Voltage Regulator: High Temps and Fuel
Lorne Goldman at the eMog Pub

The symptoms are: Car appears to warm up normally, taking about 5-10 minutes of driving in ambient temps of about 9C to warm up to normal temp of 90C. But then the temp gauge just carries on climbing. After half an hour or so it's nudging 140 C.

But: The fan doesn't automatically cut in, and on opening the bonnet there's none of the associated smell or sense of an overheating engine.  Cheers, Tim

Firstly, you are not overheating. Forget that possibility. You would know it, you would smell it, your rad fan would be on continually and your rad would be overflowing.

Your water temp gauge is effected by a limited number of things.

1. Your voltage stabilizer. This items alters and maintains your 12 Volt to a 10V supply to your fuel gauge and water temp gauges. If the screw holding it at the tab looses, the stabilizer's earth wire, held by the same screw, can lose a proper contact and the current to the gauges will jump to your alternator's output (14 +or-.5V). That will make your fuel level and water temp jump considerably. First, verify that the stabilizer is earthed (grounded) by testing it on the body of the stabilizer. 90% of problems are resolved right here.

2. Your voltage stabilizer has failed. I had a run of 4 (all sourced from the US while mogging there. They all failed. [i](This is not an indictment of the US automobile aftermarket industry..which I dearly love but it is best to buy this type of British parts from Britain. It is also much cheaper.[/i] I then switched to another source, namely Caerbont who now manufactures Smiths) (They sell to Holden, Moss, Europe, Morgan..etc etc) and this resolved the problem.

You can test this item. You cannot detect the proper output from a stabilizer, but you can substitute something you know is putting out 10V. If you passed #1, get another battery and let it run down to 10V. Earth it on the car and have it feed your fuel of temp gauge directly (no stabilizer in between). If your temps now behave, the stabilizer is blown.

BTW, I purchased the other 10V stabilizer (1.5 amp) from http://www.digikey.com #UA7810CKC They are 40 pence each rather than 8 pounds. Read this article . Try your neighborhood Radio Shack or computer repair place.

3. The wire from the temp sender on the engine block is earthed somewhere. To test, you could simply put in another wire from the sender temporarily and loop it around to the gauge.

4. If you pass..#1-3 the sender is gone. Not unheard of. It must be replaced.

5. I have not seen this yet, but I imagine the gauge itself can go.

The key is that if it is the voltage stabilizer is abnormal in some way, both the fuel level and the water temp will be effected.

from Tim at Ron Davis Racing

A voltmeter capable of reading both AC and DC currents is required to test cooling systems. The meter needs to read zero to the maximum voltage of the system being tested in tenths of a volt. The meter leads must be long enough to reach between the coolant and the ground side of the battery. An ohm function of a voltmeter is very helpful to pinpoint areas of resistance in as electrical system that will cause an electrical current to ground through the coolant rather than the engineered electrical circuit.


1.  Attach the proper meter lead to the ground side of the battery, negative-to-negative or positive-to-positive.
2.  Install the second lead in the coolant touching the coolant only.
3.  Read the DC and AC voltage with all systems off. If a block heater is present, also take a reading with the heater turned on. If an automatic battery charger is present, as a standby system, also take a reading with this system running.
4.  Read the DC and AC voltage with the electrical starter engaged.

5.  Read the DC and the AC voltage with the engine running and all systems turned on: lights, coolers, fans, heaters, air conditioning, cell phone, two-way radio, including the phone and radio on both standby and transmit.
6. The above procedure will test a complete system except for an electrical current, which can be generated by the rear end transmission. This is particularly true with air bag suspensions, rubber pad suspensions and rubber-mounted transmissions. Any current generated will travel up to the drive shaft to ground through the engine coolant. Grounding rear ends and transmissions is strongly recommended.
7.  Voltage of zero to .3 is normal in a coolant of cast iron engine. Such an engine will be destroyed with time by .5 volts, and engine manufactures are reporting .15 volts will slowly destroy an aluminum engine.
8. The current will be AC if the problem is due to static electricity.
 9. If the coolant shows an electrical problem with all the equipment turned on; turn off one system at a time until you finally turn off the system that stops the electrical current. When the current stops, this will indicate the electrical system causing the problem.
10.  Be partially careful of starters. They can cause as much damage to a cooling system as much as a direct connection to an arc welder! This is due to the amperage present.

11.  Always change the coolant if a current is detected. The electrical current will destroy the protecting chemicals in a properly inhibited coolant.

Whys & Wherefores by J.F. Frot
Electrolysis needs an anode, a cathode and a conductor between the two. In an automobile, the coolant plays the role of the latter. Pure water, distilled water and (even BETTER) demineralized (aka deionized) water are, in theory, non conductive (although no money would make me hold a power drill in a bucket of any water and press on the button). Hence no current and no electrolysis between the different metals. Great benefit there.The coolant additives are supposed to neutralize the salts collected from the rock they run through or in the reservoirs and that you get from the tap or in bottled water. Interestingly, the suppliers pride themselves on the good salts that their water contains: calcium, magnesium, potassium, added fluoride, etc. which are indispensable for our metabolism to function correctly. However they have a disastrous effect on metal machinery and the bi-metal corroding effect. As soon as you pass the 65C mark. The calcium carbonate and the like precipitate and build up on the walls of the circuit and take the role of insulator to the heat transfer.  
The efficiency of the additives to neutralize the coolant is questionable. I used to run very large superheated steam boilers on crude oil carriers when I was in the merchant navy and it takes far more than a bit of glycol and a drop of hydrazine to avoid the destruction of a turbine in record times. So, again, demineralized water is a better option than relying on the coolant and its additives. 
Personal Note: In my Edwardian and vintage cars, coolant had an adverse effect. For a reason I do not know, they corrode the nickel that the outside of the radiator is plated with. So I do not use them and as soon as the roads are gritted, I empty the circuits and they stay in the garage until late March. As they work on the thermosyphon principle and under no pressure, they consume a fair amount of water. So I use the stuff from the water but, rain water, (you could call it cold distillation or evaporated water) with very good results for the last forty years. We also collect rain water at the steam museum where we run a 600 HP compound engine with very little corrosion of the boiler or the cylinders. 
On modern engines, the coolant is indispensable to avoid frost damage to the engine block as before but essentially to raise the boiling point and cope with any summer traffic jam (if the water pump and the fan are in good order). So, great benefits to be had there. My tuppence on the subject.        

by Lorne Goldman

After a period of usage, the effectiveness of coolant deteriorates. This can be tested in two ways, both convenient.

1. There is the conventional method with a coolant tester..an inexpensive device that can be purchased at any auto store or online. Looks a little bit like a turkey baster and functions the same way. You simply have it suck up some coolant and read what it tells you.

2. The second, newer method is to buy inexpensive tests strips. Dip the strip into the coolant and the color it changes to will indicate the effectiveness of the coolant. The strips are also inexpensive and easily found.