Johnson 90 rev-limiting. HELP

[waddy]

I have a 2000 Johnson V4 90, new to me, and I believe it has a problem with the rev-limiting 'SLOW' system. It will reach a certain rpm - est 2000 - 2500, and the motor will seem to me like it rev-limits. Sounds and feels exactly like a revlimit, and it wants to jump right off the transom. I removed the starboard brown/white wire to the cylinder head heat sensor, and then tried removing the brown/white and white/blk wires to the port sensor but the problem remains. I heard the problem MAY be a related diode. Does that sound right, and if so, where exactly is this diode? Is there any way to disable this S.L.O.W. system entirely? This is happening while on the earmuffs, so a third question (unlikely I think) is could this be that due to intake vacuum, the motor 'knows' it isnt under load, and it may be OK on the water?? Any advice would be appreciated. I'm unfamiliar with OMCs. Thanks - John

 

[noelm]

Re: Johnson 90 rev-limiting. HELP

OK, first things first, DO NOT rev it up on the flusher, you may get a "runaway" and trust me, you will remember it very well, so your motor does not have a limiter that will stop it reving when not in the water, I would suggest you buy a genuine manual before you start anything, remove the plugs and inspect them, are they all the same? try a complete new tank. hose and fuel, do a compression test, after all these easy low/no cost tests we can go further with the aid of your manual.

 

[wilde1j]

Re: Johnson 90 rev-limiting. HELP

Shakes hard

Engine will not rev up above 2500 RPM and shakes hard (S.L.O.W. activated):

1. Verify the engine is not actually overheating by using infrared pyrometer.
2. Check the routing of the tan temperature wires, an example of a bad location is shown below. The tan wires need to be located as far as possible away from the spark plug wires.
3. Disconnect the temperature sensors and see if the engine performs normally. If it does, check both temperature sensors and replace the defective one.

S.L.O.W. is typically engaged when the system thinks the engine is running too hot and is there to prevent engine destruction from running hard while overheated. You don't want to disable this feature. Noelm has some good suggestions, especially the shop manual and warning about high, no load revs. It isn't pretty when a rod comes thru the block.

 

[waddy]

Re: Johnson 90 rev-limiting. HELP

Thanks for the quick replies. Let me try to respond:
1. Verify the engine is not actually overheating by using infrared pyrometer.
Problem is occurring right after startup, engine is nowhere near hot.
2. Check the routing of the tan temperature wires, an example of a bad location is shown below. The tan wires need to be located as far as possible away from the spark plug wires.
I also read this in the manual, and was hopeful, but it did not work, so I moved on to step 3 as you noted.
3. Disconnect the temperature sensors and see if the engine performs normally. If it does, check both temperature sensors and replace the defective one.
First tried disconnect of starboard brn/wht, then port brn/wht and wht/bk without luck.

Does anyone know exactly where the sensor diode is located so that I can bug it out? I am pretty darn sure I have a problem with the 'SLOW' system. Is there a way to easily disconnect this system for troubleshooting purposes?

 

[wilde1j]

Re: Johnson 90 rev-limiting. HELP

This IS NOT specifically for your engine, but for similar year range 120-140HP motors (an OEM shop manual would have detail for your motor):

ENGINE STAYS IN QUICK-START:
1. With the engine idling, check the Yellow/Red wire for DC voltage. If there is DC voltage over 2 volts on this wire while the engine is running, the Quick-Start will not disengage. A voltage of 5 to 7 volts will not engage the starter solenoid, yet will engage Quick-Start.
2. Short the White/Black temperature switch wire FROM the power pack to engine ground. Start the engine, if the Quick-Start drops out after approximately 5 seconds, replace the White/Black temperature switch.
3. Disconnect the Black/White wire from the power pack. If the Quick-Start feature is not now working, replace the power pack.

 

[marinemech1]

Re: Johnson 90 rev-limiting. HELP

locate power pack...under cover behind coils
Disconnect the port plug and test
if problem remains power pack is likely culprit
P.S> YOU HAVE CHAMPION SPARK PLUGS IN THIS RIGHT
And as others have said NEVER REV UP A 2- STROKE ON FLUSHERS
Wayne

 

[waddy]

Re: Johnson 90 rev-limiting. HELP

This morning I went through same procedure as noted earlier, with one exception: I shut off and restarted motor before and between disconnecting each of the two cylinder head heat sensors. On my earlier trials I had pulled the connectors while the motor was idling. That procedure does NOT work; it seems the S.L.O.W. system must be 'reset' by restarting each time. Anyhow, now she spins up no problem. I could tell right away with one blip up of the throttle. No need to over-rev, its immediately obvious. I believe I have one defective heat sensor, although still not 100% sure on that since it ohmed out as 'open' to engine ground, as it should. We'll see when the new sensor is installed.. Thanks