Alarm with RPM reduction

[Cuz]

I have twin counter rotating Yamaha TXRP 150's which are 1991 models. I have replaced both water pumps and the oil injection system checks out good and seems the sending units are working properly. The problem, which is intermittent, is that the alarm goes off on the starboard engine and reduces RPM. Seems to be no specific condition that triggers this, but it never seems to occur at idle or *****ing speed no matter how long it is run. Pulled the thermostats and tested them and they are opening around 140 deg. Read the post about checking the temp. switch on an 85 hp yammie. Could this possibly be the problem I'm having with this engine? If so, how can I test? Any advice would be greatly appreciated.

[Ken]

Cuz,
You are correct about the thermostats - they should be fully open at 140 degrees f. - should be approx 0.120 open.
You can test the thermo switches in the same manner. There is a thermo switch mounted in each head assy. It is just a bi-metallic strip housed in a small metal cup that bends when heated, and when it gets to approx 180 degrees, it will close a pair of contacts. This closure causes a ground signal to the CDI and the alarm immmediately sounds, and a few seconds later the motor will go into rpm reduction mode. You throttle back to neutral and let the motor cool off at idle and try to find out what the heck is wrong - that's how it is suppose to work.
I have personally experienced a faulty thermoswitch close at 130 degrees, giving me a false overheat signal.
You can pull the thermoswitch and test it in a pan of heated water with a thermometer. Use an ohm
meter to monitor the two leads for continuity (closure). The thermoswitch is behind a small round rubber plug that is mounted toward the top of each head assy. The plug has two wires coming out if it. Some are held in by a single bolt/tab, while others are just a "twist-and-lock". The rubber plug is about the size of a nickle, maybe a little bigger. When you pull the plug, the thermoswitch will come out with it. Leave the switch mounted to the plug when you water test.
Now, if your thermoswitch tests ok, then, indeed the head temp at that location is getting up to 180 degrees and you need to trouble shoot further the overheat condition. Hopefully, it will be just a bad thermositch, but they rarely go bad.
Good luck, and let us know what you find out.
Ken K [img]smile.gif[/img]

[Cuz]

Thanks for the great troubleshooting info Ken. I'll go through the steps as you describe and keep my fingers crossed that it is a thermo switch. Looking at my repair manual it appears that I may have to remove the head cover plate on this particular motor to get the switch out. This seems like a silly way to engineer something like this, any thoughts on it? It would be great if I don't have to do so.

Thanks,

[Ken]

Cuz,
Looking at the thermo switch for your motor in the online catalog above (Electrical Parts 2 - Item #26) - I can't tell how it is mounted in the head assy, or what you are going to have to remove to get to it - your manual should give a better drawing of it.
Good luck [img]smile.gif[/img] ,
Ken K

[Hamfisted]

Cuz, just a quarter turn will release them. Turn 'em 1/4 turn counterclockwise and pull them out. When you re-install turn 'em 1/4 clockwise to secure 'em. You don't need to remove any covers. Check for any signs of corrosion on 'em and replace if there is.


Mike

[Cuz]

Thanks Ken and Hamfisted for the info. I'll let you know how this turns out when I get to the bottom of the problem. Appreciate the help and advice.

[Cuz]

Hi folks,
Getting back to you to follow up on the problem I was having with the overheat alarm. I went throught all the diagnostic checks, as described for the t-switches, t-stat and PC valve. didn't really see any noticeable problems so I pulled the impeller again. It seemed to be installed backward, but I can't see how it would have simply turned the right direction when the engine was run. Also, found a shorted battery and replaced it. I really don't know if any of these cured the problem, but was able to run the boat pretty good on Saturday and didn't get the alarm. I'll keep my fingers crossed that one or the other of these cured the problem. On another note however, when almost back to the dock, I had diddiculty shifting my port motor back to neutral. The control lever moved, but the engine stayed in gear running. After some aggravation pulling the lever back and hitting reverse, then forward again and back I was able to get neutral. Maybe it was just acting up, but have any of you ever experienced this? '92 Yamaha 150's with dual Yamaha controls. Cables and connections all seem to operate normally. Sorry to be so long winded.

[Hamfisted]

Cuz, disconnect both the throttle and the gear shift cable back at the motor and check the cable adjustments. With the engine in exact neutral and the console control lever in exact neutral adjust the cable end so that it just drops over the shift pin without any interference. That the throttle cable end does the same thing, slides over the pin with no interference. Do both engines. Also check that your gear lube is in good shape in the lower unit. If the shift rods have never been changed they are still the original non-stainless steel type and prone to rotting in half. Look in the gap in the midcase at the shift rod and see if it looks like it's rusting out. You have to pull the powerheads to change them. After '93 they went to stainless rods. The replacement shift rods you buy from Yamaha now are all stainless.


Mike