Fuel Gauge 1800 Information and Notes
First published: Nov 2019  R. Kwas  Revisions On-Going

Late 1800 (Injected) Fuel Gauge [Wiring Diagram excerpt only.]

Link to 122 Fuel Gage Notes:  https://www.sw-em.com/Fuel%20Gauge%20Notes.htm


The Smiths supplied 1800 fuel gauge (early, carbureted) is different from both 122 (based on the VDO supplied Drehspulinstrument) gauge (but not in operating Principle, see below!) and later (injected) Voltage Stabilizer associated gauges of the E and ES vehicles, therefore these three gauges types must be considered separately. 

Slight variation in the face exist between the pre Chassis No. 7000, and post Ch. 7000, but the internals of gauges are the same, and in operating principle, similar to gauge of the 122.  See also: Drehspulinstrument  



A page from the Factory Manual [correction for Fig. 2.3:  Connection for Ignition Power (not "positive terminal of battery"!).  Fuel Gauge is an Ignition function...why would you want it ON all the time, draining the Battery?  You wouldn't!...more auto-electrical wizardry courtesy of Lucas? ]  


1800 Fuel Gauge and circuit detail.


As noted, 1800 and 122 instruments are similar only in operating principle. Picture below shows two Magnetic Coil Assemblies fixed to the housing, and generating magnetism at right angles.  In the 122 instrument, they are part of the rotating Needle Assembly, see:  Drehspulinstrument

All subsequent pictures by James R. and used with his kind permission: 


What the 1800 Fuel Gauge looks like from the business end, installed in a Dashboard.  Red/White wires are Instrument Lighting (Lamp in holder is a friction-fit, and simply pulls out, for checking or bulb replacement),  (Double) Greens are Ignition Power from Fuse 2, Green/Black is the Sender connection, and (double) Blacks visible are some instrument chassis connections (these may be located on Fuel Gage, or elsewhere)...depends on if it was raining that day in Britain...


Tools used to gently nibble (by bending) away at the Chrome bezel to allow gentle removal of it, and gauge face (notice the generous and repetitive use of the word gentle here!).   Note evidence of the bending on the instrument edge to allow this. Note also slotted holes (at Yellows) for precise position adjustment of the Magnetic Coil Assemblies during calibration (see text below!).  There is evidence of a white lacquer on securing nuts.  Don't loosen or reposition these, but verify securing nuts are snug and have a dab of locking paint.  In the case of James' instrument, something in this system failed, allowing coil assemblies to move out of position, which caused a complete failure (non-function, but not terminal!) of the instrument. 


Internals of gauge exposed. 

Blue circle highlights movement arc of Needle Assembly, and matching arcs on the endplates of Magnetic Coil Assemblies are clearly evident.  Precise position of these determines magnetic influence on the Needle Assembly and instrument calibration. 


PLACEHOLDER FOR LATE 1800 FUEL GAUGE NOTES (fitted in vehicles with Fuel Injection and Voltage Stabilizer):

Until I can add some details on the later version instrument, here is a Wiring Diagram excerpt of the Fuel Gauge wiring.  Note that Fuel gauge is supplied by the same circuit (White/Green Wire) which supplies Voltage Stabilizer (Item 28), this should not be confused with the VStab output!  

1800E and ES Fuel Gauge is item No. 68, supplied by Fuse 4 with White/Green Wire. 



Thread where James R.  repairs a non-functioning 1800 Fuel Gauge:   https://forums.swedespeed.com/showthread.php?610015-Fixing-P1800S-fuel-gauge

Apparently, the Magnetic Coil Assemblies on his gauge became dislodged and were out of position...he notes:  "There was also a circle at the base of the needle with a similar diameter of the crescents, but my crescents were no where near lined up with the base of the needle. ").   

My response to that thread

"Those pieces with the crescent shapes are the two Magnetized Coil Assemblies (at right angles to each other, this is the basis of both the 122 and 1800 instruments. Note in the 122 instrument, the coils are part of the rotating armature, in the 1800 instrument, they are fixed to the instrument case.  See:  https://www.sw-em.com/Fuel%20Gauge%2...spulinstrument), which act upon the Needle (more specifically, what the Needle is mounted to)...it sounds like mechanical securing of those coils became dislodged, so the magnetism was out of place and because of this, ineffective to work on the Needle as intended. By moving the poles to where it seemed they should be, you have brought back the function, but I expect the calibration will have less than its original accuracy and should not to be highly trusted until you get a few tank-fulls of experience with it! Congrats on the repair!" 


Link to another thread:  http://forums.swedespeed.com/showthread.php?547874-Gas-tank-level-replacement-recommendations&p=6709266#post6709266


Reference Information Related to Reliability of Smiths Gauges: 

Question:  Where did workers fired from the Lucas factory go? 

Answer:  Down the street to the Smiths factory. 


External material sources are attributed.  Otherwise, this article is Copyright © 2019.  Ronald Kwas.  The terms Volvo, VDO, and Smiths are used for reference only.  I have no affiliation with any of these companies other than to present my experience, and highly opinionated results of the use and care of their products, for the purpose of helping other owners keep their vehicles on the road, safe and reliable.  The information presented is my own, and can be used or not, or ridiculed and laughed at, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you don’t, you’re just a lazy, scum sucking plagiarist…so The New York Times wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and maybe wise-a** comment. 


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