K-Series high coolant temperature gauge reading - earth issue

[IanJ]

Following a gearbox change on my 1.6 K-Series, I was seeing much higher readings than usual on the coolant temperature gauge.

From starting the engine the temperature gauge would climb to around 90C, stay there for a while and then head on up to around 115-120C at which point the fan would cut in and bring the temperature down to about 100C. Even with a constant flow of air at motorway speeds the temperature wouldn't drop much below 95C.

I reviewed lots of useful threads which boiled down to it likely being an airlock, thermostat, sender or gauge issue. This in particular was a very useful thread: https://www.caterhamlotus7.club/forum/techtalk/coolant-temperature-question and I can confirm that gauge 71229 is indeed a suitable replacement for 71164.

However, the one very interesting "feature" I noted (which nobody else seems to have reported) is that the gauge would read around 10-20C higher with the engine running than with the engine stopped. I still don't have a good explanation as to how that might happen so answers on a postcard please!

Anyway, to cut a long story short (and after some very astute advice from Rob Clay) the issue was traced to a missing earth connection between the engine mount and chassis on the right hand side of the engine. I'm kind of surprised that was the only symptom, but once the earth strap was reconnected readings returned to normal throughout the temperature cycle, so big sigh of relief!

Other useful things I found out:

• It's quite normal for my IR thermometer to read 103-107C on the water rail of a K-Series next to the temperature sender when the fan comes on, and 93-97C when the fan switches off.
• IR thermometers tend to read low on shiny surfaces, so stick some masking tape on them before take a reading.

 

 

[Jonathan Kay]

Very interesting.

Have you posted about this before?

Just to be sure: did you conclude that there was only ever a problem with the temperature readings, not with the actual temperature?

Thanks

Jonathan

[IanJ]

Hi Jonathan,

No, I haven't posted about this before. I just wanted to share my findings in the hope it would save somebody else a lot of time and frustration!

Yes, I concluded it was only a problem with the temperature readings and not the actual temperature. I compared IR readings from the water rail next to the temperature sender between my car and a friend's. On my car the fan switched on at 103C and switched off at 93C. On his car the fan switched on at 107C and switched off at 97C. Then I ran a test with the sender in a mug of boiling water alongside a thermometer. The sender was surprisingly accurate with the engine stopped, but reading way to high with the engine running. Reconnecting the missing earth strap immediately dropped the reading when the engine was running to match the reading when it was stopped.

Given the battery -ve is connected to the gearbox bellhousing, the poor ground connection must have been to the gauge, rather than the sender. Still not sure why that would cause the gauge to read high.

[Jonathan Kay]

Thanks

Jonathan

[revilla]

If the battery negative connects to the bellhousing and we are assuming a good electrical from there to the "earthed" casing of the temperature gauge sender, but there's no strap from the engine to the chassis, then the chassis "earth" voltage (and therefore the gauge's "earth" voltage) may not be the same.

Can we deduce which one would be higher? Well the battery and alternator combination are providing the overall supply to the circuit, so unless there's another active voltage source all voltages in the car will fall somewhere between the voltages at the two battery terminals, so where there is a difference, the chassis or gauge "earth" will be a higher voltage than the battery or sender "earth".

Now view it from the point of view of the gauge. The sender's earth reference is seen as a voltage lower than earth, a negative voltage.

As the sender gets hot, it's resistance falls and it pulls the gauinput input terminal to a lower voltage. If the sender is pulling down to an even lower reference voltage, it will look to the gauge like it is pulling down down more strongly and the gauge will therefore read higher.

There must have been some sort of earth path between the engine and chassis for anything to work in the first place, but without the strap it was probably just a leakage path through things connected to both the engine and chassis ground, for example ECU drivers. As such it would probably have a low current flow and therefore voltage drop with the engine off and a higher voltage drop with the engine running, so the effects whereby the gauge over-reads would increase with the engine running.

Hope that makes sense!

[IanJ]

Hi Andrew,

Many thanks for clearing that up. My confusion was down to 2 misconceptions:

1)

The sender would have a higher earth reference than the gauge due to higher resistance via the leakage path to the gauge.

2)

The resistance of the sender would increase with temperature.

Kind regards, Ian.