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LX dash lights and quad gauge issue


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#1 toryman76

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Posted 05 April 2020 - 05:17 PM

Hey guys and gals. I've had an issue with my LX ever since I finished it. I've fitted a slr cluster which was overhauled prior to fitment. As far as I know all gauges work as they should except for when I turn on the dash lights the oil pressure and water temperature gauges go up by around 1/8 to 1/4 on the dial. This only happens with the car running. This has me stumped. I know it's most likely related to an earthing issue but I'm not sure where to start looking. Does anyone have any advice?

#2 hanra

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Posted 05 April 2020 - 06:59 PM

Mine did the same. I had also fitted a 5v regulated power supply instead of the bi-metallic strip regulator.

Disconnect the ground wire of the regulator from your existing harness and just run a totally dedicated ground wire for the voltage regulator and that will solve your issue.

Edited by hanra, 05 April 2020 - 07:06 PM.


#3 toryman76

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Posted 05 April 2020 - 07:23 PM

Yeah my bi-metalic regulator was also replaced with a modern version which I was told is one that is failsafe and won't damage the gauges if there's a short. I pressume this is much the same as what you have in yours. Running a new ground wire where would be the best place? Is there somewhere neat up under the dash or did you run it back into the engine bay?

#4 hanra

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Posted 05 April 2020 - 07:46 PM

Yeah just disconnect the exisiting ground wire for the regulator and just solder on a new ground wire with a ring terminal and connect it to a clean section of the metal dash frame in behind.

When I re-did my entire harness I cleaned up each and every individual terminal and socket of every connection point. So I was surprised to see an issue with grounding.

#5 toryman76

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Posted 05 April 2020 - 08:31 PM

How did you go with the ground wires in the engine bay after fresh paint? I've been warned most grounding issues occur after painting. Is it worth considering a dedicated strap from block to body?

#6 hanra

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Posted 05 April 2020 - 08:42 PM

All my earth points on the body have the paint scrapped back for a clean connection.

There is already a dedicated earth cable from engine block to NEG on the battery. And then as part of the same cable, from NEG on the battery to the body where the radiator mounting bolts are. The headlights share the earth points from memory.

All the bare earth points on the body and every single plug and socket was treated with Deoxit spray.

https://www.mektroni...spray-142g.html

Edited by hanra, 05 April 2020 - 08:46 PM.


#7 toryman76

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Posted 06 April 2020 - 05:57 PM

Thanks heaps for the tips Brad. Looks like the new dedicated ground wire from the voltage regulator to the body has resolved the issue.

I am still curious to understand what an acceptable voltage drop would be across the various grounding points on the car. In my process of trying to resolve this issue I measured voltage drop with headlights on high beam, heater fan on speed 2, dash lights full brightness and from battery negative to:
- chassis ground it was 0.05v
- driver headlight ground 0.075v
- steering column bolt 0.08v
- new voltage reg ground 0.08v
- heater fan ground in engine bay 0.1v
Am I right in thinking these are too high?

#8 hanra

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Posted 06 April 2020 - 09:07 PM

Have you measured those same points for resistance and take into account the resistance of your meter leads.

#9 toryman76

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Posted 07 April 2020 - 07:22 AM

Yes. I am using an analogue multimeter. It has the ability to dial in the resistance to 0 so even though I've extended the leads to get to the various points it should rule out any resitance in the leads yes? Figures of the same points:
- chassis ground 0.4ohm
- driver headlight ground 0.6ohm
- steering column bolt 0.7ohm
- new voltage reg ground 0.8ohm
- heater fan ground in engine bay 0.8ohm

Edited by toryman76, 07 April 2020 - 07:23 AM.


#10 toryman76

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Posted 15 April 2020 - 04:39 PM

Well I kept tinkering away and found a few simple things made a huge difference. I relocated the driver and passenger headlight grounds to the underside of the bonnet stop adjuster screws. I found this to be a better more consistent location and splitting the passenger side away from the battery to body lead made a difference too. I also found that using a small amount of dielectric grease on the threads for all bolts / screws into the body in the engine bays various grounding locations made a difference. Doing these simple things I was able to reduce voltage from battery negative to all grounds easily accessible to around 0.05v to 0.055v. I still wanted to get it better than this so I added another 6mm earth strap from the battery negative to the body. This halved the voltage drop to 0.025v to 0.03v at the various locations. I am thinking now that it may be a good idea to make up a new earth lead with a single thicker gauge wire from battery to body but for now my extra wire will do the trick, just not exactly easy on the eyes!

#11 yel327

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Posted 15 April 2020 - 05:36 PM

One mistake people often make is to eliminate the 0V strap from the -ve terminal to the body. I’ve found over the years if you remove it and just have the big cable to the engine all sorts of weird things happen with temperature and fuel gauges plus dash lights. So even if the connection gets dirty or has paint under it you can see some weird stuff. I always run a big 0V cable from the battery to the trailer socket for voltage drop reasons, and also from the trailer plug to all points, makes a difference especially for electric brakes.

#12 toryman76

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Posted 15 April 2020 - 08:53 PM

It's interesting that most things I read and suggested to me was to add an earth strap from the block to the body. For some reason this made no difference. It was only when I added the extra wire directly from the battery to the body did it make a noticeable difference.

#13 claysummers

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Posted 15 April 2020 - 09:39 PM

Doesn't make sense does it. You wouldn't think it would make any difference. But then why make those poor wee electrons travel through the body, then a lead to the block, then another lead to ground. You eliminate two connections and therefore potential for extra resistance.

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