I can't get my mercruiser over-temp alarm to shut off

[rodgeml1]

Engine: Mercruiser 4.3LX, serial number 0F586331
Outdrive: Alpha one, Gen II

Scenario: After completing some work on the outdrive, I took the boat out for a test run. The boat started great, shifted great, and for the most part ran great (it was a little slow getting out of the hole and getting up on plane). After running for about an hour, We decided to stop and drift for a while. Upon starting back up to move again, I got an (obnoxious) alarm indicating the engine was overheating. Turns out the hose leading from the water pump popped loose right after it came through the transome. Engine cooled off quickly after that.

Problem: Though the engine temperature came down and was staying down while running, the alarm never completely stopped. It did slowly get softer and softer and changed from a solid tone to more of a chirping sound (kind of like a hen turkey chirp) but it never completely stopped. Additionally, the rev limiter (slow mode....whatever you want to call it) was engaged and the engine wouldn't get above 2800 RPMs. We were, however, able to start making our way back in. Several times when coming in, after running steadily for 30 mins or us, the engine would choke out and die, then wouldn't restart. If I'd wait 10 minutes or so, it was start right up and cruise for a while and choke out again.

Troubleshooting accomplished: As far as the engine choking out is concerned, I anticipate a new fuel filter will help with it. I've got one on order and will change them out as soon as it gets here. I pulled the wire off of the temperature sender and started the boat. The alarm still persisted though it was still muted and chattered. I removed the coolant temperature sending and tested it using the method of submerging the tip in water, heating it to specific temperatures and checking the resistivity. Below is what the manual says the resistivity should be at given temperatures, and what observed on my sender:

140 degrees - 121-147 ohms.....mine read at 156 ohms
194 degrees - 47-55 ohms....mine read at 72 ohms
212 degrees - 36-41 ohms....mine read at 55 ohms

Question: Is the discrepancy between the expected value and actual value enough to anticipate the temperature sender is bad? If not, what else can be causing the persistant alarm? Is there a code that needs to be reset to clear the fault?

Any help y'all can provide would be greatly appreciated.

 

[dubs283]

Re: I can't get my mercruiser over-temp alarm to shut off

check to see if any wires were melted when the engine overheated

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

check to see if any wires were melted when the engine overheated

Sorry, I meant to indclude that I had checked that before I pulled the temp sender. The insulation on the entire length of the wire was intact with no evidence of overheating or charring.

 

[wellcraft-classic210]

Re: I can't get my mercruiser over-temp alarm to shut off

Are you sure the sender is tied into your alarm circuit and that is the way it was wired originally?

A temp alarm normally has its own temp switch and the gauge normally has a variable resistance sending unit.

The alarm unit would normally need a full 12 volts and only its own resistance in the path to gnd to operate properly ( loudly). A variable resistance in the ckt would be expected to cause the alarm to be louder as resistance lowers / & quieter as the resitance raises.

You may also want to see if any relays in the ckt are sticking & do some voltage checks on the input to the alarm.

 

[Don S]

Re: I can't get my mercruiser over-temp alarm to shut off

Do you have the Thunderbolt IV or V ignition? From the parts breakdown, it appears you have a TB4.
Neither has a "Slow Mode" built into them.

How do you know the alarm was caused by Temperature? There is an over temp switch (Not the gauge sender). a low oil pressure switch (Not the oil gauge sender or fuel pump switch) and a low lube oil reservoir that can set off the alarm.
Any one of the 3 can cause the alarm.
To find out which one is causing the alarm, with the engine running, un-plug them one at a time until the alarm stops.

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

Do you have the Thunderbolt IV or V ignition? From the parts breakdown, it appears you have a TB4.
Neither has a "Slow Mode" built into them.

How do you know the alarm was caused by Temperature? There is an over temp switch (Not the gauge sender). a low oil pressure switch (Not the oil gauge sender or fuel pump switch) and a low lube oil reservoir that can set off the alarm.
Any one of the 3 can cause the alarm.
To find out which one is causing the alarm, with the engine running, un-plug them one at a time until the alarm stops.

I have no idea which ignition I have (I've only had the boat for a couple of weeks). I'll have to do some research to figure out which I have. Since the alarm started when the hose coming from the water pump to the engine popped off, I just assumed it was due to temperature. Also, while we were limping back in, the alarm kicked into full mode again and which coincided with the hose popping off again (putting on a hose clamp to fix that). I traced the wire from the oil pressure sender and found that it had been disconnected by the previous owner as had the wires coming out of the lube oil reservoir. So when I unplugged the temp sender, all three circuits should have been open.

I do have some water intrusion into my lube oil. I had just serviced the lube oil and the level in the reservior was right at the line. When I got back home, I noticed the oil was milky. When I opened the lid to the reservior, oil started oozing out, and there were air bubbles that gurgled up. I've taken a sample from the lower drain plug and have it set aside to see if it separates.

 

[Don S]

Re: I can't get my mercruiser over-temp alarm to shut off

Which of these distributors look yours? Specifically the modules on the outside of the housing #12 in the first picture, a and b in the second picture.

 

[Don S]

Re: I can't get my mercruiser over-temp alarm to shut off

I traced the wire from the oil pressure sender and found that it had been disconnected by the previous owner as had the wires coming out of the lube oil reservoir. So when I unplugged the temp sender, all three circuits should have been open.

