Sunday, December 24, 2017

bimetal automotive gauges

My neighbor asked me to help him trouble shoot an intermittent problem with his auxiliary gauge cluster on his circa 1980s Isuzu Trouper. At times the fuel level gauge and the engine temperature gauge would stop functioning. The most peculiar symptom is they stop functioning and start functioning in tandem.  Seems simple enough, I thought until I have a chance to see the assembly first hand.

this mSATA SSD is for to upgrade my Lenovo T400s notebook; it will need a mSATA to 1.8" HDD adapter to be arrive in a few days; I scored an Amazon Warehouse Deal hence the broken packaging

mSATA to 1.8" HDD adapter

compared to this 128GB microSDXC card the mSATA SSD is very expensive

The auxiliary gauge panel has 3 gauges. At time the temperature and fuel gauge will both stop working. When not failing, all three gauges will function. The oil pressure gauge never fails.

I have to remove all there gauges in order to get to remove the PCB for further inspection

all three gauges are carefully removed; they are all of bimetal thermal type

there are a (appears to be ) voltage regulator; a few resistors, one capacitor, and an inductor on the PCB

the water temperature gauge has only two terminals; same also is the oil pressure gauge

the fuel gauge is most puzzling as there are two bimetal assemblies and 4 terminals

It is amazing how little information I could find on automotive bimetal gauges by googling. Additionally, many are simply wrong and confused with milliamp gauges. I have to resort to try figuring this mystery problem myself. My neighbor found a closest wiring diagram for this aux gauge panel but it shows all three gauges has 3 contacts.

Of the few "schematics" of bimetal automotive gauge that I found on the web, this one is the most trippy. It uses a bimetal sender instead of a common potentiometer. On the surface, it seems it is unworkable but I can see why and how the design works (as a fuel level gauge).

As I don't yet have the luxury to trouble shoot this gauge pod in the vehicle, I have formed my reverse-engineered theory of operation. I soldered up a few probing wires so a number of signals can be monitored when the gauges function, and malfunction. My hunch tells me the likely cause of the failure is the extra bimetal subassembly in the fuel gauge.

mini's festive decoration

icy mini

this is a simple and low calories dish I made with the Shanghai salted vegetable; some of the simplest dishes are the most endearing

Here in Portland, Oregon it is a big deal to have a white Christmas. It is very rare but today we technically get one.

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