Bad ground?

[Per]

eh - have been doing fairly well without too many incidents this year.
however on our last trip, i early on noticed my sb alternator was out of range pushing over 15volt, had to slow down to apprx 1000 rpm to keep it in range.
once i got to our destination, I snipped the yellow wire to take it out of action in order to get back home at our regular cruising speed. the return went without incidents.

i ordered a new alternator and had my electrician swap the two...
on test ride i noticed if i click on the engine gauge lights, all gauges go full right. so i kept the gauges light off but now seems to be mushrooming..

i turn the ignition to on on sb and gauges comes on normal, when i also turn on the port ignition all the gauges goes full deflection (fuel gauges, oil pressure etc).
my electrician is sorta lost, any advise appreciated.

 

[Barking Sands]

After replacing the alternator did your over voltage situation get corrected?

 

[STB]

The alternator problem sounds more like the regulator, possibly located within the alternator, than the alternator itself. You may want to save and fix the old one as a spare.

A very common cause of this is moving the battery switch while the engine is running. If it is break-before-make, which means thst it has a brief open period while switching, the alternator voltage can jump through the roof during this time with no load eating diodes, etc. I'm not sure if this happened to you or not, but please do be careful.

If you need to switch while running, there are make-before-break that make one battery connection before breaking the other. They are less common but out there. You can swap if you don't have one already.

In one paragraph you mention the gauges going "right" and in the other going full "deflection". I'm imagining full deflection right. If it is left, that's a different matter.

As for the gauges, what type are they? Are they, by any chance, VDO?

If not, they are probably US standard gauges in which case the senders are connected to ground at the engine and as resistence at the sender goes down the gauge needle goes right. See the attached table from a Sierra gauges document, for example.

This suggests that something is shorting the signal/sender wire to ground.

Think of the gauges as being ammeters measuring current. As the resistence of the sender goes down, there is more current flow from the power connection at the gauge, through the gauge, through the sender (that changes resistence with whatever it is measuring) to ground at the engine block.

If the gauges are pegging right, the gauges are sensing a lot of current passing through them. Since the voltage they are getting isn't likely going up, the resistence to ground must be going down. It is a bit strange to see it to so many gauges at once. But it suggests the sender wires are being grounded somehow.

I asked earlier about VDO standard gauges. They are less common, but definitely not unicorns. This situation makes a lot more sense to me if your gauges use VDO standard. See the attachment. With VDO gauges, the gauge deflects left with more current/less resistence. As a result, if the wire from the sender to the gauge becomes is loose the gauge pegs right.

Also, many VDO senders are so-called isolated ground senders. They don't ground themselves at the engine block. They ground themselves with a separate wire. This allows the sender and gauge to have the same ground reference.

If you have isolated ground VDO senders, the ground is originating in the panel, and the ground wire had been disturbed and is now open, I have a possible model for what's causing the problem and how to fix it.

Having said that, it is a somewhat unlikely model. Isolated ground senders are rare. One way of telling is to look for a small wiring harness at the gauge and see if there is a blue/black wire in use. If it is an actual VDO gauge, that would be the signal ground.

At any rate, I think I need a lot more information about your senders, gauges, and set up to understand this situation and be of more help.

Pictures of the senders, the fronts and back.od the gauges, and the wiring connections at the sender and gauges would be particularly helpful.

Good luck!

 

[Per]

thanks for your reply.
indeed these gauges are VDO
the problem does not seem to be related to one gauge, its across the board.
strange as this sounds it started with the instrument light switch, when i turned it on position, all the gauges deflected full right.
so you are saying there is a bad ground wire in the panel.
if its not grounded to the engine - how is it grounded?

 

[STB]

Can you look and tell me if there is a little wiring harness at the back of each gauge with a blue/black wire? If so, is it connected or just hanging out? Thanks!

Edit: Blue+black? Not plain blue.

 

[STB]

What I'm trying to figure out is if you have classic style gauges with push or screwdown connectors or Viewline style guages with a wiring harness.

If you have newer Viewline style gauges, that blue/black wire will tell me a lot.

If you've got older style guages, we are going to have to poke some more.

But one model for the failure is that you've got isolated/floating ground senders, which don't necessarily ground to the engine block. They have their own ground terminal.

Some people wire that to the engine block, others to a ground bus bar in the engine room And others connect it to a wire that runs up with the wiring harnass to the same ground as the gauges use.

Given that you have vdo gauges, the simplest model.for the failure is that they've lost their ground. Given that they are all bad, this means that they've all lost their ground. Since that many independent failure events at the same time is super unlikely, it is unlikely that each one is failing to independently ground itself to the engine block.

So, we have two straight-forward possibilities: Both engines have become ungrounded, which seems unlikely. Or, you have floating/isolated ground senders connected to the same grounding point and either that grounding point or their connection to it had failed.

Given that this happened while you were working in the console by the gauges, my current theory is that they were grounded to the same point as the gauges, somewhere near where you were working, for more accurate readings -- and they have become disconnected from it.

Other things are possible. Butnthay seems to be the most straight-forward failure mode. So, I'm going with Occam's Razor until something else seems more likely.