Water Pump Johnson 1988 120 HP

[crabby_bob]

Thanks to everyone who offered advice regarding the S.L.O.W mode problem my brother and I have been having with our motor. We replaced the water pump impeller and defective heat sensor this past weekend but the problem is still there. We're positive that both heat sensors are working properly and that the horn and associated wiring works all the way back to the sensors.

We're not sure about the isolation diode though. In the reverse bias direction it measures an infinite resistance but in the forward bias direction it measures about 7k on a 20k scale. Does that sound right?

We found that as people predicted the motor runs much cooler at high speed than it does at a trolling speed. At high speed both heads remain cool enough to keep my hand on them indefinitely. At trolling speed the port side head gets hot enough to limit me to a five and a half count. The starboard side only permits about a 2 and a half count. So it seems like the starboard side might be getting too hot. However, I thought S.L.O.W. mode was triggered by the heat sensor and the I'm positive the heat sensor and alarm work so why doesn't the alarm sound?

We forced S.L.O.W. mode by briefly shorting the heat sensor to ground. The alarm sounded and the motor, which had previously worked fine, wouldn't get over 2600 RPMs. We only grounded the sensor long enough to hear the alarm make a short beep but found that we had to turn the engine off to clear S.L.O.W mode. What would happen if the head is heating up to just below the sensor threshold? Is it possible that the sensor can trigger and reset fast enough to set S.L.O.W mode but not fast enough to trigger the alarm?

Thanks again,
bob

 

[imported]

Re: Water Pump Johnson 1988 120 HP

Why not just replace the power pack? Sounds like its broken.

 

[crabby_bob]

Re: Water Pump Johnson 1988 120 HP

Why not just replace the power pack? Sounds like its broken.

Thanks for the suggestion. I looked into it and it's a fairly expensive part. I'd hate to change it out before I've finished troubleshooting. Someone suggested in another thread that I change the thermostats. I guess there's a newer type that works better. I might try that next.

 

[imported]

Re: Water Pump Johnson 1988 120 HP

seeing as no one has responded thus far to your issue i would take that as a hint that your current engine is to old and outdate to fix. You should repower with a Honda of the same size.

Just my opinion.........

 

[reeldutch]

Re: Water Pump Johnson 1988 120 HP

so if i understand you when you are on plain your overheat alarm will go off but the motor is in fact not overheating?and its going in slow mode? has that been verified with temp gun?


and at low speed let say around 1500 rpm the motor will run all day?? no alarms?
verifie the temp at slow speed with a temp gun.

the overheat alarm is triggered at 185F

the motor should normaly run around 140F depending on the situation.

good luck

 

[Silvertip]

Re: Water Pump Johnson 1988 120 HP

If the sensor is triggering the SLOW mode it is also triggering the alarm horn. It can't distinguish between the two. On is On and Off is Off to both circuits. The sensor works like a light switch. If turning on the switch lights two bulbs, the switch is not the problem if one of them does not light.

 

[crabby_bob]

Re: Water Pump Johnson 1988 120 HP

so if i understand you when you are on plain your overheat alarm will go off but the motor is in fact not overheating?and its going in slow mode? has that been verified with temp gun?

The alarm never sounds. The original horn was burned out (melted) when we got the boat so we replaced it. The port side heat sensor was burned out as well so we replaced it. Then we verified the alarm by grounding the tan wire going into the power pack. It's in working order.

At high speed there is no problem. There's no alarm and the boat runs fine. The engine never gets to hot to touch.

The problem is at low speed. There's no alarm but the starboard side engine head gets too hot to touch for more than 2 seconds. The port side gets hot but can be touched for maybe 5 seconds. The engine runs fine but when we try to get back on plane the engine won't go beyond 2600 rpm. We haven't manually taken the temp yet but the alarm doesn't sound. We did test the sensors and according to the manual they both work.

and at low speed let say around 1500 rpm the motor will run all day?? no alarms?

Right, the motor will run at low speed all day with no alarm.

good luck

Thanks for your help and well wishes.

 

[crabby_bob]

Re: Water Pump Johnson 1988 120 HP

If the sensor is triggering the SLOW mode it is also triggering the alarm horn. It can't distinguish between the two. On is On and Off is Off to both circuits. The sensor works like a light switch. If turning on the switch lights two bulbs, the switch is not the problem if one of them does not light.

