Engine water temp gauges merc 200 ob

[NYJim]

Back again folks

Just got another houseboat. 38 coastal cruiser with 2 mariner 200 outboards 1997.

Was in storage and fogged correctly so I hooked up fresh gas and batteries. They started right up, very happy.
Problem is after running them for maybe 5 minutes or so. The temp gauges on both engines did not move up at all. Like they are dead. I shut them down so as not to have a overheating issue. These gaugesbdo not look originals like the other gauges.

Can I just get a cheap gauge at an auto store to check if it's the gauge??? Should I replace the sending units on each motor??

Thanks in advance.

Big Jim

 

[Silvertip]

Re: Engine water temp gauges merc 200 ob

No you can't use auto gauges. You diagnose the issue. To test the gauge, jumper the "S" (send) terminal to ground which should peg the needle. Yes = gauge is good. No = something is wrong but other checks need to be made.
1) Check for +12 volts on the "I" terminal with the key on. Yes = good No = fix the reason why.
2) Check for good ground on the gauge. Yes = good. No = fix it.
3) Disconnect the send wire at the "S" terminal on the gauge. With key OFF, measure resistance of that wire to ground. Depending on the air temperature the reading should be somewhere between 33 and 240 ohms. 33 ohms = hot, 240 ohms = cold (about ambient temp). If no reading = wire is bad, or connection at sender is bad. If reading is normal and gauge checked ok earlier, then the gauge calibration cannot measure low end temps accurately which is not a problem -- many gauges do this and do not start registering until well oveer 100 degrees. That is why I will not use a temp gauge unless it has actual numbers and starts at 60 degrees.
4) Temp gauges and senders must match so auto gauges typically require vastly different senders. You can however, rent or borrow and Infrared Temp gun and measure cylinder head temp at the hottest cylinder near the plug.

Lastly., the fact that both gauges read low, says it is highly unlikely they are defective - but rather just calibrated to high on the low end of the scale. The fact that they are different style than the other gauges says they were added by a previous owner.

 

[NYJim]

Re: Engine water temp gauges merc 200 ob

Silver tip. You always have the answers. I think they were replaced with auto gauges. Will test on Monday. Thanks for your help

Jim