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Dart Temperature Gauge

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Kbeal
Posts: 148
Joined: Mon Feb 15, 2016 9:01 pm

Re: Dart Temperature Gauge

Post by Kbeal »

Hans, usually such issues are caused by poor earth connections, very common with glass fibre cars. In the SP only one gauge has an earth wire, (in mine it's connected to the oil pressure gauge) and this then earths all the other gauges through the metal panel. The best way to solve the problem is to run a separate earth wire to each gauge individually.

Kevin

Soundmike
Posts: 76
Joined: Thu Mar 17, 2016 8:54 am

Re: Dart Temperature Gauge

Post by Soundmike »

Check earthing between engine & chassis. Adding an earth wire so will soon ascertain if this is the problem.....

Bud
Posts: 318
Joined: Fri Feb 12, 2016 9:06 pm
Location: Rochester, NY

Re: Dart Temperature Gauge

Post by Bud »

After checking through your recent flasher install for proper location in the circuitry, correct voltage sources and groundings and still having these dips and spikes:
For powered meter or gauge fluxuations not associated with groundings I would think one or more circuits (check the fan relay source and add the headlights if not relayed) are in need of a relay controlled direct battery source.
A full battery voltage source away from the original circuitry distribution blocks so that switches are electronically assisted in the full battery voltage distribution. Using relays reduces losses through older insufficient wires and poor contact points all dropping voltage and loosing needed amperage. The relays are readily available and easy enough to confidently install.

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heh101353
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Joined: Sat Sep 30, 2017 11:34 am
Location: Basel, Switzerland
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Re: Dart Temperature Gauge

Post by heh101353 »

Looking closely behind the dash it turned out, that somebody in the past already noticed the problem and added a separate earth wire to each gauge.
Looks a bit like overkill.
Especially as the temperature gauge has already its dedicated earthing by means of the wire to the sender.
Two years ago a kill switch was added and all heavy gauge wires between battery, solenoid and starter have been replaced. All copper bands as well. So no luck there.
It must have something to do with the fan drawing a lot of amps when starting up. The fan is directly operated by a switch under the dash. Not a brilliant idea because every time the switch is is triggered it will carry all the load and this will for sure burn the contacts more sooner than later.
So I went down the route which Bud suggested.
Bought a relay, altered the wiring accordingly and surprise, surprise. The needle still jumps towards hot but it does not hit the limit stop anymore. It takes about two seconds 'til it swings back and settles to the usual 10° more. I can live with that.
It still puzzles me, how adding a relay could alter the behaviour in such a way. What's the difference between a manually operated switch and a relay :?: :?: :?: :?:
Forgot to mention, she still runs on a dynamo.

Thanks for your ideas. Very much appreciated.
Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Brian-H
Very Wise Man
Very Wise Man
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Joined: Fri Aug 09, 2019 6:04 pm
Location: UK

Re: Dart Temperature Gauge

Post by Brian-H »

I presume that all the gauge earth wires that were added, all go to the same point somewhere. If the earth wire for the fan switch also goes to the same point, then that point is the problem.

When the fan switch turned the fan on directly, the fan current ran down the earth wire from the switch to that point, and if that point does not have a very very very low resistance back to the battery, then when a current is put across that point, it creates a voltage drop. By putting a relay onto the fan, you've now moved the fan's earth current away from that point, so you've removed the voltage drop caused by the fan. But it seems that there's probably the same problem, albeit relatively minor now that the fan's current has been taken out.

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heh101353
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Location: Basel, Switzerland
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Re: Dart Temperature Gauge

Post by heh101353 »

Brian-H wrote: Sun Jul 05, 2020 4:17 pm By putting a relay onto the fan, you've now moved the fan's earth current away from that point, so you've removed the voltage drop
I see where you are getting at.
And you are right as well in your assumption that all those earth wires run to the same point.

Thanks
Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Brian-H
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Location: UK

Re: Dart Temperature Gauge

Post by Brian-H »

Of the ground wires that were added - at the point where each wire attaches to its gauge - you remove the wire and measure the resistance between the wire-end and the battery terminal that's the "earth" terminal (i.e. if the car is -ve earth, measure to battery -ve or if +ve earth, measure to battery +ve). Same for the fan switch ground wire.

If the resistance is say 0.1 ohms, then when the switch directly carried say fan current of 10 Amps, this indicates that all the gauges were losing 1 volt (because V = I x R = 10 x 0.1 = 1). This is because, when something else increases the current through the bad connection, then all the devices lose the same voltage drop caused by the current, even though its not their own current.

The reason for checking down the earth-wire on the gauge (or the switch) is because we assume the earth wire itself is extremely low resistance, so by measuring there you're taking a better measurement of the resistance of the ground point itself. If that ground point is on the chassis, then it's probably developed a bad connection on the chassis. This is the reason why the gauges aren't measuring correctly, because they're losing small voltage drop across the bad connection, which may also alter as the cumulative currents pass through it.

A diagram would be a lot easier if there is one.

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heh101353
Posts: 157
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Location: Basel, Switzerland
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Re: Dart Temperature Gauge

Post by heh101353 »

Thank you for your detailed explanation.
I will check if Ohms law applies to my Dart as well. :lol: :lol: :lol:
If the check turns out to be positive I'll see what can be done to improve the earthing of the gauges.

But this will have to wait until seasons end. Now it's time to head to the Alps ... :mrgreen:
It's one of my and the Darts favourite pastimes doing three to four Alpine passes in a day despite the fact that it takes us two hours before reaching the first peak.

Image
On top of Gotthard Pass, June, 23 2020

Image
On top of Nufenen Pass, June, 23 2020

Thanks again
Hans
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

Sydsmith
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Location: Aberystwyth Wales

Re: Dart Temperature Gauge

Post by Sydsmith »

As we are digging into this a bit deeper, the other consideration is the size of the earth wire used.

The original earth and other cables were designed to carry the original current load, if we increase that load, then the earth wire must be increased in size or an additional earth wire run to support the extra current.

The problem with a 12 volt system is that Ohms law, electric current is proportional to voltage and inversely proportional to resistance, means that what appears a small change in one parameter will have a big effect on the other two. The resistance of a cable depends on the cross section of the copper and the cable length and will increase with increased current. If we increase current drawn we must reduce resistance by increasing cable size/cross section or the voltage at the end of the cable will fall. Syd

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JohnM
Posts: 206
Joined: Thu Feb 11, 2016 1:31 pm
Location: N W London

Re: Dart Temperature Gauge

Post by JohnM »

Loved the pictures, Hans - we were due to do the "Grand Tour" in September, but all cancelled now :( :( :(
Next year at Spa looks like our next meet-up now.......

best regards
John (& Pat)
John M in Middlesex, NW London
1962 SP250 "B" Spec
DLOC Herts Region Sec

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