System Check Gauge

[Beaux38]

Hello all,
I just bought a 98 Bayquest which originally had a 98 Johnson on it. That motor was replaced with a 98 Mercury 200 Offshore. Now, I know the trim gauge for Mercury/Johnson are not compatible, as exhibited by mine working backwards. Don't know if you can reverse wires to fix that or anything else, but that's not my biggest concern. I just replaced the water pump impeller because I wasn't sure how old it was and I needed piece of mind. I also replaced both thermostats. I get ~8PSI at idle and about 15psi at WOT. I have a good stream from the tattle tale with no indication of steam. Myself and some fellow boating guru's of mine feel that the motor is not running hot, but the temp light comes on, as it did before the impeller and thermostat replacement, and the audible goes on at WOT even though I can barely hear it and goes off at idle. My question is with the System check gauge. If the trim gauges are different, why shouldn't I believe the system check gauges are different as well. My best guess is that it is the original system check gauge and I'm only saying that because its the same Farria Brand as the rest of the gauges. I've seen on Great Lakes Skipper where certain system check gauges are "set up for Johnson/Evinrude", would this explain the temp light and the audible? Any input will be greatly appreciated.

 

[Chris1956]

Re: System Check Gauge

The temp alarm for the Merc and the Johnnys should be the same. The temp switch should ground upon detecting overheat. Maybe you have a temp gauge-type sender on the Merc? That sender would partially ground when the motor warmed up. Check it with a ohmmeter.

The System Check Tach might (might) work with Merc Motor.

 

[Beaux38]

Re: System Check Gauge

Chris, thank you for your response. I didn't mention that I'm relatively new to the outboard world , but I think I understand what your saying. The sender may be for a temperature gauge and sending variable signals vs the on or off signal that a high temp sender would output? I'm not quite sure how to test that, one lead on the sender terminal and the other to ground with the engine running? If its a high temp trip sender, resistance should just suddenly zero and if it's a temp gauge sender it would slowly change it's resistance value? Also, I'm not sure I understand the tach speak, the tachometer is totally separate from my system check gauge. I have not checked for bad insulation from the sender to the gauge yet, that's next, but considering the audible comes on only on WOT, I'm not banking on finding a bad wire. I guess the throttle linkage could be rubbing. It will be a couple of days before I can dig into it anymore, but I do appreciate your thinking and It even makes sense to me. Please clarify this resistance check procedure for me. Thanks again.

 

[Chris1956]

Re: System Check Gauge

Sounds like you understand the high temp circuit. The automotive world uses the same concepts for detecting overheat. An idiot light uses a sender that changes from open to grounded when detecting an overheat. A temp gauge sender changes resistance levels, as the motor warms up, causing the gauge to register the correct temp. Merc often puts an overheat sensor on one cylinder head and a temp sender on the other cylinder head on a v-engine. You cannot connect them together, or you have conflicting signals on the idiot light or gauge.

When you said System Check gauge, I thought of my Faria System Check Tachometer, hence my comments. In that gauge, the four system check lights are combined with the tachometer.

 

[Beaux38]

Re: System Check Gauge

OK, I pulled both sending units off the motor, brought them to the lab at work, found out which was variable and which was on or off. Sad thing is, the high temp trip sender trips at around 180 deg. So, I am running hot at WOT. I have replaced the guts on my poppet valve and have not had time to see if this is the cure. My question concerning my system check gauge is this. The High temp sender sounds the audible and the gauge sender turns on the light. This does not seem right. Shouldn't the high temp sender trip both the light and the audible? Remember this is a Mercury wired through OMC pigtails, I would think that there should have been some reconfiguring somewhere am I wrong?

 

[Chris1956]

Re: System Check Gauge

Yes, the high temp switch should indirectly trigger the alarm and trigger the light, by grounding. Specifically the system check gauge will detect the grounded wire and turn on the light and buzzer. Only wire the high temp switch to the system check gauge. There is no place for the temp sender to be wired.

 

[Beaux38]

Re: System Check Gauge

Chris, I never thanked you for your time. You definitely put me on the right track. I solved the problem by the way, just fyi. What I learned is that an OMC 4 Light system check has no legitimate business on a mercury engine harness. I could have made the lights work right for the temp and one of the oil light functions, but it wasn't worth it in my opinion. I finally realized the audible was coming from a Mercury buzzer like it should and yes, some numb who did not wanna invest research time wired the temp sender to the omc temp light. So I was getting a temp light at anything above idle and when I opened her up, I was getting a true high temp buzzer alarm from Mr. Mercury. That being said, I found the cooling water hose from the top of the engine block to the poppet valve was cut and plugged so I fixed that, replaced the poppet valve guts, replaced the entire water pump assembly, installed the thermo's and dropped my jackplate 1/2 inch for ****s and giggles. Voila, one or more of those actions fixed my problem. I run around 140 deg like I should. Plus, I've learned a lot about my Old Merc. I'm loving this boating stuff. Thanks for helping. Ken Scoville

 

[Chris1956]

Re: System Check Gauge

Ken, Good luck with it. Sounds like you learned some stuff and had some fun. At least looking back on it, it appears to have been fun.

On my fun side, I had to make a new rear cover support for my Merc 135HPV6. The POS stamped steel one rusted away. A replacement was avail but I was too lazy to pull the powerhead to replace it. Not a great design, but one who lives on to this day. I ended up fashioning a piece of aluminum, and found a way to bolt it on. If it lasts, it stays. If it falls off, I pull the powerhead, and install an OEM one. Of course after the boating season ends.....