Running hot? Or gauge issue?

zachtyler · Aug 22, 2017

Just upgraded my 89 Laredo to an open cooling system. New radiator, cap (16psi), fan clutch, thermostat, hoses, and coolant bottle. Operating temp appears to be 215 according to the dash gauge. On the highway it creeps to about 220-225. My jeep always gets much warmer when going up long grades, or any incline where I really need to push it. However, I have a Renix engine monitor that reads the computer data. The water temp (different sender than dash gauge) says around 195. Am I overheating? I inspected the temp sending unit, the wire looks to be in not-so-good shape. Could this cause an inaccurate gauge reading? Will work on either repairing or finding a junkyard replacement to splice in.

lawsoncl · Aug 22, 2017

A poor connection might change the resistance and hence the reading. The dash guages on the Renix era aren't terribly accurate to begin with.

8Mud · Aug 22, 2017

The lower the resistance in the sender, the higher the reading temp. Approximately 1600 ohms for 240F, 300 ohms for 180F and 110 ohms for 240F.

Remember also the coolant comes into the block cooler than when it leaves the block. The temp isn't the same by the inlet, the middle and the exit.

My temp sender/gauge reads around 20F low.

A nice trick you may want to use, most ohm meters either come with a thermo couple or you can buy one extra. *Shiny metal* and a good hard contact to the themro couple will give you a good temp reading, better than an IR gun. Especially useful are copper or brass fittings, they transfer heat really well (and quickly). You can even use a dab of computer heat transfer paste, for a better reading, but it really isn't necessary, you mostly need a good hard pressure contact and a clean surface.

Just a side note it takes a few or half a dozen heat and cool down cycles to get all the air out of the system, even with an open cooling system.

ehall · Aug 22, 2017

There are multiple ground wires involved with the temperature gauge, not just the sender ground but also a ground circuit on the gauge itself (same for oil pressure). Might want to clean the printed circuits and contacts on the back of the gauge cluster, and check resistance on the cluster ground. Mine has 6 ohm between the connector and the fender wall, something bad in my dash wiring, temperature and oil pressure gauges are both high.

Ecomike · Aug 23, 2017

I would not trust either Renix Temp sensor reading to be accurate.

I use a hand held, $20 IR InfraRed temp tester to get local spot temps of every location and part under the hood to verify how far off my Renix dash gauge and CTS readings are. It gives me Radiator inlect, outlet, T-stat housing, block and head temps at multiple locations and even the oil filter/oil temps in seconds.

Then I adjust the gauge temp in my head with a correction factor. One of mine reads 110 F at 200 F, the other reads about 220 F at 200 F

Ecomike · Aug 23, 2017

Correcting what I think is a typo in your post?

"Approximately 1600 ohms for 140F, 300 ohms for 180F and 110 ohms for 240F."

8Mud said:
The lower the resistance in the sender, the higher the reading temp. Approximately 1600 ohms for 240F, 300 ohms for 180F and 110 ohms for 240F.

Remember also the coolant comes into the block cooler than when it leaves the block. The temp isn't the same by the inlet, the middle and the exit.

My temp sender/gauge reads around 20F low.

A nice trick you may want to use, most ohm meters either come with a thermo couple or you can buy one extra. *Shiny metal* and a good hard contact to the themro couple will give you a good temp reading, better than an IR gun. Especially useful are copper or brass fittings, they transfer heat really well (and quickly). You can even use a dab of computer heat transfer paste, for a better reading, but it really isn't necessary, you mostly need a good hard pressure contact and a clean surface.

Just a side note it takes a few or half a dozen heat and cool down cycles to get all the air out of the system, even with an open cooling system.

Running hot? Or gauge issue?

zachtyler

NAXJA Forum User

lawsoncl

NAXJA Forum User

8Mud

NAXJA Forum User

ehall

NAXJA Member

Ecomike

NAXJA# 2091

Ecomike

NAXJA# 2091

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