Oil & water gauges

[pogi]

Hello, I hope someone has an idea about what is going on with my gauges. When I turn on the key but don't start the engine both oil press. and water temp gauges jump up, oil to 45psi and water temp to 180. Now the boat engine is cold and not even started yet but thats the reading i get. Could it be the senders and is there a way to test them. Mercuriser 898. Thanks

 

[HT32BSX115]

Re: Oil & water gauges

Howdy,


Yes there is. First of all, both gages going "up-scale" is sometimes indicative of a bad ground connection.

The gages are simple ammeters that measure the current going thru the respective "sender" which is just a variable resistor connected to ground (the engine block)

You have 12v DC connected to one side (+ side) of the gage (switched on & off with the key) under the dash.

Then you have a single wire running from the other side of the gage to the single connection on the sender.

Current "flows" from the + side of the gage, thru the gage and wire to the sender to ground.

The sender resistance varies with oil pressure/temp respectively.

If you short either sender to ground, either gage should go "up-scale".

The senders are pretty standard. If you have an accurate multi-meter that will measure resistance, you can test the temp sender in hot water.....and you can test the pressure sender using a simple connection to an air compressor with a regulator and a mechanical gage.


Those senders don't fail very often.....They do fail sometimes but indication problems are frequently poor connections and /or frayed wires in the associated wiring and poor engine/battery ground connections.


I believe Mercruiser service manuals have actual resistance ranges for senders..... It's somewhere around 20-300 Ohms for most of them.....

Here's a link to Teleflex senders http://www.teleflexmarine.com/library/marinedirectory/TMTechCat-Instruments(v4_0)Senders.pdf

Regards,


Rick

 

[Silvertip]

Re: Oil & water gauges

Stick your head under the helm and look carefully at the back of the two gauges. Since the ground wire for each gauge is a "daisy chain" connection (one wire from one gauge jumps to the same terminal on the nexdt) it may be that the ground connection on one or both of the gauges has loosened and is touching the send line.