View original topic: engine bay wiring q's + coolant light not working...?
NorthOf50 Sun Nov 08, 2020 12:02 am

Hi all,

Recently purchased my '84 WBX, been without one for a long while and happy to be back. I'm working through my list of"defects" and next up is to sort out why the coolant temp warning light does not seem to work. It does not light up when you start the engine (self test). The temp gauge operates as it should just not the blinky light!

I have pulled the coolant level sensor off and verified that when that happens, the gauge pegs (as I think it is supposed to do). While I had the sensor plug off I started testing ground resistances as the connector looks in bad shape. I am getting a very high resistance short from the ground (brown) wire @ the connector to the "normal grounds" (tried a few to be sure). I opened up the wiring junction box (left engine) and split the connector open there and I have a perfect ground from that location. I'm pretty sure there is a poor connection @ the connector ground. Will this affect the warning light? I'll go looking for a new connector to crimp in place when I have time this week.

Also I'm trying to make sense of a few wires in my engine bay, one in particular doesn't seem to make sense. One wire, hard to make out the colour at night under headlamp but this is when I have free time (kids in bed) but it comes out of the same loom as the coolant level warning circuit & coolant temp (yellow) but it has at some piont been crimped through to a brown ground wire....any idea what this is?

There is also a mystery green wire hanging out there with no friend to plug into. Any ideas there?

Many thanks for any help!

Bad connector to coolant level sender

What are these wires??


bobhill8 Sun Nov 08, 2020 7:15 am

Green one might go to the O2 sensor?

crazyvwvanman Sun Nov 08, 2020 7:31 am

The coolant temp gauge led should blink several times each time the key is turned on. Starting the engine has nothing to do with it.

If it does not blink some each time the key is turned on then the likely problem is in the instrument cluster.

Since the temp gauge needle pegged when you disconnected the coolant tank level sensor that shows that the level sensor and its wiring was working. If you want to get the temp gauge led to work focus on the dash/instrument cluster. I'd start by tightening the 3 nuts on the back of the temp gauge. One of the nuts is the ground connection for the led circuit board that lives inside the temp gauge.


NorthOf50 wrote: up is to sort out why the coolant temp warning light does not seem to work. It does not light up when you start the engine (self test). The temp gauge operates as it should just not the blinky light!...

NorthOf50 Sun Nov 08, 2020 7:50 pm

Brilliant, thanks Mark. I've got to pull the dash tonight anyhow to replace a burnt out brake light switch anyhow and will have a good look at the guage and associated connections. Here's hoping it's that easy!

NorthOf50 Mon Nov 09, 2020 11:49 pm

thanks bobhill8.
As you can imagine, replacing the rusted out o2 sensor was coming up on my list of to-do, now I know where the tail should go! seems the Bosch OEM only come with a splice type but I have found a Walker @ Rock Auto which comes with a full OEM style tail. Anyone have experience with non-OEM or should I just stick w/ Bosch and splice a new one in place...?

NorthOf50 Tue Nov 10, 2020 3:39 pm

crazyvwvanman wrote:

If you want to get the temp gauge led to work focus on the dash/instrument cluster. I'd start by tightening the 3 nuts on the back of the temp gauge. One of the nuts is the ground connection for the led circuit board that lives inside the temp gauge.


Unfortunately I have struck out here. All 3 nuts were realtively tight, but I gave 'em an extra bit just go be sure.

Following the electrical schematic, I notice the high beam LED also shares the same ground, turns out this doesn't work either. I verified voltage @ the wiring loom through terminals 14/9(everything) and 14/5 GND, 12.3 volts there, all the other gauges and lighting works. I tested @ 14/4 for the high beam, got voltage there as well. I then tested 14/5 and grounding against a body screw and have 3ohm this something that would make a difference. it shouldn't in my mind as everything else using the 14/5 works.

What are the odds both the high beam and coolant warning LED are both burnt out? Could the voltage stabilizer be on the fritz? But if so would that not also affect my fuel gauge & temp gauge (which are working)? Is there another way to test there or perhaps something I have missed??

Thanks for any input,


Crooked Designer Tue Nov 10, 2020 3:51 pm

FWIW, when my blue foil crapped out, lots of things went south on my instrument cluster. It was a bit pricey, but definitely worth replacing the blue foil with one of the kits (can't remember which one I got, but I think they're all the same idea.. replace that frail foil with actual wires). Everything worked perfectly after that and I've had no issues in 2 years now.

*edit. Just did a search and the one is the one I used.

crazyvwvanman Tue Nov 10, 2020 4:54 pm

You should be able to determine which of the 3 nuts on the temp gauge is the ground. Then you could jumper a wire from there to chassis ground, on the dash metal.

The early cluster like 80-84 has a screwy ground setup that often gets fried on the blue foild near the 14 pin connector. You should look that area over carefully. The 14 pin actually has 2 brown wires going to it but only 1 of them is a true ground. The other brown SHARES the cluster ground with other things in the dash and this sharing can result in the blue foil getting fried near the 14 pin. VW fixed this after 84 and stopped sharing the ground cluster with external stuff.


NorthOf50 Sun Nov 15, 2020 9:53 pm

well I found some time to continue investigating today. I jumpered from chassis ground to the nut attached to foil run #5, I'm 99% sure it is the correct one. the middle of the 3 screws for the gauge. unfortunately it did nothing. I will try the other 2 nuts to be sure when I get time again.

upon further inspection of the wiring diagram, I am unsure of this anyhow as it shows the instrument panel lights and the analogue clock sharing the same 14/5 ground and yet both of those work without problem.

back to square one when I have time and light.


