Mercury 850 sensors

[Lonepiper]

I have a <>1973 Merc 850 85 Hp motor and I was wondering if there are water temp and oil pressure sensors on it. I was told there were but it doesn't quite make sense to me.

Thanks

 

[Karl_Childers]

Re: Mercury 850 sensors

I really doubt there are oil pressure sensors on it, since it's a 2 stroke. But water sensors I believe.

 

[Moody Blue]

Re: Mercury 850 sensors

For sure you don't have an oil pressure sensor since its a two stroke.

There is provision to install a temp sensor but they didn't come from the factory with one. So, unless someone installed one, you won't have one of those either.

 

[Chris1956]

Re: Mercury 850 sensors

Lone, Those motors do not have any sensors, except the trigger on the ignition. They do not have a thermostat. The only worthwhile gauges to have are tach, gas gauge and water pressure gauge. Temp gauges are useless, since there is no thermostat.

 

[Lonepiper]

Re: Mercury 850 sensors

Is it possible (and easy and cheap) to add water pressure and temp sensors to the Merc 850? Reason I ask is since the motor is 30+ years old, this is probably good insurance to keep an eye on it.

Thanks for the help all!

 

[Moody Blue]

Re: Mercury 850 sensors

Chris, I keep reading on these forums about the benefits of water pressure gauges and I can't argue about the reasons for using them BUT, I have also read in many cases that the water pressure is soooo low that the gauges just don't register.

I have done alot of research and can't find a definitive answer on the best place to plumb them into so that they will work. I went to the trouble of putting one on my motor and with a brand new pump assy & impeller I get about '0' PSI showing on the gauge. Have tested the gauge with compressed air and reads fine to 15psi. Plumbed a "T" into the telltale line and ran to the gauge.

 

[Chris1956]

Re: Mercury 850 sensors

Moody, The inlines do indeed have low water pressure, especially at idle. If you have a threaded plug on the head water jacket, I would use it. If not teeing into the tell tale is the only other option. Use a brass tee that threads into the block. One side of the tee is the pressure fitting the other side is the elbow fo the the tell tale hose. It will work, I have done it.

 

[Moody Blue]

Re: Mercury 850 sensors

Chris, the second option (telltale), is the approach I took and plumbed exactly as you described.

At idle to about 3000rpm my gauge reads 0. At WOT it shows maybe 5PSI. Not much use.

Will try changing the take-off to the port on the head waterjacket midway between cyl 2 and 3 and see if it makes a difference.

Sorry Lonepiper, don't mean to hijack your thread, but the discussion seems relative to what you are asking.

 

[Lonepiper]

Re: Mercury 850 sensors

I do not mind at all, I would be very grateful if you or someone would tell me exactly to hook up said water pressure sensor.

 

[Moody Blue]

Re: Mercury 850 sensors (water pressure gauge)

Re: Mercury 850 sensors (water pressure gauge)

Finally able to re-plumb the pressure gauge since it did not work when plumbed into the tell-tale line.

I installed a T into the port on the block and ran one side to the pressure gauge and the other side to the temp gauge. Worked very well and was happy with the results. Gauge reads 3-5psi at idle and about 12psi at wot. Finally some valid readings.