Temp/Pressure alarm not working

[bleu30]

I'm trying to resolve an issue with my temp/pressure buzzer alarm on a 1978 Catalina 30 A4.

After some hours of amateur sleuthing I still have no resolution, but may have enough clues for someone far smarter than I to give me a clue. Admittedly, electrical troubleshooting is not my strong suit.

The alarm unit behind the instrument panel is new and confirmed to work both when direct connect to battery and also when connected to its wires behind the panel in the troubleshooting test configuration described below. The oil pressure sending unit appears to work fine. Oil pressure increases after engine start and grounding the unit with key in the ignition position pegs the gauge to the right.

I believe the temp sensor and gauge also work although I don't specifically remember anymore whether grounding directly to battery pegged the gauge to right or not. But I confirmed that both senders have good ground to the engine. (side questions...why would grounding a sender unit directly to the battery negative post be any different than the unit getting its ground from the engine itself? Is the elec connector tip insulated from the body of the sender unit?)

The only test condition under which I have been able to get the buzzer to alarm when installed behind the panel is when the temp sender is grounded directly to the battery and key in ignition position or engine started, but in this configuration the buzzer remains on, regardless of oil pressure or water temp (although I didn't run the engine long enough to significantly increase the temp). I'm hoping that this last test result provides the clue to resolution, but if so, it has escaped me so far. Would that simply be an indication that the temp sender is bad? I'm trying to avoid random swapping of parts.

Much appreciation for any guidance on this.

Thanks
Eric

[joe_db]

I'm trying to resolve an issue with my temp/pressure buzzer alarm on a 1978 Catalina 30 A4.

After some hours of amateur sleuthing I still have no resolution, but may have enough clues for someone far smarter than I to give me a clue. Admittedly, electrical troubleshooting is not my strong suit.

The alarm unit behind the instrument panel is new and confirmed to work both when direct connect to battery and also when connected to its wires behind the panel in the troubleshooting test configuration described below. The oil pressure sending unit appears to work fine. Oil pressure increases after engine start and grounding the unit with key in the ignition position pegs the gauge to the right.

I believe the temp sensor and gauge also work although I don't specifically remember anymore whether grounding directly to battery pegged the gauge to right or not. But I confirmed that both senders have good ground to the engine. (side questions...why would grounding a sender unit directly to the battery negative post be any different than the unit getting its ground from the engine itself? Is the elec connector tip insulated from the body of the sender unit?)

The only test condition under which I have been able to get the buzzer to alarm when installed behind the panel is when the temp sender is grounded directly to the battery and key in ignition position or engine started, but in this configuration the buzzer remains on, regardless of oil pressure or water temp (although I didn't run the engine long enough to significantly increase the temp). I'm hoping that this last test result provides the clue to resolution, but if so, it has escaped me so far. Would that simply be an indication that the temp sender is bad? I'm trying to avoid random swapping of parts.

Much appreciation for any guidance on this.

Thanks
Eric

First off, the gauge senders and alarm sensors are not the same things. Pegging the gauges is a crude test, but hard on the gauges. Testing them with a meter and testing the gauges with a resistor or two is way better.
The temp and oil pressure alarms fitted to most engines are one wire devices that ground the connection when triggered. They are not variable resistors. The oil pressure sensor is easy, it should be 0 ohms to ground with the engine off and infinity when the engine is running. The temp sensor is a bit harder, it should be infinity until it gets too hot, then 0. That isn't east to test in place, you would need to take it off the engine and boil it.
You really need to trace all these wires out and diagram the system. The alarm wiring and gauge wiring are not interconnected or shouldn't be.

[bleu30]

Thanks Joe. You pointed me in the right direction. Further troubleshooting today leads me to conclude that both the pressure switch and temp switch are bad.

[bleu30]

What is the recommended A4 water temperature at which you would want to alarm? I see temp switch options that range in the temperature they would close. 200, 225, 235, etc.

Thanks
Eric

[edwardc]

What is the recommended A4 water temperature at which you would want to alarm? I see temp switch options that range in the temperature they would close. 200, 225, 235, etc.

Thanks
Eric

Not sure what is recommended, but mine is set at 200 and is located on a tee at the manifold coolant exit.

[joe_db]

What is the recommended A4 water temperature at which you would want to alarm? I see temp switch options that range in the temperature they would close. 200, 225, 235, etc.

Thanks
Eric

200 would be my choice.