NorthOf50 Thu Dec 17, 2020 9:58 pm

Apparently having 2 young children really eats up any time a guy has to work on his aging van!

Some further diagnostics today while I was installed a new 12V outlet (the original lighter plug had it's wiring removed...?)

I have tried running a ground wire to all 3 of the nuts @ the back of the gauge cluster with no luck, although it was easy enough to follow path #5 to figure out which should be the ground. No luck with getting the warning light or high beam indicator to light up.

Next step was to check the V stablizer, but since the gas gauge and temp gauge are working was pretty sure not an issue there. I had 12V at one side, 8V at the other and 0V at the other prong. I'm assuming these are the expected readings since the other 2 gauges that share that V path work. Unfortunately after all my poking and prodding today my gas gauge seems to no longer work. frig!

Does this point towards the foil being the culprit and failing? w/ visual inspection is "seems" fine. I get all the correct V & gnds @ the harness...

any other ideas before I have to bite the bullet and order a foil replacement kit?


crazyvwvanman Fri Dec 18, 2020 1:36 am

This diagram shown the pins on the dash cluster voltage stabilizer.
TCA700 is original part, (other is common replacement with switched pins)
The middle pin is GROUND.
Power in, varies but we call it 12v.
Output is supposed to be 10 volts, almost exactly.

Looks like you have a cluster ground problem, at the least.


NorthOf50 Fri Dec 18, 2020 10:33 pm

Thanks Mark,

To be clear are you suggesting that there is a ground problem w/ the stabilizer or the gauge cluster as a whole?

I've tested the ground from the gauge connector and they are solid. If I get time this weekend I will try to confirm that the ground from the back of the temp gauge is actually getting a ground path. I did already try a direct ground to bypass everything and I did not have any success...

Thanks as always.

NorthOf50 Sun Dec 20, 2020 11:25 pm

OK, so I have created a new problem for myslelf, thought I found and fixed another but no luck. sorry for what appears to be a convoluted train of thought below...

VStab is kaput. I must have fried it during all my tinkering, will see if the local VW shop has one available tomorrow, if not I'll order one online. It did previously work though as I did measure 12v, 0v and 8(ish)v across the pins. if it's supposed to be 12, 0, 10 then perhaps it's been dying for a while?

As for the ground. I tested the circuits along the blue foil. O have 0ohm along circuit #5 from the harness to the ground of the Vstab and the ground lug of the warning light circuit. I have 25ohm from the gnd of the Vstab to #5 which I think should be the value of resistor #2 (it measure 25ohm, so I hope).

I measured voltage @ the high beam indicator pin #4 (11v) and @ Vstab input 12v, so that should be fine...

with the cluster connected I measure roughly 200ohm from the grounds to chassis ground. here is a problem. I know it isn't in the foil as that all measures 0ohm and I know it isn't from pin 5 on the harness as that also measure 0ohm. sooo can I be getting a dirty connection (200ish ohm) @ the harness to cluster connection?

I installed a ground jumper from chasis to one of the nuts on the rear of the cluster to bypass circuit #5. no change in operation. at this point the warning light should not work as my Vstab appears to be dead, but I should get the high beam LED to light. unless it is burnt out, it does not light. this all shares ground circuit #5....

So my thoughts are:

high beam LED could maybe be burnt out? Any way to verify? if I can get this to work it will verify my ground path #5. which I am certain is OK as the analog light and instruments lights share as wel. and these work fine, jumped to chassis ground or not.

vstab is dead, get new one, maybe this will be the fix?

anecdotes: I found the ground screw for the Vstab to be quite loose @ the foil. I removed and replaced with a #8 screw and it is good and tight with a solid 0ohm ground path now. gas gauge no longer works as a confirmation Vstab is dead.

I love electrical gremlins....

djkeev Mon Dec 21, 2020 6:36 am

This plug has serious corrosion issues.

Look at the green permeating the white plastic plug!

It might be working fine, it might not be or could soon fail.

Bad connector to coolant level sender


NorthOf50 Mon Dec 21, 2020 11:26 pm

Thanks Dave. despite the ugly looking connector the circuit works properly. it's a ways down on my list but eventually I'll get a new connector on there.

Further to that. I replaced the voltage stab today. my fuel and temp gauges now work again, hurray. however I am no closer to getting my temp warning light working.

Next troubleshooting breaks into 2 parts in my mind:

1 - what actually triggers the self test warning light? following the wiring diag, is it the coolant low level warning control unit, the coolant low level warning switch or coolant temp sending unit, or perhaps something else entirely. could one of these have failed which makes the self test not happen?

if I disconnect my ugly low level warning plug, the gauge pegs as it should...

2 - is there another way to get the warning light to flash? ie. could the LED be faulty and the circuit is actually working?

Less important. Why does my high beam ind light not work? shares the same ground (which is good but I have it jumped to chassis gnd for testing). I have voltage at the input pin for it but no light up when hi beams on.

Thanks again.

crazyvwvanman Mon Dec 21, 2020 11:32 pm

The high beam indicator is by far the most common indicator to burn out.
It isn't an LED in most vans, just a tiny old fashioned white bulb with a blue cover.


NorthOf50 Mon Dec 21, 2020 11:41 pm

Great! Finally something "easy" to fix. Thanks Mark, that I can tear into tomorrow I hope.

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