temp guage/sender problem

[jam1ej20]

I have a 1995 4.3 mercruiser and the temp gauge isn't working. Here's what i've done, any advice is appreciated.

-Gauge tests good per merc manual.

-With key in run position, gauge bounces slightly when lead is placed on and off of temp sender (indicating connectivity of lead wire)

-When temp sender and switch leads were switched, low tone could be heard from warning bell. After the problem was realized, the leads were corrected and the gauge worked for the first time for about 30mins. The gauge then dropped back to cold and has not worked since. I assume that the sender is (or at least WAS) working since the warning bell emitted the low tone while connected to the sender instead of the switch.

I have not actually tested the sender yet, and just wanted to sound off here before I go to the trouble of running that test to see if anyone had any insight to other potential problem areas.

Thanks!

 

[Chris1956]

Re: temp guage/sender problem

The warning system and gauge use different senders for both oil pressure and temp. Are you using the correct sender?

The temp sender for the warning warning system will read open until the motor overheats. it will then ground.

 

[jam1ej20]

Re: temp guage/sender problem

The thermostat housing has two different temperature "fixtures." These are now properly connected, and always were except for a brief period when trying to diagnose the temp gauge problem. I understand the sender/switch principle. This is why is only a LOW tone was heard while connected to the sender. The switch is either ON or OFF while the sender is a gradual increase. Therefore the low tone indicated that some signal was received, however not a full ON response such as would be received from a closed and overheated switch. Right?

 

[Bondo]

Re: temp guage/sender problem

Ayuh,...

Go back to the test with the ket "On"....
If the gauge Pegs when grounding the wire, Replace the sender....

Make sure the T-Stat Housing is properly grounded too...

 

[jam1ej20]

Re: temp guage/sender problem

Ayuh,...

Go back to the test with the ket "On"....
If the gauge Pegs when grounding the wire, Replace the sender....

Make sure the T-Stat Housing is properly grounded too...

Gauge test good per merc manual. It does, indeed, peg when grounded. I was leaning towards the sender, but wasn't sure since it had worked for a short time.

How is the t-stat housing grounded? The metal inserts in the gasket between the housing and the block?

 

[sdoomaha]

Re: temp guage/sender problem

My '94 4.3 would not "heat up" this Spring after getting it out of storage. Would climb very slightly, but never reach operating temp. Tested the gauge it was good. Tested sending unit with 190 degree water and a jumper for ground....gauge read perfectly. Thought maybe the thermostat was staying open all the time. Wrong.....it was actually staying close all of the time. Note where your temp sending unit is....it is on the cool side of the thermostat. If the thermo is frozen closed, the heated "coolant" never reaches it. The slight temp increase I was seeing was just from the thermostat housing the sender was located in heating up. Kind of scary thinking if I had ran it hard could have easily overheated it because of the design. Seems to me the sending unit shoud be on the "hot side" of the thermostat. Anyway....replaced the thermostat and been running and reading an appropriate temp ever since.

Not sure if this is your problem....but thermostats are pretty cheap...and if it is...would hate to see you overheat your boat thinking you are actually running on the cool side. Not hard to tear apart...get 2 gaskets if you do it...one to mount the housing back to the bock and one for the housing cover.

Good luck.

 

[jam1ej20]

Re: temp guage/sender problem

The thermostat is new...in fact, most of the motor is new. This is a rebuild of an improperly winterized motor. I really think the thermostat is working properly and have seen the gauge function and show the "expected" output for some period of time. It just will not read consistently. The motor isn't overheating and is running great. It also has the warning buzzer temp switch on the other side of the thermostat housing for redundancy.

 

[Chris1956]

Re: temp guage/sender problem

Yes, the housing grounds using the two brass gromets in the gaskets. The bolt may also touch the housing as well.

 

[Rat Capri]

Re: temp guage/sender problem

do a continuity check between the center post of sender and the case

 

[jam1ej20]

Re: temp guage/sender problem

do a continuity check between the center post of sender and the case

That reading will be based on temperature, no? And will require various temperature readings corresponding to set, known, ohms values?

 

[Rat Capri]

Re: temp guage/sender problem

no, just to verfiy the 2 circuits are separated

 

[jam1ej20]

Re: temp guage/sender problem

no, just to verfiy the 2 circuits are separated

To do a continuity check between the ceter post of the sender and the case of the same sender? This is the same test as outlined in merc manual 18, page 4D-7.

140 F = 121-147ohms
194 F = 47-55ohms
212 F = 36-41ohms

Maybe I do not understand you. What would this test tell me at ambient temperature? I just wanted to avoid heating up a water bath and all the hassle if someone had a similar experience with something like this.

 

[sdoomaha]

Re: temp guage/sender problem

Understand....please be aware your warning buzzer sending unit is also on the cool side of the thermostat, so while redundant...it is redundant except for a thermostat frozen shut since once again the hot circulation going through the block will never reach it.

Hope the great folks here help you find your problem.....take care.

 

[jam1ej20]

Re: temp guage/sender problem

UPDATE: I tested the temp sender and it also tests "good" per the merc. manual. I have a suspicion that this is a grounding problem now...any other ideas?

Will test grounding tomorrow and update.

Thanks!

 

[jam1ej20]

Re: temp guage/sender problem

RESOLUTION: Bond-o wins the prize. The problem was, in fact, a grounding issue.

My synopsis is that the gasket sealant I used with the t-stat housing gasket somehow interfered with the metal grommets' ability to ground the housing properly, thereby inhibiting the sender from grounding. Regardless, the issue is now solved. Thanks for all the advice!

 

[Bondo]

Re: temp guage/sender problem

RESOLUTION: Bond-o wins the prize. The problem was, in fact, a grounding issue.

Alllll Righty,....
Could you deliver it to the 6th house on the right, at the mouth of the Chaumont River,..??