Under reading temperature and fuel gauges

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jagwit
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#1 Under reading temperature and fuel gauges

Post by jagwit » Wed Apr 04, 2018 6:49 am

Both fuel and temperature gauges rely on a “voltage stabiliser” which is just an inline switch that automatically cycles on and off. This applies to S1, S2 & S3. The duty cycle between on and off is meant to provide the equivalent of 10V DC.

The temperature and fuel gauges on a '65 S1 I’ve been helping a friend with, were under reading. I measured the output of this voltage “de-stabiliser” with my Picoscope and it looked like this:
27 Ohm.png
27 Ohm.png (79.31 KiB) Viewed 3979 times
This trace was taken with a 27Ohm load on the ouput of the voltage “de-stabiliser”. It can clearly be seen that the on vs off time is greatly biased towards the off cycles and thus the average voltage output was way below 10V.

I installed a LM7810 which gives a constant 10V DC ouptut and now both gauges read correctly.
Best Regards
Philip
Jag: 72 S3 XKE, 74 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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MarkRado
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#2 Re: Under reading temperature and fuel gauges

Post by MarkRado » Wed Apr 04, 2018 10:49 am

Philip,

thank you for your research! On google I found this for LM7810,
Image
I am wrong that this is no solution for pos ground cars?

Mark
Mark
1963 OTS 880436

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#3 Re: Under reading temperature and fuel gauges

Post by jagwit » Wed Apr 04, 2018 11:00 am

For positive ground, you would use a LM7910.
Best Regards
Philip
Jag: 72 S3 XKE, 74 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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Polse7317
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#4 Re: Under reading temperature and fuel gauges

Post by Polse7317 » Wed Apr 04, 2018 11:15 am

Hi Jagwit, is there a connnecting diagram about the 3 connecting pins ? also for a positive earth :shrug:
I suggest one for the live, one for the earth and the third for......?
Yves, happy XKE 63 fhc , w113 280sl owner
Looking for a OTS 4.2 serie 1....! :scratchheadyellow: and now have found a fhc xk 140 :lol:

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#5 Re: Under reading temperature and fuel gauges

Post by MarkRado » Wed Apr 04, 2018 11:23 am

Thank you, perfect,
gonna buy one immediately, see engine temp rise and start restoration of cooling system :oops:

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1963 OTS 880436

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#6 Re: Under reading temperature and fuel gauges

Post by Polse7317 » Wed Apr 04, 2018 11:33 am

Thank's Mark diagram perfect... :salute:
Yves, happy XKE 63 fhc , w113 280sl owner
Looking for a OTS 4.2 serie 1....! :scratchheadyellow: and now have found a fhc xk 140 :lol:

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#7 Re: Under reading temperature and fuel gauges

Post by cactusman » Wed Apr 04, 2018 4:18 pm

And for those with positive earth cars the 79xx pin out is NOT the same so check t'interweb . Gnd is the left pin, input is middle and out is right. It also means that on the 79xx series the heat sink tab is connected to the input so must be isolated from any grounded metal.
Also both input and output pins of both should have small capacitors to the gnd pin for device stability. Somewhere from 0.1 to 1uf is fine... :bigrin:
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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#8 Re: Under reading temperature and fuel gauges

Post by Heuer » Thu Apr 05, 2018 11:50 am

There is a thread with components and diagrams here: viewtopic.php?f=9&t=5011

You really need to consider adding a 0.33uF capacitor on the input to the transistor to smooth out any transient voltages from the charging system.
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#9 Re: Under reading temperature and fuel gauges

Post by jagwit » Thu Apr 05, 2018 12:07 pm

Heuer wrote:
Thu Apr 05, 2018 11:50 am
You really need to consider adding a 0.33uF capacitor on the input to the transistor to smooth out any transient voltages from the charging system.
From engineering principles, one can not argue against that recommendation for a truly stable output. However, considering how crude the original voltage "stabiliser" is, and how acceptable the resulting display with it, would it really matter if the 7810 output is not as smooth as it could be with a smoothing cap? Both my own V12 and this 4.2 S1 are now running on the 7810 alone with perfectly good readings resulting from it.

But, for our entertainment, I will scope the output as it is now - without smoothing caps. And maybe I'll add smoothing caps to see the changes. It might be weeks before I get to do this on the S1 though. In the mean time, I'll scope my V12.
Best Regards
Philip
Jag: 72 S3 XKE, 74 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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#10 Re: Under reading temperature and fuel gauges

Post by jagwit » Thu Apr 05, 2018 12:58 pm

jagwit wrote:
Thu Apr 05, 2018 12:07 pm
But, for our entertainment, I will scope the output as it is now - without smoothing caps.
Here are the results measured on my S3 V12 with a 7810 installed inside the voltage stabiliser housing - without any smoothing caps on either input or output. Note that I had both channels set to DC, both channels unfiltered (i.e. high frequencies are not filtered out)

To see the traces really nice, click once on the pictures, then right-click on the picture and select "view immage" and then press F11 to make it go full screen.

12V input trace is blue, calibration on the left.
10V output trace is red, calibration on the right.

We can see the input voltage at 12V (ignition on, fuel pump running, engine not running) and the 10V output signal. One can see rather huge spikes on the 12V signal coming from the OEM fuel pump, but the 10V signal is clean, for all it matters.
10V unfiltered.png
10V unfiltered.png (58.51 KiB) Viewed 3867 times
Now we look at the two signals with both probes set to AC, so that we can only see the AC component in both signals. Amazingly the 12V input signal has spikes (very briefly) to 8V (ABOVE the 12V input) and to -3.4V ( very briefly pulling the 12V line down by that much)
10V AC unfiltered.png
10V AC unfiltered.png (67.86 KiB) Viewed 3867 times
Finally, I zoom into one of those spikes (trace time base now at 100us/div) to see more detail of what happens there. Now we can see that each of those spikes is not not one spike but several!! I presume this is due to the mechanical contacts within the fuel pump "bouncing" (or arcing??) as it opens - making and breaking several times - before finally breaking.
10V AC unfiltered zoomed.png
10V AC unfiltered zoomed.png (67.7 KiB) Viewed 3867 times
Marek did invite me to look at how noisy the 12V input is. Now we can see, indeed it is. But we can also see how good a job the 7810 is doing at putting out a steady 10V despite that noisy input with a ripple of only 0.2V - and then, only during those spikes.
Best Regards
Philip
Jag: 72 S3 XKE, 74 S3 XKE OTS, 80 XJS (Megasquirt + 5sp manual O/D)
Jensen: 74 Interceptor (EFI by Megasquirt + O/D 4sp auto)
Chev: 59 Apache std, 70 C10 (350V8, 700R4)

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#11 Re: Under reading temperature and fuel gauges

Post by Heuer » Thu Apr 05, 2018 1:01 pm

Texas Instruments recommend caps on both input and output but I reckon only the input poses a risk more to the transistor than anything else. I put mine into the original VR can so no special installation is required.
David Jones
S1 OTS OSB; S1 FHC ODB
1997 Porsche 911 Guards Red

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#12 Re: Under reading temperature and fuel gauges

Post by johnetype » Fri Apr 06, 2018 7:28 pm

The 7810 regulator works fine as you've shown but for a few pence more I'd suggest using the LM2940-10. The reason being that the 2940 series was designed to operate in the automotive environment and is better protected than the 7810 from voltage spikes and also has a much lower drop-out voltage of 0.5 volts. As your traces shown, even in the basic E type environment there's quite a lot of noise and spikes.

I'm not aware of a +ve earth version of the 2940 so for that environment it would have to be the 7910.
John

1969 Series 2 FHC

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