1991 Mariah voltage drop at dash

[wolfe1]

Hello everyone. I'm new to the forum looking for some help. I've got a 91 Mariah 240Z with a 7.4L Merc motor. I have an electrical issue involving a voltage drop at the dash.

I've just installed a new battery and have 12.3V at the posts. With the engine off and the ignition on I only see 10.5V on the volt gauge mounted in the dash. With the engine running, and all accessories off, the voltage does come up to over 13V. The voltage at idle does not stay constant, it flickers for lack of a better term, as if there it an itermitent drain. When I switch on the blower the voltage drops below 12V, as I turn on other things such as lights, docking lights and others, the votage drops below 10V. I've even had the motor shut down at idle before with the voltage at the gauge this low.

I've been dealing with the problem for years and after some resent issues I'm determined to find the cause. My thoughts are loose or poor grounds and possible loose spade connectors behind the dash creating resistance and power loss. I've had the alternator tested and it's fine, and as I mentioned the battery is new. Any help would be appreciated. Thanks, John

 

[seabob4]

Re: 1991 Mariah voltage drop at dash

You probably have a single, fairly heavy gauge hot and ground that feed your helm. Check both those for corrosion, and make sure all your connections are tight. At 19 years old, I'm sure there is a significant amount of corrosion in your wiring. Down here on the Gulf Coast of Florida, it can start to show up VERY quickly...

 

[wire2]

Re: 1991 Mariah voltage drop at dash

One thing going for you in this situation; where there's current through a resistance, there will be heat. Mr. Ohm passed a law that says so.
e.g. if there's 10 amps of current through a poor joint (corrosion) and you lose 2 volts, there's 20 watts of heat being produced there.
So, run your lights, electrical equipment for a while, then feel the wires, connections for hot spots.

Or, connect a long reference wire to the + battery post, connect it to the + lead of a DC voltmeter, then use the - meter lead to test the + connections at various places. If you read 1 v or more, work backwards to find the cause of the drop.

 

[wolfe1]

Re: 1991 Mariah voltage drop at dash

Thanks for the replys. Here's an update. I started tearing the dash apart and found that the heavy gauge positive wire coming from the engine harness goes straight to the dash mounted circuit break and then to the ignition switch.

The ground on the other hand was tougher to trace due to it's location under the dash. It appears the ground feeds to the blower switch and then is daisy chained throughout the dash. The dash mounted volt meter is way down the line. Is I common to have a negative/ground switching on a boat or am I missing something? Looks like the positives go through the circuit breakers and the grounds are switched.

With the motor and ignition off I measured the voltage at the ignition and the back of the blower switch and got 12.34V, same as at the battery. When I started the blower the voltage dropped to 11.4V at the dash. At the battery with the blower on I got a drop from 12.34 to 12.24V. Does this seem normal, why the 1V drop from back to front? Resistance in the wire?

With the dash mounted volt meter in the circuit so far down the line, it's reading the voltage drop throughout the whole dash. Is that it's purpose or is it supposed to be reading battery voltage? Seems like the positive for the gauge should come directly from the ignition switch, and the ground from the battery. Also, is it normal to have a 2-3V drop throughout the dash with a lot of accessories/lights on? Sorry for the book, but thanks again, John

 

[seabob4]

Re: 1991 Mariah voltage drop at dash

John,
In a typical small boat setup, the wire feeding the helm will first go to a 40A breaker close to the battery or battery switch, then run forward and either terminate to a positive bus and feed the breakers from there, or go straight to a breaker and then daisy chain to the other breakers down the line. The ground wire coming from the back end should go to a ground bus, and the grounds from each individual appliance, be it light, horn, pump, whatever, should terminate to that bus. Normally there are no grounds on switches unless the switches have an indicator light in them, then the ground is only needed for the light. You can disconnect the ground and the switch will still function properly.

The voltmeter typically gets it's power from the engine harness, in a Mercruiser's case, the purple wire, as this measures the voltage the engine is seeing, which is what you want. If it is wired otherwise, someone has been in there somewhere along the way.

And, no, it is not unheard of to have switches in the ground path, although I haven't seen it in any production boat. 12VDC is simply a path from the battery to the appliance (12V+), and path from the appliance back to the battery (12V-). Interrupt either path with a switch, and you achieve the same results. It's just really not done on the negative side too often...

 

[bruceb58]

Re: 1991 Mariah voltage drop at dash

To find out where the voltage is being dropped use your multi meter. You will need long leads. Start with putting one lead on the battery and one on the fuse panel positive side and measure. Then put one on the negative post of the battery and the ground at the fuse panel. They should be exactly the same with a load turned on. Should just be a few tenths of a volt. The side that has the most drop is the one that needs to be investigated.

 

[Silvertip]

Re: 1991 Mariah voltage drop at dash

You will see ground wires on switches IF the switches have internal lights. Those ground connections are merely the ground for the indicator light. Otherwise it is simply bad practice to switch a ground lead.

 

[wolfe1]

Re: 1991 Mariah voltage drop at dash

Took another look today and you guys are correct, the grounds on the switches are for the lights. I could barely get the switch panel out from the face of the dash far enough to see all the terminals and missed seeing the red lead originally. I also went and pulled off all of the terminal connections and cleaned and recrimped all of them throughout the dash. No change in the voltage drop, I see 12.34V at the blower switch with it off and about and about 11.5V with it on. Would you guys consider that normal? I would think it's normal to see a progressive minimal drop in line voltage as load increases on the circuit.

Also, my drop at the gauge isn't as bad as originally thought. Turns out my dash mounted volt gauge reads 1V low from actual. At this point I'm not even sure I have a problem, now that I know the gauge reads low. I've owned the boat since '96 and it's always been this way. What prompted me to mess with it at this point was I installed a radio that shuts down when the voltage gets low (I believe 10.5V). I hooked it to power under dash. I think I'm going to run a line straight to the battery and see if that solves the problem. Thanks for all the input.

 

[bruceb58]

Re: 1991 Mariah voltage drop at dash

Try what I suggested. Its a real easy way to find drops.

One thing you should always do when you tracing down various voltages is use one ground reference. This may require a long lead to the battery but then everything is referenced to one point. If you measure voltage at the battery and then measure at the helm, your measurements may throw you off the right track.

As far as your blower causing a voltage drop like you are seeing, that won't be due to the ground going to the helm since the blower's ground is coming from near the engine more than likely. Again...use one reference.