Club 80-90 Forums

So, my coolant gauge LED does not illuminate.  No flashing at start up, no flashing with level sender unplugged or 42/43 relay removed.  
Also, no illumination when I short Signal to Ground on the back of the gauge (mimicking V high temp, temp needle goes to Max).

Ground and 10V supply at the gauge is good.  Temp gauge itself is working fine.  I have tried a spare 43 Controller unit just in case.

So...I suspect an issue in the gauge itself (LED itself or another component) but thought i'd run it past the forum in case i've missed something obvious?


My understanding from the *numerous* threads on the topic on here and samba is the control module at the fusebox (42/43) pulses a signal to the guage when the coolant level sender on low coolant but this doesn't do the led flashing itself, that's handled by the circuit in gauge itself (which also detects V high temp to light up the LED solidly). There's capacitor that can go wrong, but tends to lead to erronous flashing.

Not sure if relavent, but I have SHC PCD replacement kit fitted and have had recent issues with a couple of Voltage regulators going bad in quick succession.  I found a 'better' one (10V, 2A rating), slapped the biggest heat sink I could find on it and i'm getting solid 10V to the fuel/temp gauges.

As a temp measure I'm contemplating cutting the K4-D29 link at the fusebox, fitting a normal relay and running a standalone LED from K4 to provide a low coolant warning, but that doesn't solve losing the high temp LED.

If you unplug the coolant temp sensor and bridge the terminals with a bit of wire, the needle should shoot high, and the LED should flash.
If the needle goes full scale, but the LED doesn't flash then ordinarily the culprit could only be the gauge itself.
HOWEVER you mention you have a PCB replacement.

Double and triple check you have the polarity of the wires going to the gauge correct. If they're flipped, the needle can still move but the LED won't light (and you might damage the circuit inside the gauge itself).

Unfortunately you can't even trust that the PCB replacement was manufactured with the correct coloured cables. I'll dig a cluster out and do some measurements off it for you so you can work out which of the gauge terminals should be ground - do you have a revcounter or not?

I did similar already - shorted the signal to ground terminals on the gauge itself (simulating V high temp).  Needle goes to full scale, no LED.

The SHC PCB replacement has been in the van for 4+ years working perfectly fine (with test flashes of LED on start up etc).

Regarding polarity, I don't think it possible to have it the wrong way around.  It's a small connector plug from the SHC board, not seperate wires, i'm pretty sure it's keyed to prevent it being connected the wrong way around. 

But...i've been wrong before so I'll go outside and double check.

Thanks for the pointers!

Mines a 1990 with the big clock, no factory rev counter.    The SHC mini board that connects to the gauge itself has the posts Labelled.

If you (even very briefly) connect the "sensor" terminal to ground, the LED should flash for about 5 seconds

cobblers wrote: ↑22 Aug 2025, 09:39

If you (even very briefly) connect the "sensor" terminal to ground, the LED should flash for about 5 seconds

Double checked and it's wired correctly.  No LED when shorting Signal to ground.

I note you have a non stock engine, so how exactly is the temp gauge getting a signal ?
The Subaru gauge sender is different to the VW one

Oh man I can barely remember now...running factory subaru ECU with modified subaru surgery loom hooked into the body loom at the usual black relay box at rear corner of bay.  IIRC the Temp sensor feed is from the subaru ECU and Level sensor loom is as per VW.

But i'm not convinced the issue is related to the subaru setup.  My Subaru ECU is reading Coolant temp fine (checked via ODB/SSM) and the gauge has been working fine up until a few weeks ago (installed for >2 years).  Ie test flashes on start up then settling at just below LED when running.