Alarm buzzes, temp gauge ok
 

A friend has a 454efi, the engine block temp sensor is causing ECM to activate alarm. The temp gauge/sensor is reading 140 at time of occurance. Happens when he gets up on plain after about 30 seconds. He jumped out block sensor and it stopped the buzzing from occuring. Gauge sensor in mounting on t-stat housing, engine block temp sensor is mounted below and screws into the block.

Please help, thanx in advance....

This doesn't sound like a stock
Mercruiser setup, both senders are
in the t/stat housing on stock EFI setups,
From the sounds of it, the temp gauge
sender is mounted in the water pump
bypass fitting port on the front of the intake,
correct ?
Has the recirculating water pump been
removed, and replaced with a crossover ?

jt

I am the guy that "Spent" is referring to in the orig post.
'99 7.4 EFI Merc BIII - completly stock setup
Here is my problem, happens since I put in this season:
I can idle around all day, no problem. I can even get the boat just planed out and I'm still OK. As soon as I get up in speed (or even accelerating hard) the idiot buzzer goes on, and will stay on until I turn the ignition off and restart. Upon restarting, as soon as I give it some reasonable throttle (not WOT) it starts buzzing again, always within 1-2 minutes of restarting. When drive is disengaged and I rev the motor the buzzer does not go on, only when drive is engaged does this happen.

as formulafastech said it can be one of 3 things:

-Drive oiler is full. Sensor operates open when full and shorts to ground when empty. I disconnected this sensor to verify it was not the problem.

-oil pressure. Oil Pressure has 2 senders, one for the gauge (resistive) and one for the buzzer (open-closed). As with Oiler, when you have pressure the sensor opens and shorts to ground when you have no pressure. I disconnected this sensor to verify it was not the problem.

one sensor left
-temp sensor. There are two temp senders, the one for the gauge is a one wire resistive type and screws into the starboard side of the t-stat housing (small manifold with 4 hoses). The gauge appears to be reading correctly, with max temp never going above 140 F (lake Michigan is 60 F so cooler running temperatures are normal). Engine feels like it is running at this temp when I touch the block. The second temp sender (labeled Engine Coolant Temp sensor "ECT" in book) is a 2 wire sensor, resistive type that screws into the block from the front. It is just below (about 1") the t-stat manifold and is the only sensor in the buzzer circuit that is resistive (as opposed to open-closed).

All 3 buzzer senders plug into J1 (all on separate pins) on the ECM, which controls the buzzer.

After reading thru my Mercruiser Service Manual (#23) I saw the troubleshooting section for the temp guage sender, with the resistance vs temp charts you need to verify the sender is working correctly. Merc does not give the same table for the ECT sender. I do have some approximate values of the ECT:

80 deg F - 2200 ohm
140 deg F - 470 ohm

Since the ECT is resistive, you cannot unplug / short it like the oil pressure and oiler senders to bypass it for troubleshooting. I did, however take a potentiemeter and try to simulate the ECT using the above resistances as a baseline.

2200 ohm - same problem as original
500 ohm - same problem as original
5000 ohm - problem went away, drove WOT for a good 30 minutes without buzzer turning on.

Does this sound like the ECT is bad or is there a deeper problem causing this to happen?

thanks
dave

Canyonero,
I easy test would be to borrow a EFT sender from another EFI set up and see what happens.

Everybody I know nearby is running carb's

It seems it would be the sensor or the ECU. I would replace the sensor first. Is this a twinn setup? If so switch one from the other engine. Is the engine running OK? lean or Rich? Just curious because the sensor reports the water temp to the ECU and controls the fuel flow.

Did you unplug all the sensors on the J1 plug circuit and it still sent an alarm?

Spent/Canyonero,

I've got a scan tool. PM me and we can set something up.

Regards,

Mark S.

I will double check the temp next time I am out. I have a portable thermocouple I can hook up to verify the correct temp.

Raypanic - I disconnected both oil pres and oiler senders and the buzzer turned on. I then used the 5K bypass resistor for the ECT and the buzzer did not sound anymore

Did you disconnect the harness to the ECM and check for bent pins? Check you wiring to the ECM, should be the brown wires for the discrete circuit. Sounds simple and stupid, but has happened to many.

I probably can get a scan tool also if you need it. I'm in Antioch.

Did you disconnect the harness to the ECM and check for bent pins? Check you wiring to the ECM, should be the brown wires for the discrete circuit. Sounds simple and stupid, but has happened to many.

I probably can get a scan tool also if you need it. I'm in Antioch.