Merc Audible alarm issues and no temp reading

[Rake722]

a couple of days ago I started my engine (1978 470) and got a panic moment when the audible low oil / high temp alarm didn’t stop after starting the engine. A quick glance at the oil pressure gauge received the panic as it showed 40psi pressure.

i then letvtje engine heat up - alarm still going - to find that the coolant temp gauge wasn’t reading at all.

i guess the 2 issues could be related ?

i tested continuity from the tan water temp wire to the gauge - tested good. I see 12v between love purple and ground at the gauge.

i tested resistance between the 2 terminals on the sender which went from 1.1k ohms down to 700 as the motor heated up.

i guess the wiring gauge and sender are ok ?

that leaves the oil pressure switch connected to the oil pressure sender.

am I on the right lines here ?

many thanks

ron

 

[wrench 3]

The alarm system on that should be a basic add on one. There is one wire running from the horn assembly to the engine and attaches to the nearest sender. Usually the temp switch. The wire is usually tan with a blue stripe (however I can't find any information on it before 1980). From there another wire jumps to tho other switch. The horn sounds when one of the switches grounds the wire. To test remove the wire from one sender at a time and see if the alarm is silenced. If it still sounds with both senders disconnected then check for a short to ground in the wiring.
I'm curios, you should have a single terminal temp sender for the gauge. Is the second terminal a ground for the sender or a switch terminal for the alarm?

 

[achris]

Alarm and gauge use different senders. The temp gauge not working has NO relationship with the alarm... You have 2 faults to deal with.

Chris.....

 

[Rake722]

Thanks chaps.

Temp sender and oil sender are of course seperate but there is a wire connecting them.
If I disconnect the temp sender alarm still rings but if I disconnect the oil pressure switch alarm stops.

Although not impossible it would be highly unlikely for both senders to fail toghether which suggests a commonality.

I amy be wrong but thats how it looks

 

[wrench 3]

I'm still curios as to how you came by a two terminal temp sender.

 

[Rake722]

I changed gauges and this was sold to me by the vendor who thought it needed 2 terminals for the alarm kit.

 

[achris]

Nope. Sender is a variable resistance (33-240 ohms) that the gauge (actually a voltmeter with a different scale) displays as a voltage. Alarm is a switch set at a particular temperature to close, grounding the minus side of the 'siren', completing the circuit and making the siren sound..... 2 terminal senders are usually digital, used with an ECU.

Get a Merc gauge sender and start again...

Chris........

 

[Rake722]

Thanks chris - seemed to work just fine up to now.
i checked faria and the sender is a 2 station sender for 2 gauges according to their site.

ill get a merc sender, I have one in the old log manifold. I also have a matching detmar vintage gauge

 

[peterzabriskie]

Hello Rake722, I have been reading your posts on your 470 rebuild with interest. I have the opportunity to get into boating in a Regal Sebring 1988 running the same, I think, engine. Curious minds want to know what has been the time and cost (ballpark #) you have into this effort, and would you do it again?

I am retiring and moving into boating (home-built-ish) from flying (experimental-home built). Hoping to live longer on the water than in the air! I also play bass guitar and drive a 1997 Jeep ZJ with 353000 miles.