Trouble Shooting "Low" Operating Temperature

[Smalljobs]

There are some excellent threads on this and they all helped me. I think it might be helpful to others if I post my own situation. The machine now has 100 hrs and the gage always read low on the first mark off the pin. I removed the thermostat and found it to be conventional and unlike others that have the well documented vent/bypass problem. The OD fits into a shallow counterbore inside the lower housing while the upper housing clamped it in securely. The factory stat was marked 75C. I bought a new Stant PN 45358 rated at 82C and put them both in a sauce pan and checked with a digital thermometer. All worked as expected.

Based on threads by others, I measure the resistance on the sensor and found it to be .79 ohm from the lead to the case. Resistance from case to block was near zero.

Then I removed the radiator cap and put cardboard in front of the radiator.

When the radiator coolant reached 52C/125F, resistance was .04 ohms (tractor off, key off) but the gage was still on that first mark.

At a measured 75C/167F in the top of the radiator, sensor resistance was .05 but the gage was still at that first mark. I also removed the lead from the sensor, found it to be bright and clean and cleaned it again with no improvement.

At this point, I'm not sure if I have a sensor or gage problem and am working my dealer to solve it. I'll post the solution when we solve it.

 

[greg_g]

Have you replaced the OE pressure cap? Because a common mistake is simply to buy one that fits - rather than buy one that matches the correct pressure release point. Most of the time it's probably a 15 pounder. The Jinma 200 Series - at least the ones I've either owned or worked on - use a VERY low pressure cap. The metric equivalent is about 4.3 psi. There are very VERY few American aftermarket caps (that fit) in this range, so about the best one can do is a 7 pounder.

If you're still using the OE cap, this probably won't help. But if you've gone aftermarket - your test results may have been skewed by the cap you're using.

//greg//

 

[Smalljobs]

Greg - tks. The cap is OE. My coolant temperature readings were with the cap off and a digital meat thermometer immersed in the top of the radiator.

 

[SPYDERLK]

Have you replaced the OE pressure cap? Because a common mistake is simply to buy one that fits - rather than buy one that matches the correct pressure release point. Most of the time it's probably a 15 pounder. The Jinma 200 Series - at least the ones I've either owned or worked on - use a VERY low pressure cap. The metric equivalent is about 4.3 psi. There are very VERY few American aftermarket caps (that fit) in this range, so about the best one can do is a 7 pounder.

If you're still using the OE cap, this probably won't help. But if you've gone aftermarket - your test results may have been skewed by the cap you're using.

//greg//

The cap pressure wont be a factor with coolant temps so far below 100C. However, the use of a low pressure cap on oem suggests that they run cool normally, and so dont need much pressure to prevent boiling. Could be the system cant stand much pressure - so caution is good.
larry

 

[greg_g]

Greg - tks. The cap is OE. My coolant temperature readings were with the cap off and a digital meat thermometer immersed in the top of the radiator.

The sending unit is a thermistor. As coolant temps rise and fall, resistance falls and rises. A cold engine will be cause max resistance, a hot engine will cause minimum. So to test whether your problem is the sender or the gauge, simulate a hot engine. That is, no resistance.

Remove the wire from the sending unit, turn the keyswitch to the ON position. Watch the gauge, and briefly ground the wire to the block. If the needle pegs, the meter's good - replace the sending unit. If the needle does the same old thing, replace the gauge.

The cap pressure wont be a factor with coolant temps so far below 100C.

Disagree - the wrong pressure cap can skew gauge indications because it causes leakage elsewhere in the system. But since OP is still using an OE cap, the point is moot.

//greg//

 

[Raw Dodge]

Greg - tks. The cap is OE. My coolant temperature readings were with the cap off and a digital meat thermometer immersed in the top of the radiator.

Temps in the top of the radiator arent always the same as engine temps.Even with rad blocked,you will get cooler than actual readings in the top of rad.Esp true if the t stat isnt fully opening.

 

[SPYDERLK]

Disagree - [[[the wrong pressure cap can skew gauge indications because it causes leakage elsewhere in the system.]]] But since OP is still using an OE cap, the point is moot.

//greg//

What do you mean? Leakage from the system or within the system?
larry

 

[Smalljobs]

Greg - Tks, will try that tomorrow and post. Bolton Power recommended the same approach.

 

[greg_g]

What do you mean? Leakage from the system or within the system?
larry

I don't know the technical reason that manufacturers increased cooling system pressures over the years. But 4 PSI systems were common in years past. See RADIATOR 4 pound Pressure Cap - Wayne WR-5 New Old Stock #2. And we all know that much of the engineering in these Chinese tractors dates back many years as well.

So on this Jinma, we're talking quite low coolant system pressure - about 4.3 PSI. The pressure that pushes against the relief spring in the cap is the same as that which pushes against all the seams, hoses, fasteners, seals, and gaskets. I've seen a 15 PSI pressure cap on a 200 Series Jinma cause hot coolant weep profusely; underneath hose clamps, through radiator tank crimps, past sending unit threads. Stop them, and it starts pushing on the pump seal.

As I'm sure you're well aware, cooling system leaks influence operating temperature - and that operating temperature is reflected by the temperature gauge. Hence, a 15 PSI cap on a 200 Series Jinma can skew temp gauge readings. But I only asked the OP the cap question, as one step in the troubleshooting process. Since he's still using the OE cap, this doesn't apply.

SMALLJOBS - Having said all that, the OE cap springs aren't very robust. As part of your repair effort, I personally recommend a low pressure aftermarket cap. Don't hold me to this number - I don't have those Jinmas anymore. But I'm thinkin' I used a Stant 10282 on them. If you hunt long enough, you may eventually find an old 4 PSI cap to fit. Short of that I found the 7 PSI Stant to stop the weeping caused by it's stouter 15 PSI cousin.

//greg//

 

[Smalljobs]

Today I turned on the key, had my wife watch the gage while I ran a jumper from the sensor to the block. It pegged the gage at the other end. Will replace sensor and report back.