There is a difference between senders and switches. Don't confuse them. The senders that operate the gauges, do not operate the alarms, switchs do that. Noting but on and off.
All of your alarm switches will have the same color wire going to it. Tan with a blue stripe.

When I opened the lid to the reservior, oil started oozing out, and there were air bubbles that gurgled up. I've taken a sample from the lower drain plug and have it set aside to see if it separates.

You have a leak in the drive somewhere, don't waste your time waiting for the oil to seperate, it won't. If it's milky, it's water.
Pull the drive and check the bellows for water, Drain the gear lube and pressure test the drive to find the leak.

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

Which of these distributors look yours? Specifically the modules on the outside of the housing #12 in the first picture, a and b in the second picture.

View attachment 141538 View attachment 141539

I don't know the answer off the top of my head. I'll check first thing when I get off of work and answer your question.

I will comply on pressure testing the drive. I know the O-ring for the cap on the upper is broken but I'm not sure if that would be sufficient to have that much water intrusion on a single trip.

 

[Don S]

Re: I can't get my mercruiser over-temp alarm to shut off

I know the O-ring for the cap on the upper is broken but I'm not sure if that would be sufficient to have that much water intrusion on a single trip.

:facepalm:

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

:facepalm:

It seems as though a man's desire to be out on the water can sometimes circumvent common sense. And it seems as though a man's common sense will never be mourned more than when said man's boat is at the bottom of the ocean.

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

Which of these distributors look yours? Specifically the modules on the outside of the housing #12 in the first picture, a and b in the second picture.

View attachment 141538 View attachment 141539

Ok, After looking at it at home, I do have a Thunderbolt IV ignition. It looks the most like the second picture except the tab from part b is rotated town instead of out the side. So this would mean there is no "slow mode" built in? Seems odd that it was so consistantly limited to 2800 rpms after the overheat if that is the case right?

 

[Don S]

Re: I can't get my mercruiser over-temp alarm to shut off

The picture on the right is the Thunderbolt V. Post a picture of your distributor if you will.

 

[riptorn]

Re: I can't get my mercruiser over-temp alarm to shut off

I think you got water in your oil from the oil cap. Bad design caps. Should flush the oil and change filter. You got a blowby. How old is the motor (rebuilt?)

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

The picture on the right is the Thunderbolt V. Post a picture of your distributor if you will.

Absolutely. I guess I misinterpreted the pictures you attached....but my igniter does say Thunderbolt IV.

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

I think you got water in your oil from the oil cap. Bad design caps. Should flush the oil and change filter.

I think you're probably right. I drained it tonight and will give her a new oil bath and hope I can get the alarm issue and reduced rpm issue resolved.

 

[riptorn]

Re: I can't get my mercruiser over-temp alarm to shut off

I think you're probably right. I drained it tonight and will give her a new oil bath and hope I can get the alarm issue and reduced rpm issue resolved.

Are your water temp and oil pressure gauges ok ? If ok disconnect alarm for now.

 

[riptorn]

Re: I can't get my mercruiser over-temp alarm to shut off

Are you sure the sender is tied into your alarm circuit and that is the way it was wired originally?

A temp alarm normally has its own temp switch and the gauge normally has a variable resistance sending unit.

The alarm unit would normally need a full 12 volts and only its own resistance in the path to gnd to operate properly ( loudly). A variable resistance in the ckt would be expected to cause the alarm to be louder as resistance lowers / & quieter as the resitance raises.

You may also want to see if any relays in the ckt are sticking & do some voltage checks on the input to the alarm.

I would check into what wellcraft-classic210 is pointing at. I didn't have an alarm on my old 78 Silverline so you may have dual sensor.

 

[achris]

Re: I can't get my mercruiser over-temp alarm to shut off

I know the O-ring for the cap on the upper is broken but I'm not sure if that would be sufficient to have that much water intrusion on a single trip.

Are you serious?!?!?!??? A leaking 'O' ring is enough to totally destroy a drive, and you're going out KNOWING you have a broken one.... My god man!!!

Now, the alarm circuit is incredibly simple. 12V goes from the ignition switch (in the ON position) to the alarm module (the siren under the dash), and has 1 Normally Open and 2 Normally Closed switches on the engine, one switch for water temperature (NO), one switch for oil pressure (NC, but open when oil pressure present) (these are NOT the sensors for the gauges), and one switch in the bottom of the leg oil monitor bottle (NC, but open with oil in the bottle). From these switches the current path is to ground.. If any one of these switches closes during normal operations, the alarm sounds. FYI, no relays or even fuses in this circuit...

If the alarm is sounding quietly after you had copious quantities of seawater hose down your engine, then I would suspect one (or more) of these switches has got wet (surprise, surprise!) and is now providing a resistive path to ground.... Wet electrics will also explain all your other symptoms.... Dry out the electrical system, clean all the switches and sensors....

You have a Thunderbolt IV system, but what I didn't see there is the ground wire from the distributor to the engine block... you should find a spare ground wire in the harness next to the distributor. Remove one of the module mounting screws and put the ground wire under that....

Chris......

 

[rodgeml1]

Re: I can't get my mercruiser over-temp alarm to shut off

Are your water temp and oil pressure gauges ok ? If ok disconnect alarm for now.

Yep....the gauges all show everything is ok. Disconnecting the alarm will obviously make it less annoying, but right now I'm using it as a possible indicator as to whether I've tracked down my problem. And I wont' be taking it out on the water until I can figure out why it won't go above 2800 rpms anyway so the alarm isn't a problem.