Instrument guages, again...1980 300SD

[gdemoss01]

I solved a previous problem of an inoperative tachometer by replacing a reference sender unit mounted on the flywheel. A major PITA, by the way due to difficulty accessing bolts, etc.

New guage problem: Temperature. Replaced themostat and sending unit with brand new. Also replaced guage (used but functional). No register on temp guage now. Engine is heating up. Auxilliary fan comes on after engine warming. Voltage to both sending unit and guage in 14 to 14.5 range when engine running.

Is this a sending unit problem? What does anyone thing? Spent most of the day yeasterday screwing with it. I'm tempted now to just get an aftermarket temp guage and hook it up. Any ideas?

[Matt L]

Originally Posted by gdemoss01

Voltage to both sending unit and guage in 14 to 14.5 range when engine running.

That would seem to point to the sending unit.

A couple of questions. How do you know the used replacement gauge is functional? What happens to the gauge if you ground the temperature sensor wire to the engine block? Have you measured the resistance between the temperature sensor and ground (both hot and cold)?

[gdemoss01]

Originally Posted by Craig

A couple of questions. How do you know the used replacement gauge is functional? What happens to the gauge if you ground the temperature sensor wire to the engine block? Have you measured the resistance between the temperature sensor and ground (both hot and cold)?

I know the replacement guage works in another car--it was working when I pulled the cluster.

The second part. I haven't measured resistance but will go do that momentarily. I'm not real literate on car elctrical stuff (I can wire a house though, lol). What resistence should I be looking for. This is ohms, I'm guessing, right? I'll check and let you know.

Originally Posted by gdemoss01

I haven't measured resistance but will go do that momentarily. I'm not real literate on car elctrical stuff (I can wire a house though, lol). What resistence should I be looking for. This is ohms, I'm guessing, right? I'll check and let you know.

I don't remember the actual value (in ohm), but I believe the resistance should be higher when cold and lower when hot. I would also expect the temperature gauge to jump to maximum if the sensor wire is grounded. That should help narrow down the problem.

Just to make sure, we are talking about the sensor on the drivers side of the head (near the glow plugs), not the sensor near the thermostat.