Instrument Voltage Regulator

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Instrument Voltage Regulator

Postby Simon_P » Wed 09.01.2019, 17:12

Fed-up wit the fuel and temp gauges being inconsistent I have decided to change the voltage regulator.

I'm sure that there was an article somewhere that that showed how to change it and gave the part number....

After looking I found this
viewtopic.php?f=37&t=20752&p=280711&hilit=LM2937ET#p280711
which wasn't the post I remembered it also shows LM2937ET-5 and I recall it being -10

After some more searching for instrument bulbs I came across this in the wiki
http://wikilec.com/view/Instrument_panel
which states LM2937ET-10

The package appears to be TO-220
but is the output voltage 5v or 10v ?

Thanks,
Simon
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Re: Instrument Voltage Regulator

Postby Brit-Car-Nut » Wed 09.01.2019, 17:43

Simon:

I ordered one years ago from Lotus (A100N6007S) and I received an unidentifiable voltage regulator. Although I expected a TO220 package I got a chip I couldn't cross-reference.

I never followed up on it as 10V would be more expected. I will dig it out of the boxes and post a picture. I will also pull a facia assembly down and take a picture of the back-side to see if I can find any details.

I know the GM tank unit that Lotus used wasn't exactly linear in it's design so the gauge could rarely be trusted. I gather Lotus used GM sensors but a different brand of gauge and their electrical properties don't really match.

Do both of your gauges read consistently high or low? That would be an indication of a bad voltage regulator. Didn't Geoff work on a way to correct the fuel gauge mis-readings?
Last edited by Brit-Car-Nut on Thu 10.01.2019, 14:13, edited 1 time in total.
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Re: Instrument Voltage Regulator

Postby GeoffSmith » Wed 09.01.2019, 17:56

Simon, it's 10V. I'll be in touch tomorrow when I get home. About this and the lighter...
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Re: Instrument Voltage Regulator

Postby Simon_P » Wed 09.01.2019, 20:54

Hi John,

Yet that was my reason for repeating the package I've also ordered IC's before and received something much smaller than expected.

Not too fussed about linearity, the Blue car has got to the stage where the low fuel warning comes on when the gauge shows near 1/4 full, and its ok when the fans come on just above half, but just over 3/4 gets me a little worried. I'm using the care every day and on long journeys so I need it to be consistent.

Thanks Geoff, no hurry I'm fixing the mirrors at the moment so I can see other traffic, then I'll do the dash light so that I can see what's going on in the dark.
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Re: Instrument Voltage Regulator

Postby Brit-Car-Nut » Wed 09.01.2019, 22:31

Sorry for the confusion. Up until recently, I didn't have a spare cluster so had not seen the installed regulator.

Here are some pictures. The stock Lotus regulator is an ITT chip and the other numbers don't identify it. Since it ended in 5, I mistakenly assumed that was the voltage.
The correct voltage is 10V, not the 5V I thought I got

I used the non-Lotus one to show the difference between the original and the TO220, not to show the voltage.
Click on any of them for a larger view

I lined the original one with the 78 series for reference.

Cluster1.jpg

Cluster2.jpg

Voltage Reg1.jpg

Voltage Reg2.jpg
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Re: Instrument Voltage Regulator

Postby GeoffSmith » Thu 10.01.2019, 12:53

Back to a proper computer now, so just a few quick points.
1. My Wiki info says it all: The original regulator is +10V low dropout voltage (i.e. will operate with only half a volt between input and output) and will tolerate input voltage spike is of up 36V. My suggested replacement (as far as I can tell the same spec apart from reduced 0.5A output current) is the LM2937ET-10.
2. The part discussed in the thread referenced by Simon is wrong - I have added a warning.
3. It looks like Lotus are new shipping LM7805ACT as a replacement. The spec. for this:
Output voltage: 5V - completely wrong!
Dropout voltage: 2V - gauges may read low at tickover.
Maximum input voltage: 25V - small risk of going pop with spikes from e.g. rough alternator output.
Operating temperature: 0°C to 125°C - don't expect the gauges to work in the snow.
4. The originals are nearly 30 years old (date coded 90XX) and we will probably see more failures over the years. The LM2937 seems to be NLA (no longer available) from reputable suppliers and leaded (TO220 package) components are becoming obsolete fast. I will look into this when time allows.
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Re: Instrument Voltage Regulator

Postby Brit-Car-Nut » Thu 10.01.2019, 14:09

False statement...
"It looks like Lotus are new shipping LM7805ACT as a replacement."
End of False statement

I got it wrong a few years ago when I received the original one in the Lotus package.

The most recent picture is just me using a spare Voltage Regulator to show the SIZE difference between the correct 10V one from Lotus and the the industry standard TO220 package.

I am removing my mistakes as fast as I can.
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Re: Instrument Voltage Regulator

Postby GeoffSmith » Wed 10.04.2019, 14:50

Well it looks like there's another bum steer in all the information!

I have tracked down the GM part number to the ITT generic part number and found a datasheet and compared it to the purported replacement regulators: THE INPUT AND OUTPUT PINS ARE TRANSPOSED. I have confirmed this on the instrument panel pcb so I can't believe that anyone has successfully replaced one with a LM2937 or similar.

I will have a solution in a couple of days. If you want to try it out Simon then send me your address by e-mail.
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Re: Instrument Voltage Regulator

Postby Simon_P » Wed 10.04.2019, 15:13

As it happens I'm sorting through problems... As I type I have the steering (doesn't) lock in my left hand.

When I've put it back I'll take some readings from the Voltage (doesn't) stabiliser.
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Re: Instrument Voltage Regulator

Postby Simon_P » Wed 10.04.2019, 20:46

I thought taking a few readings would be straightforward....
Couldn't reach the voltage stabiliser so took the fascia panel out.

Readings were 10.03 volts +- 0.01 volts over about 15 minutes with various loads switched on. Not much wrong with that then.

I did notice that the temp was at the correct mark and the volt gauge read near 14v rather than 12 1/2 v so perhaps I disturbed a bad connection.
DSC_0759.JPG

In situ
Top leg Vbatt
Middle Ground
Bottom +10v
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Re: Instrument Voltage Regulator

Postby lotusflasherman » Thu 11.04.2019, 16:23

Simon_P wrote:I thought taking a few readings would be straightforward....
Couldn't reach the voltage stabiliser so took the fascia panel out.

Readings were 10.03 volts +- 0.01 volts over about 15 minutes with various loads switched on. Not much wrong with that then.

I did notice that the temp was at the correct mark and the volt gauge read near 14v rather than 12 1/2 v so perhaps I disturbed a bad connection.
DSC_0759.JPG

In situ
Top leg Vbatt
Middle Ground
Bottom +10v


Instrument Wiring Diagram (Section MJ Sheet 24) shows Voltage Stabiliser to give 10V +/- 0.1V.
If you are running the engine the alternator regulated output voltage should be between 14.2 to 14.8 with no significant electrical load and that will show on the Volt Gauge.
When the engine is stopped and ignition on Volt Gauge will show battery volts, hopefully 12 + a bit.
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