Sticky topic please: Gauge calibration

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Sticky topic please: Gauge calibration

Post by AJL Electronics » Tue Sep 03, 2013 10:56 am

This is from a Jensen Healey forum, but relevant to our cars in the main. http://www.jensenhealey.com/forums/view_topic.php?id=14

A Note on Voltages. In automotive usage, the term "12 volts" means the output of a charged six-cell lead-acid storage battery in good condition. As it happens, the actual output of such a battery is around 13.6 volts. But the terminology is unlikely ever to change, so one must get used to the discrepancy.

General Operation. The volts, fuel, and temp gauges all operate on the same basic principle: an electric current flows through resistance wire inside the gauge. This current causes heating of the resistance wire, which is wrapped around a bimetallic strip. This strip bends in response to the heating. The gauge's pointer is attached to the strip and the more the strip is heated, the more the pointer deflects. The response time of these gauges to a change in the current is typically about 3 to 10 seconds.

The fuel and temp gauges work in conjunction with external sensors (senders) that vary the current flowing through the gauge. The volts gauge is its own sensor and is connected directly to the switched 12 volt supply.

Voltage Stabilizer. There are two types: mechanical and solid-state. The mechanical stabilizer works something like a turn signal flasher, producing 12 volts for a while, then 0 volts for a while, so that the average output over time is about 10.0 volts. This switching effect confuses many digital and some analog voltmeters.

The solid-state stabilizers are externally the same as the mechanical ones but rely on solid-state devices for their function, and produce a constant output of about 10 volts. You'd expect them to be far more reliable than the mechanical ones. However, these stabilizers do not seem to have any protection against a short circuit on their output, and thus can be expected to fail instantly upon the occurrence of a short.

If the output of the voltage stabilizer increases, the readings of those gauges connected to it will normally increase. Conversely, if the output decreases, gauge readings will normally decrease.

Oil gauge. Smiths PL 2312/04. Unlike the other gauges used in a JH, this gauge is purely mechanical and uses a Bourdon tube to deflect the pointer. Some non-JHs use electrical oil pressure gauges, however, and their operation follows the same principles discussed below.

Voltage gauge. Smiths BV 2204/05. This should be connected to the switched 12 volt supply. A good gauge will measure about 120 ohms between its two terminals when disconnected from the wiring.

Fuel gauge. Smiths BF2201/27. This should be connected to the output of the voltage stabilizer. A good gauge will measure about 60 ohms between its terminals. The fuel tank sender varies from circa 30 ohms (full) to circa 370 ohms (empty).

Temp Gauge. Smiths BT 2204/24. Like the fuel gauge, this should be connected to the output of the voltage stabilizer. A good temp gauge will measure about 60 ohms between its terminals. In a properly operating system, the mid-range of the gauge is about 80-85°C or 175-185°F -- the temperature at which the thermostat opens.

The correct temp sensor is the Smiths TT6811-01, which has an 1/8" x 27 pipe thread. The resistance of the sensor varies with temperature as indicated below.

°C 25 50 70 80 90 100 110

°F 77 122 158 176 194 212 230

Ohms 820 306 170 142 106 79 53 All plus or minus perhaps 10%

Interchangeability. Most electrically operated Smiths gauges are substantially identical internally. This means that you can very often repair a burned-out temp or fuel gauge by installing the mechanism from a Smiths gauge scrounged from the junkyard. This is just as true for MGs, Jags, and Lotus as it is for JHs.

Calibration. A close look at the face of a Smiths gauge will usually disclose three pairs of dots that align with the low, middle, and high ends of the gauge's range. These are calibration markers. When the gauge is correctly calibrated, the pointer will lie between the two dots of a pair at three specific inputs.

Gauge calibration can be checked by applying several accurately known DC voltages across the gauge terminals. For 2.0 volts, the gauge pointer should lie within the low pair of dots. For 4.8 volts, the pointer should lie within the middle pair of dots. And for 7.6 volts, the pointer should lie within the high pair of dots. Unfortunately, there does not seem to be any publicly available information on how one adjusts the internal parts of a gauge so that everything will line up correctly.


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Re: Sticky topic please: Gauge calibration

Post by philhoward » Tue Sep 03, 2013 11:02 am

Stickied ;)

Would be very handy added to the Wiki entry on Gauge calibration (Jim/Geoff?) which covers the last line of that article ;)


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Re: Sticky topic please: Gauge calibration

Post by scimjim » Tue Sep 03, 2013 11:15 am

Can we confirm that this (part numbers and figures) is applicable to Scimitars:

Voltage gauge. Smiths BV 2204/05. This should be connected to the switched 12 volt supply. A good gauge will measure about 120 ohms between its two terminals when disconnected from the wiring.

Fuel gauge. Smiths BF2201/27. This should be connected to the output of the voltage stabilizer. A good gauge will measure about 60 ohms between its terminals. The fuel tank sender varies from circa 30 ohms (full) to circa 370 ohms (empty).

Temp Gauge. Smiths BT 2204/24.

The correct temp sensor is the Smiths TT6811-01, which has an 1/8" x 27 pipe thread. The resistance of the sensor varies with temperature as indicated below.

°C 25 50 70 80 90 100 110

°F 77 122 158 176 194 212 230

Ohms 820 306 170 142 106 79 53 All plus or minus perhaps 10%


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Re: Sticky topic please: Gauge calibration

Post by AJL Electronics » Tue Sep 03, 2013 11:21 am

Data not checked but I intended it as a general article, particularly the calibration voltages in the last paragraph.


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Re: Sticky topic please: Gauge calibration

Post by AJL Electronics » Tue Sep 03, 2013 11:28 am

The gauge I have just taken out reads 56.3 Ohms. Temp sender which MAY BE FAULTY reads 757 Ohms at ambient today, ~ 26C in the shop. Actually that resistance reading has just increased to 794 and is slowly climbing. Strange... It has stabilised at 875 Ohms at 26C. Another one tested reads 714 Ohms and seems stable.

BT2204/29 is the type number of the SE6a temperature gauge. It has a paper sticker on the back 258 but the one out of the GTC is 459 on the sticker.
Last edited by AJL Electronics on Tue Sep 03, 2013 11:53 am, edited 3 times in total.


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Re: Sticky topic please: Gauge calibration

Post by philhoward » Tue Sep 03, 2013 11:29 am

From when I was looking at fuel gauge compatibility to mine (forgetting the reversing effect):
windy wrote:Just tested a NOS Lucas SE5a sender & the resistance is 29 to 240 ohms.


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