Thanks Silvertip. I wasn't sure exactly how the the sensors triggered the alarm but suspected that they worked as you describe. Thanks for confirming that.

 

[crabby_bob]

Re: Water Pump Johnson 1988 120 HP

seeing as no one has responded thus far to your issue i would take that as a hint that your current engine is to old and outdate to fix. You should repower with a Honda of the same size.

Just my opinion.........

I don't know about getting a Honda. I'd kind of like to hold on to my Johnson. My opinion is that a lot of Honda stuff is over priced, no offense. If we swap it out I'm thinking either a Mercury or a Tohatsu.

 

[bktheking]

Re: Water Pump Johnson 1988 120 HP

I don't know about getting a Honda. I'd kind of like to hold on to my Johnson. My opinion is that a lot of Honda stuff is over priced, no offense. If we swap it out I'm thinking either a Mercury or a Tohatsu.

Just fix the one you have, unless you are rich.

 

[wilde1j]

Re: Water Pump Johnson 1988 120 HP

Sounds like an obstruction in the stbd bank or a vernatherm problem. Have you checked to see what temp they open at? You may need to use water or air pressure to make certain the coolant passages are clear.

 

[reeldutch]

Re: Water Pump Johnson 1988 120 HP

i would when the engine goes in slow mode verify the temp on both banks.

if its indeed overheating than you have to fix the alarm + the reason it overheats.

when was the last time waterpump was done and tstats?

i removed on several engines the old style with the high and lowspeed waterflow valve , and replaced it wit the 1993 suffix s tstat + tstat cover.

works great.

good luck

 

[bktheking]

Re: Water Pump Johnson 1988 120 HP

Top of the post says they replaced the impeller last weekend.

 

[bktheking]

Re: Water Pump Johnson 1988 120 HP

the overheat alarm is triggered at 185F

good luck

In the water or test tank start the motor and rev to 3500 rpm.

The overheat alarm is triggered at 203F according to the SLOW section of troubleshooting in my manual. They state in the book that first the port side should be tested by disconnecting the wire going to the sensor and then going to ground. The rpm of the engine should drop to 2500. Shut the motor off and then do the same process for the starboard sensor. The rpm should drop as well. If the rpm drops with only one sensor then the wiring harness and connectors.


I put the laser thermometer to the heads of my 90 two days ago just to find out what 140F feels like, I couldn't keep my hand on for long without it starting to burn, 140 is quite hot, my 9.9 on the other hand runs at around 113 and I could keep my hand on it all day long. Big difference over 27 degrees.

 

[reeldutch]

Re: Water Pump Johnson 1988 120 HP

what you will have to do is check the temp with your gun when the motor drops in slow mode.
run the engine so the problem happens then check the temp wih gun.

your system might be working perfectly fine

 

[crabby_bob]

Re: Water Pump Johnson 1988 120 HP

I agree, the next step is to actually measure the temperature of the motor. I don't understand why the alarm doesn't sound if it's a heat issue but that will probably make sense once we find the problem.

Thanks for the help.

 

[crabby_bob]

Re: Water Pump Johnson 1988 120 HP

My brother picked up a temperature gun last week and we took the boat to a pond to do some more trouble shooting. We were hoping to isolate the problem to overheating on one of the heads so that we could work on it over the winter. Unfortunately we weren't able to reproduce the problem. The boat ran great! We idled around the lake for a couple of hours while monitoring the temperature of the heads and neither of them ever got above 155 degrees. We measured the temperature at different points on the head including near the sensors. The starboard side was about 20 degrees cooler than the port.

We think that the water temperature has gotten cold enough to keep the engine from overheating so we won't get a chance to go further with this until next year. We don't want to do any work on the engine until we can isolate the problem to either one or both heads.

One thing we noticed when we took the lower unit off a few weeks ago was that the end of the water pump pickup tube was corroded and jagged. I don't know if that would be a problem. Also, I'm not sure but I believe we have the newer thermostats. They're black and don't have an elongated nose. There's no part number on them though so I can't tell for sure. I think someone said the old style parts are white. Is that correct?

Thanks for the help.

bob

 

[reeldutch]

Re: Water Pump Johnson 1988 120 HP

i dont know if you have the original system with the waterflow valve that changes the waterflow at high speed or low speed?

if you do and still have overheat problems in the spring ,get rod of that old system, and get the 1993 suffix s thermostats and thermostat cover and springs and gaskets.
i guarantee you it will be so much better and less prone to failure.

good luck