for 1 minute. This neutralizes the acid. Then submerge it in plain water, or just rinse it WELL under the tap to remove the baking soda solution. Blow dry.
Now use a Dremel tool with a wire wheel and go over all the brass /copper pieces to make them shine like new!
To clean individual connectors and terminals on the harness just dip each one in the vinegar solution for 1-2 minutes, then 30 seconds in the neutralizer and then 30 seconds in the plain water. This cuts the oxidization from the brass terminals in the connectors.

FYI
If someone could post some pics of what the fusible links on a 1979 HJ45 look like, I might be able to build new ones.

Thanks, great info.

If you PM me your email I can send over the file.

Great info on the cleaning of the fuse box, did something similar with an old Land Rover 101 fuse box.

With our problem we have the voltage drop on both sides of the fuse box so it looks OK.

 

[jvoelcker]

Actually, the schematic I am working from is a 1981 one, but pretty close to the vehicle I am working on.

 

[jvoelcker]

Well thanks to @Coolerman , the problem was traced to the wires on the back of the Ammeter that were a little worse for wear.

They have been cleaned up and now everything is working as expected.