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UWSkier

Voltage loss... but where? Check my logic.

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UWSkier
Posted (edited)

So since I got my boat, I've noticed that the voltage to both the PP and MDC display is lower than is ideal.  With key on and engine off, my PP display says 10.0-10.3V.  Measuring at the battery gives me a solid 12.6-8v based on my cheapie harbor freight DMM.

Measuring between the hot lead on the ignition switch and the ground bus bar under the dash shows 12.5v.  About what I'd expect and rules out bad grounds, bad batt connections.

When I key on, voltage measured between any of the switched posts on the ignition switch and the bus bar shows about 10.5v.  Not what I'd expect.

So the question I have is this.  Is my ignition switch faulty, or is there something downstream of the switched posts on my ignition switch causing this?

Boat runs great other than that.

I'm suspecting that I'm going to have to pull the ignition switch and disconnect wires one at a time to find the offending resistance.  The only stuff that comes on ignition is:

  1. Fuel pump
  2. ECM
  3. PerfectPass Servo
  4. PerfectPass Display
  5. MDC
  6. Gauges
  7. Depth finder and transducer

Gotta be one of those if not the switch itself.

Edited by UWSkier

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formulaben

Ignition switches are relatively inexpensive.  I'd swap that out before going any deeper since it's fairly easy.  Just take a picture before you start taking the wires off...it's easy to mix them up.

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MLA
21 minutes ago, UWSkier said:

When I key on, voltage measured between any of the switched posts on the ignition switch and the bus bar shows about 10.5v.  Not what I'd expect.

If this includes the 12V input to the switch, then issue is the before the switch. A bad switch will still have battery voltage in the input, but drop through the switch. 

 

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UWSkier
43 minutes ago, MLA said:

If this includes the 12V input to the switch, then issue is the before the switch. A bad switch will still have battery voltage in the input, but drop through the switch. 

 

I measured the 12v input to the switch.  Between that and ground bar I get 12.5v.  But I didn't measure that post with key on.

Once key was on, I measured other posts to ground bar and was seeing about a 2v drop.

Things I've eliminated:

  1. Perfect Pass
  2. ECM
  3. Fuel Pump
  4. MDC
  5. Gauges
  6. Depth finder transducer
  7. Ground post at back of engine (disassembled, filed down to clean contact, reassembled, no change)

I eliminated the ECM, fuel pump, and MDC by pulling the kill switch.  My PP is powered off a different circuit but was still only reading about 10.3v.

I eliminated the PP by pulling the power to the PP Master Module.  PP does suck close to 1v due to the large resistor on the servo, but that's not the culprit.

Things it still might be:

  1. Key switch itself (might go ahead and swap it)
  2. Depth finder display (unlikely)
  3. Something on the charging circuit as I believe that's available with kill switch pulled.  (Exciter for alternator perhaps?)

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MLA

Id retest at the switch. Once the switch is on and carrying a load, see if the input voltage also dropped or just the output. 

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UWSkier
14 minutes ago, MLA said:

Id retest at the switch. Once the switch is on and carrying a load, see if the input voltage also dropped or just the output. 

Input did drop with key on.  I started pulling off the wires 1 by 1.  Turns out it is my PerfectPass pulling about 1.5v.  If just powering up the Master Module and display, that drops to only about .5v.  I think I'll start by running heavier gauge grounds for the display and master module.  The wire gauge is really light considering the servo and everything is being fed off of that.

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MLA

My opinion is the issue is the supply to the switch and not whats happening after it. Heavier wire from a switch output to the "load" or from the ground BUS to the load,  will not change the current draw of that load. 

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UWSkier
23 minutes ago, MLA said:

My opinion is the issue is the supply to the switch and not whats happening after it. Heavier wire from a switch output to the "load" or from the ground BUS to the load,  will not change the current draw of that load. 

I'm looking to eliminate as much resistance loss as possible.  These older boats are notorious for being under-grounded.  I'll look to improve the supply to the ignition as well.  I'm currently only seeing about a .1v drop from the battery to the ignition switch input though.  PP + comes immediately off the ignition switch.  It's grounded directly to ground bus.

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UWSkier
Posted (edited)
On 4/23/2019 at 5:25 PM, MLA said:

My opinion is the issue is the supply to the switch and not whats happening after it. Heavier wire from a switch output to the "load" or from the ground BUS to the load,  will not change the current draw of that load. 

You were right on this.  Something is a bit screwy with the feed to the switch when there's a load on it.  With no load, voltage to the switch looks fine.

I moved my PerfectPass to a relay instead of taking the power off the keyswitch.  Now instead of seeing only 10.3v at key on, it's seeing about 12v.  Much happier that way.

Edited by UWSkier

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UWSkier
Posted (edited)

I'm pretty sure on the '01s, the "battery" input to the key switch is the red wire in the cannon plug bundle.  That's a fairly long run and not historically strong connections.  For '02, they removed the Batt and Ground pins from the cannon plug and moved them up to the power block and ground bus which go direct to the battery.  Not sure whose bright idea it was to pull all accessory power via the ECM plug...

 

SC 2019-04-28 at 3.58.19 PM.png

Edited by UWSkier

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formulaben

Wow, that's definitely now how mine is wired...I see the asterisk about 2002 model years.  I wonder if that has to do with batteries becoming standard up in the observers's compartment instead of the locker?

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Sethro

Back in 1999 or 2000 I added a direct ground from the battery terminal to the bus bar on my 1998.  I can't remember how I received the recommendation, but I haven't had any issues since.

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UWSkier
19 hours ago, Sethro said:

Back in 1999 or 2000 I added a direct ground from the battery terminal to the bus bar on my 1998.  I can't remember how I received the recommendation, but I haven't had any issues since.

That was a common mod back in the MBO days.

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Bozboat

How old are your batteries?   Maybe take them to the autoparts store and get them load tested?

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