Overheat alarm on 1990 Johnson 70hp, J70TLESB

[Lakemichfish]

Problem statement: Overheat alarm goes off after 5-10 minutes of running my engine. I'm typically leaving the launch at an idle and then increasing throttle. Sometimes I get to WOT for a minute or two, takes longer at 2-3k RPM before alarm sounds.
Knowns:
1. New impeller and cup.
2. New thermostat.
3. Relocated tell tale to top of exhaust manifold per service bulletin.
4. Water pressure around 10psi at idle and 20+ at WOT. Good stream from telltale, consistent flow.
5. Checked temperature switch, good. Closes around 208F.
6. Overheat alarm stopped when I disconnected the temperature switch.

Removed head thinking that there was a clogged water passage. Everything looks clean and clear.

Thought it could also be a bad head gasket that was allowing exhaust to get into cooling passages pressurizing it and not allowing water flow.. I did not see anything that would indicate this, the gasket area around the cylinders look good.

I did see that the lower portion of the gasket around the bottom passage ways was warped. Could this allow water to bypass head or reduce pressure enough to prevent the water from reaching the top?

The gasket warped because there wasn't enough support surface on the head. The port is much bigger than the port on the block. There is no way it can seal. Is this the way it's supposed to be?

Did i get a new thermostat that was bad?

Thanks!

 

[Sam2001]

I have the same motor and had a similar problem awhile back with the warning horn sounding off sporadically , it ended up being a pinched wire inside the side mount control box that was connected to the horn that was causing it to short out . The wiring and or the horn itself might be something to look into ? There's plenty of experts on here that will chime in with some good advise.

 

[Lakemichfish]

I took my boat out last night. I ran at about 22mph for about a minute and the engine went into SLOW mode. No buzzer, just SLOW mode. Turned it off and took the cowling off and check temp- 130F. Then I it cool off, it ran fine after 5-10 minutes. Put the cowling back on and while doing so, overheat alarm came on. The cowling must be causing a short. Trailered it and took it home and accidentally left ignition on. When I got home, the overheat alarm was going off. Loosening the cowling latches turned it off. I took the cowling off and jiggled the wires but could not duplicate alarm. Put cowling back on, still couldn't get alarm to turn on.

So somewhere between the alarm and where the brown wire terminates is a short. Strange thing is, SLOW mode works without overheat alarm turning on. How do I track down the location of the short? Or could it be the CDI or diode that shorting it?

 

[Sam2001]

I downloaded an OMC service manual from self fixer.com for under $15.00 , it has very detailed instructions on how to trouble shoot everything about this motor , its probably the best investment i've ever made Manual "1990 Johnson Evinrude "ES" 60 thru 70 Service Manual, P/N 507873" I would email it but its over 44MB to big to send.