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2004 Merc 5.7L Carb'd w/B2 drives alarm help please - I am new to the forum.

J

JimMcNeely

New member
I have a 2004 30' with twin Merc 5.7L Carb'd w/B2 drives.

Last spring I installed a custom alarm panel that shows which engine is alarming and also has alarms for the bilges. The input circuit for the engine alarms circuit draws about 35 milli amps when the alarm is sounding this is the same as the original Merc alarm buzzers.

I have been getting a intermittant alarm on one engine at about 3200 - 3300 RPM after running for three or four miles. If I come down to a slower speed or speed up I do not get the alarm.

My engine temp gauges remain steady at 175 +/- a few degrees.
My engine oil pressures runs between 40 to 55 depending on RPM.
My drive lube is full.

Engine temps measured with an IR gauge are virtually identical between the engines.

There is a single wire sensor on the thermostat housing. If I pull the wire the helm temp gauge falls to the stop.

I unplugged what I understand to be the overtemp sensor (two wires on the port side of the thermostat housing) the alarm did not re-occur. So I purchased a replacement overtemp sensor. With the new sensor I still get an alarm.

At my wifes urging I disconnected the alarm panel and reinstalled the original buzzers. I made sure the overtemp was reconnected and the alarm did not reoccur.

This got me looking at the sensors why would one system trip and the other not ?

My understanding was that the overtemp , oil pressure and drive lube are all on/off switches. But when I test the overtemp sensor it is a variable resistance 2400 ohms at 70F and about 400 ohms at 160F. This I do not understand.

The Merc circuit diagram shows the alarm which is connected to 12V when the key is on , the three sensors in parallel are potential paths to negative which can cause the alarm to sound when they complete the circuit.

So here is the point.
So why would one alarm system alarm sound and the other not when the control is simply a on/off circuit and the impedence of the two alarm systems is virtually identical ?

Why is the sensor a variable resistance and not on/off ?
Am I looking at the wrong sensor ?
Am I misunderstanding the circuit ?

I would like to disconnect and test each sensor but I am not sure about the overtemp sensor and I really do not know where the oil sensor for the alarm is at. Is the oil sensor for the alarm the single wire sensor on the Stbd side of the engine block below the exhaust manifold about center front to back ?

I am sorry for so many questions and I appreciate your help.

Jim
 
Jim, welcome to ME.com. We know each other from the BOC.

I'm familiar with Volvo's alarm system, but not the Merc.

As for your "extended" alarm panel, can you post a rough sketch of the wiring schematic as to how this ties into the OEM Merc stuff?

.
 
Test the TKS Coolant Temperature Switch
ON-ENGINE TEST
1. Allow engine and switch to cool.
2. When engine and switch reach 43º C ± 4º (110°F ± 8°), check continuity.
3. If there is continuity, replace switch.
4. Start engine and allow engine to reach 55ºC ± 4º (130°F ± 8°).
5. Disconnect coolant temperature switch connector and check for continuity again.
6. If there is no continuity, replace switch.

Based on this test proceedure (if you have the TKS system) it would appear that you got a temp sensor and not a "SWITCH".

It looks like the switch is suposed to be OPEN under 110 degree and at 130 degrees it should be closed.

Switch condition Temperature
Closes 54º ± 4º C (130º ± 8º F)
Opens 43º ± 4º C (110º ± 8º F)

Does not make a lot of sense to me...........if you are running a 160 degree thermostat and a running temp of ~ 170 the I would think the temp switch would "close" at 190............

Sending unit specs

Heat water and observe thermometer.
As temperature rises ohmmeter readings must be within the ranges specified for each temperature.
Water Temperature Ohms Reading
60 degrees C (140 degrees F) 121 - 147
90 degrees C (194 degrees F) 47 - 55
100 degrees C (212 degrees F) 36 - 41

Maybe someone who has physically handled these parts and has had troubleshooting experience with this will chime in.
I myself have not had this newer stuff in my hands yet.
 
The engines have Carbs with the Thunderbolt ignition system.

I don't think I have TKS which I think is Turn Key Start.

Here is the wiring for my alarm panel I replaced the original buzzers with. The wires that fed the buzzers , feed small relays on my alarm panel. As I mentioned the current draw on the alarm circuit with the original buzzers and my panel were virtually identical, within a few milliamps.


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I designed the circuit and the panel. I built the circuit board and the panel was made by Mobile Marine Electrical Systems (MMES) in MA.

Here is the panel at the helm.

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Well I looked at the thunderbolt V manual and they dont show the over temp switch and I may be missing your year specific manual........

either way a switch is a switch, on or off

a sender varies


what you posted has nothing for me...........so I can not comment on it.......
 
I am sorry maybe I started with too much stuff. I have a 2004 Merc , 5.7L w/ Carb & Thunderbolt 5 , Engine S# OM688355.

The big question at this point is where are the over temp and oil pressure sensors for the Merc alarm? Maybe what I was sold as the overtemp was incorrect.

My troubleshooting and voltage readings would indicate a switch in the circuit and not a variable resistance. The exception being if there is another Merc device between the engine sensors and the helm which is not shown in the wiring diagram I found.
 
Jim, I'm fairly certain that the engine coolant over-temp will be a switch... On/Off type of thing.... and likewise with the oil pressure.

You may also see an over-temp switch at each exhaust elbow.... yes/no?
 
The answer to find out wich is which is to turn the key to the run position.

go to the sender or what you suspect is the sender, very quickly touch the center post (with the sending wire connected) and the other end of a jumber or paper clip go to ground.

if it is a sending unit will peg the guage. (oil pressure or water temp)

if it is a switch it will set off the alarm......(Oil pressure switch or over temp switch)
 
I have identified the senders for the helm gauges.

There are no sensors on the exhaust elbows.

I will do the test suggested with some of the other sensors.
 
I am just trying to be sure I am looking at the correct sensors. Since I purchased the one temp sensor and it does not function as expected I am trying to really understand.

The drive lube is obvious. But I am unsure which and where the alarm sensors are for overtemp and oil pressure.

Once I have the alarm sensors identified then I can troubleshoot.
 
I have identified the sensors and operation of the Mercury alarm system on our boat.

The Merc alarm has power from the ignition switch. The path to negative is closed by the alarm circuits. When the circuit is completed the alarm sounds.

We have three alarm circuits.

Drive lube level - Part of the drive lube resevoir. A two wire sensor. When the lube is not empty the switch circuit is OPEN. When the lube is low the circuit is CLOSED creating a connection to the negative for the alarm.

Engine Oil Pressure - A single wire sensor located slightly to port of the distributor. When there is no oil pressure the switch in the sensor is CLOSED creating a connection to the negative for the alarm.

Engine coolant overtemp - This is a two wire sensor on the Port side of the thermostat housing. This is a variable resistance from 2500 to 3000 ohms at 70 F. It's resistance is about 400 ohms at 160 F. The output of this sensor goes to the Electronic Control Module (ECM) of the engine mounted on the Port exhaust riser. This sensor is used to monitor engine temp for engine operation. The ECM has an output also wired to the alarm circuit negative path, in parallel with the other sensors. If the engine goes overtemp the ECM alarm output provide a negative path for the alarm circuit to sound.
 
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