View Full Version : Troubleshooting Suspected Sender Issues

Jeff d
06-19-2014, 10:48 AM
I keep seeing posts where members say they've replaced a given sender to try to resolve a lack of or incorrect gauge reading on the dash and it didn't resolve the issue. Stop wasting your money and time doing this! First of all in most cases for boats circa 2000-2006ish it's likely not the sender but rather the gauge or the MDC. The senders are very simple to test with a multimeter so you can quickly have confidence about where the problem lies.

The following should apply to just about any MasterCraft boat with an Indmar. Most, if not all of these, likely apply to Ilmor and most other boats with American marinized engines too (i.e. PCM, MerCruiser, etc.) but I have no experience with them. Most of this has been posted elsewhere in the forum but I didn't see a "one stop shop" for all of them.

Oil Pressure Sender: 0 PSI=240 ohms, 40 PSI=103 ohms, 80 PSI=33.5 ohms

Quick Test: With the key on, disconnect the signal lead from the sender and connect it to ground. Your gauge should max out. When you remove the lead from ground the gauge should return to 0. If the gauge behaves as expected then you very well may have a bad sender. If the gauge doesn't move as expected troubleshoot further with an ohm meter but your problem likely lies outside of the sender.

Note: this is opposite behavior of a normal automotive oil pressure sender. You can go to AutoZone and get a sender that looks identical but will peg your gauge at 80 PSI with the engine off and key on. You need a marine specific sender unless you want to swap in an automotive oil pressure gauge.

Fuel Level Sender: Empty=240 ohms, half=103 ohms, full=33.5 ohms

Think about what you're doing when testing this as you're probably working in the presence of combustible vapors. Don't use an electric driver to remove/re-install the screws or do anything else that could result in a spark. Keep a fire extinguisher within arm's reach.

2 wire senders on the pre 2000 year models can simply be disconnected and tested with an ohm meter across the two pins. I'm not sure on the exact years but most 2000 and later models will have a 3 wire Centroid sender (red +12v, black ground and pink or white for the signal). These are solid state senders and do not have a mechanical float so don't go digging around in your tank looking for the missing float. You can't test the resistance with an ohm meter without power and ground being applied to the red and black pins.

Quick Test: In either case (2 wire or 3 wire) if you disconnect the sender harness, turn the key to the on position, and jumper the ground pin to the signal pin your gauge should jump to full. Remove the jumper and the gauge should return to empty. If the gauge behaves as expected then you very well may have a bad sender. If the gauge doesn't move as expected troubleshoot further with an ohm meter but your problem likely lies outside of the sender.

Water/Coolant Temp Sender: 100 degrees F=450 ohms, 175 degrees F=99 ohms, 250 degrees F=29.6 ohms.

I haven't dealt with this one personally but you should be able to test it just like the others. Disconnect the sender and connect the signal wire to ground with the key on. The gauge should jump to 240 degrees. Disconnect it from ground and it should return to 80.

Voltage: There is no "sender" for this gauge. It gets it's "signal" directly from the +12 volt bus bar under the dash on the pre 2000 boats and from the MDC on the 2000+ boats.

Test voltage at the gauge for a pre-2000 boat.

For a 2000+ boat with MDC check at the 18 pin harness for the MDC. You should have ground on the black wire, constant +12 volts on the orange wire and ignition switched +12 volts on the purple wire. If all that checks out your problem lies in the MDC, the gauge itself or another gauge on the dash creating "noise" on the data bus causing the voltage gauge not to work properly.

It should be noted that all of these senders (At least on the pre2006ish vintage boats) are purely for the benefit of the operator. There are separate pressure/temperature "switches" on the engine to tell the MEFI computer if there's a problem with oil pressure, water temp, etc. If you have a check engine light or alarm from the MEFI these senders are not to blame as the only feed the MDC/gauges to let you know what's going on.