Coolant Temp Gauge Rebuild w/parts list & photos

[MarkWard]

Plus, I believe the blue high beam indicator was a bulb not an LED.

[RawUmber]

Here's a schematic for inside a '42' pseudo relay. The chip appears to be an opamp. Props to the author.

Here's diagram from "Club 80-90":
source: https://wiki.club8090.co.uk/index.php/Coolant_and_Heating_flashing_red_light_on_dash

I have a spare '43' (which was superseded by the '42', if I understand correctly). When I wired it up on a bench, it outputs ~GND its output floats (to the gauge) so long as its input stays grounded (ie, coolant tank full). Once I disconnect ('open') the ground from the input (ie, coolant tank not full), it takes ~7s until its output starts 'blinking' from GND float to ~.6V. I don't have a coolant gauge handy to see it actually blink, but it suggests to me that there is a blink activity coming from this module when the "coolant low-level switch" goes open.

Edit: I wasn't biasing the output, so I incorrectly stated that the '43' output ground when it was instead ~floating.

[crazyvwvanman]

There are 3 versions of the coolant level "relay". The later 2 versions connect a pulsing ground to the coolant temp sender line of the gauge. This triggers the circuit inside the gauge to commence the flashing but the ground pulses don't themselves make the led flash.

The original early version of the coolant level relay put a constant ground on the coolant temp sender line and this also triggered the gauge circuit to commence the flashing.

The gauge led flashes upon power up, without the coolant level relay even present. It stops flashing after the capacitor in the circuit is full. There is a current limiting resistor in the circuit that limits how fast the capacitor gets filled. The led continues to flash until the capacitor is full. That is why a leakage failure of this capacitor causes the gauge led to flash for no external reason.


When the coolant level relays pulse a ground or in the case of early version clamp a ground to the temp sender line this also drains the capacitor which causes the led to activate. The current limiting resistor delays the refilling of the capacitor for a period of time but the next ground pulse from the coolant relay drains it again before it has filled and thus the led continues to flash but the temp needle doesn't show much effect. With the early version of the relay the constant ground it puts on the sender line keeps the led blinking but also has the effect of making the temp needle peg hot. VW used the early version for about 2 years before changing to the the pulsed ground version to allow the temp needle reading to remain accurate.

Mark

[NorthOf50]

It's a surprise how many people (despite hopefully reading your multiple other posts stating this) still think the LED pulse is triggered by the relay. When troubleshooting my gauge at the beginning of all this even 2 local VW shops thought the same.

What Mark describes is clearly illustrated on Page 3 in Jim's circuit diagram of the gauge. Jim describes how to drop in voltage triggers the LED to flash very succinctly. Hopefully this will settle it for all.

[SyncroHead]

[crazyvwvanman]

Jim, as best I can tell the taller early version was used in 83/84 and the improved version appeared during 85 model year.

The original coolant level control relay #43 for the earlier models is VW # 251-919-376
The #43 control relay for the later models (and replacement for earlier) is VW# 191-919-376
Around 1990 this became a #42 relay VW# 191-919-376A

With the blinking led oddities that occur, attempts to rectify them mean any year can have any version at this point or even no version as someone may have "solved" the problem by simply taking it out.

------------------------------------

These are the 3 versions of the coolant level warning "relay" used in Vanagon 1983-91.
The tall 43 unit is the cursed early first try and should be removed and discarded if encountered.
Either of the short ones is fine.

The tall one pegs the temp gauge hot and makes the temp led blink when coolant is sensed low in the main tank.
The shorter improved later versions only blink the led and let the gauge read what the temps actually are.

Mark

[kirtfitzpatrick]

[weekenderraf]

I suspect the IC in the Relay 42 is an L4620 (Liquid Level Alarm). The block Diagram, which you can find at "https://datasheetspdf.com/datasheet/L4620.html " is certainly very similar to the diagram in RawUmber's post of Feb. 10, 2021.

Raf

[RawUmber]

[weekenderraf]

Nicely done, I found that the IC is an L4620 here: https://www.bulliforum.com/viewtopic.php?p=732540&hilit=L4620#p732540 in a post from 'Feuerwehr01' on 02.08.2017

Here is a picture of the printed circuit board with the L4620 (Liquid level alarm by SGS Thomson), the relay is a 43 .

And here an other relay 43 with the the IC labeled "VW-FSA/0 (by ST MICROELECTRONICS)

Here a picture of the printed circuit board of a relay 42, (191919376A),
all the components (resistors and capacitors) are the same (560, 180K, 68 K, 100Kohm resistors and 47 mF, 220nF, 4,7nF, 100mF and470 pF capacitors)as with the relay 43 but the layout has been changed.

[weekenderraf]

Which component (IC 4620 = VW FSA/0), capacitor, resistor) is most likely the cause of a faulty relay (42 or 43)?

[DanHoug]

[RawUmber]

I have no experience with faulty 42/43 'relays'. While I wouldn't recommend 'fixing' a working one, I'd guess the most likely component to fail first would be the electrolytic capacitor, distantly followed by the IC. The remainder rarely fail.

Nice pictures of the 42/43 components! A pic of the bottom sides could be useful too.

Way out on a limb here, was pondering...
L4620 = Liquid Level Alarm
VW-FSA/0 = FlüssigStandAlarm ?

[weekenderraf]

Here a picture of the bottom sides

of the printed circuit boards and to make it easier also

the designation of the connectors (15,31,S and G) on the printed circuit boards

[RawUmber]

Nice! Thank you for sharing that.

[dbeierl]

[zoti]

Did anyone ever scope the output from the coolant level unit to the sensor?

Can anyone explain why you need a unit that does that instead of just checking they the sensor closes the circuit?

I found that the previous owner put a simple relay instead of the unit in my van so when you turn the key on, it just closes the circuit of the sensor.