On Oct 8, 7:58 pm, Scott Derrick wrote:
Dan,

I thought about a bad ground but ruled it out because the starter turns
so easily.

I will check the main ground straps.

The oil, temp and cht probes are single wire so they are relying on the
engine grand strap.

thanks,

Scott

wrote:
On Oct 7, 7:50 pm, Scott Derrick wrote:
I'm driving a 1986 T210R.

400 hours ago we had a reman engine installed and ever since the CHT and
oil temperature gauges(original) haver read higher than they used to.

Over the last couple months the temps have been climbing until now the
oil is above redline and the CHT near redline in cruise. We have an
engine monitor with all EGT/CHT probes and it says the CHT's are fine,
hottest(#1) is 370, lowest 315. We don;t have a secondary oil temp
gauge but the oil pressure is running 58-60 steady as a rock no matter
what the oil temp gauge reads.

The high temps displayed on the old gauges will change if the electrical
load is changed. Higher the load, higher the temps. With everything off
but the master switch, the displayed temps are close to normal but still
a tad high. I'd say the oil temp varies 50 degrees between no load and
say a 60 amp load. I tested with a digital volt meter and the voltage
stays in a range of 27.6 to 28 vdc no matter what the load is.

There are 2 alternators, a 65 amp and a 90 amp, both functional, temp
display does not change if running on both or either one. With both off
the temps drop even more, but the voltage also drops to 24.5-25 volts.
Not sure if the additional drop in temp display is because the voltage
is so low?

Anybody have a clue as to what is going on? We are stumped as is our A&P.

thanks, Scott



There's a bad grounding connection between the engine and
firewall. When the alternator is at work, its return flow is through a
heavy wire from the alternator case to the firewall. An additional
ground strap is connected between the crankcase and engine mount, and
the mount should be bare metal where it touches the firewall and bolt
heads or nuts. The alternator case is mounted directly to the engine
and this is an additional path.
If there's more than the tiniest amount of resistance between
the alternator/engine case and airframe, there will be a voltage
difference between them and some of the flow will seek out a path
through the temperature or oil pressure sender grounds and through
their circuits, increasing their flow and giving erroneously high
readings. Some of those gauges have their own ground wire to the
engine case to minimize this, but a really bad ground will soon
overwhelm that small wire anyway.
So see if those oil temp and CHT ground wires are in place, and
check and clean both the ground strap and alternator ground cable
connections. I bet that when the engine was replaced, the connections
were just bolted back up without cleaning them, and the small
instrument grounds might have been left off altogether.

Dan

We had this same problem in a 172, and it was poor
grounding. The starter still turned well. It went away when we
installed a separate ground wire from a bolt near the sender on the
case to the back of the instrument itself. Got to eliminate any
differential, see. We twigged to the solution when Cessna sent us a
retrofit kit for another 172 to replace the mechanical temp gauge, and
this kit had the ground wire and warnings to make sure it got
installed.

Dan