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Discussion Starter #1 (Edited)
My DMM showed 12.6v on the battery, whilst my Volt gauge only showed about 11v. I know gauges arent 100% but I thought that was a bit out.
The gauge has 3 terminals from top to bottom;
+
-
?

I had 12v ignition source going to "+"
earth going to "-"
and nothing connected to "?"
Forget about the light, it is an already power sourced and the bulb and the housing pushes into the gauge.

I removed the + and connected it to the bottom "?" and nothing showed on the gauge. I then wired it back to how it was and now the gauge only shows just over 10v.

Inside of the gauge

Between the middle connector and the bottom connector there looks to be a fried diode, is it a diode and does anyone know what value I need to replace with?

What is this third "?" terminal for?
I have not bent the needle and it moves freely



Restistance between "+" and "-" on the 20k scale is 0.46 / on the 2000 scale 458
Thanks in advance
Alan
 

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dunno Alan. Never had a voltage meter sorry. ( any ideas about my rear light earthing connection?)

Was waiting for Lonewolf to chime in.:cool:
Be patient;)
 

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Hi Al,

I'm the last resort ? It's flattering :) (Unless this post also ends up in a bin...)


I'd check the voltage on the voltmeter terminals with the DMM to rule out any wiring issues.
(with the voltmeter connected)

The voltmeter design seems very similar to the fuel level, and oil pressure gauges to me:
it uses two coils to generate magnetic field which moves the permanent magnet attached
to the needle.
From the photo the third (? marked) pin might be the voltage input pin, but I'm not 100% sure,
without inspecting the gauge more closely. The diode looks like a zener diode, which may serves
voltage stabilizing purposes, it maintains constant voltage in one of the coils using as reference bias.
(is the diode connected between the "-" and the "?" or the "+" and the "?" pins ?)

Is the gauge came from a Spider? By checking the wiring diagram of the car (where the gauge is
originated from) could anwser the function of the "?" pin. It is possible that pin shouldn't be connected
to anywhere. I guess the gauge may share the mechanical construction with the Jaeger oil press
gauge, which requires the third pin.
 

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Discussion Starter #6
1st choice more like :)

Yeah afterwards I thought I should have connected the gauge directly to the battery and as you say check the voltage at the gauge with my DMM, too late now I think it's broken now.
The diode goes across the middle "-" and the bottom "?" pin. Should I supply positive power to both the "+" and "?" pin then?
Do you think replaceing the diode, with what value and will that bring it back to life?
Remeber I connected power to only the "?" terminal.
 

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Well, if the diode is between the "-" and the "?" pins, and the white stripe on the diode housing
(indicates the cathode) is faces to the "-" terminal (I can't decide this from the picture correctly),
then the diode was probably blown instantly, when the "?" and "-" was connected to the battery,
because of the enormous forward current (this equals a short circuit).

If the white stripe on the diode is faces to the "-" pin, then the "?" is not the input pin.

The "+" and "?" terminals can't be connected together, in this case the diode will blow as well:
not instantly, but within 1-2 seconds.

I'd still recommend to check the wiring diagram of the original car.

Regarding the repacing: is there any writings on the diode?
 

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Discussion Starter #8
I never had the "?" pin connected when it was working, I connected it to see if it would give a more accurate reading doh :eek:
I think they came from a ser 1 33 or a sud :)
The diode looks cooked to me and I can't see anything on it :( Would this being faulty stop the gauge from working properly, or have I fried it?
Anyone else got the same dials and could you take it apart to check for me please?
I just did a voltage check at the connection to where the voltage gauge was and got the same as at the battery.
 

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Surley the third terminal is positive for the illumination circuit?. The guage was out of a series 1 33, pre made by some chap Alfa Aid may know (thats where I bought it from) The housing it sat in has a wiring note on the back, I still have it so will have a look in a bit and let you know Al.
 

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Hi Al,

I've did some research and it seems the voltmeter is from a S1 33 1500 QV model.
The "?" pin is not connected to anything originally, only the "+" and "-" terminals were used.

No info found about the diode. If no one knows the exact type, only experimenting remains.
In this case I would try to replace the original with the following ones:
- BZX55C5V7
- BZX55C6V8
- BZX55C10
Cut one end of the original diode (near to the gauge terminal) by using crocodile clips
you can connect the replacement diodes one-by-one to the "?" and "-" terminals.
The diode polarity is important. The striped side of the diode must be matched to the original one.
Then you can check the readings on the gauge.

Here in Hungary these diodes are cost about 0.02 GBP each.
 

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Discussion Starter #13 (Edited)
Thanks LoneWolf, much appreciated.
Anyone got a S1 1.5 QV 33 that can check the value of the diode on the volt gauge for me please??
 

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Discussion Starter #15 (Edited)
Surley the third terminal is positive for the illumination circuit?.
Sorry Iain, I didn't see your post for some reason.

Forget about the light, it is an already power sourced and the bulb and the housing pushes into the gauge.
The "?" pin is not connected to anything originally, only the "+" and "-" terminals were used.
Out of curosity, why is the "?" terminal not used, why can't it be connected with the "+" terminal, I have seen this in other volt gauges.
 

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Now as I getting more familiar with this gauge, I've corrected my earlier post#9.

The "+" and "?" terminals can't be connected together, this would blow the diode:
not instantly, but within 1-2 seconds. The battery voltage is higher than the diode
reverse breakdown voltage, and the current will be exceed the diode specification
within moments (because there's no shunt in the circuit in this case).

Why is this terminal not used? The oil pressure gauge needs three pins, so IMHO it was
cheaper to use the same base for the voltmeter too.
 

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Discussion Starter #17
I did mean "-" to negative & "+" wired together with "?" and both connected to positive on the battery.
Did you think I meant "+" to positive and "?" to negative lol
 

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I've attached diagram about what I wanted to say. That's how not to install.

In the video that's a different type of voltmeter. It requires an "enable" signal, which is an
ignition key triggered positive.
 

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