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Fuel and Temperature Gauge Malfunction
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kamzcab86
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PostPosted: Thu Jan 26, 2017 1:29 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

dhaavers wrote:
Specs on your VR from Mouser say minimum voltage 12.8v/100mv "line regulation" but the
item (Casey) posted from DK specs min. 11.5v/0.5v "voltage dropout" & the other (Vanderblt)
posted simply says "up to 35v" input/2v "voltage dropout"...sounds like apples & oranges?


Original stabilizer:
Manufacturer: ITT Semiconductor
Type: TCA700Y
Housing: TO202
Specs: 10V output, 220mA current limit, 12V to 16V input
Output tolerance: ±2%
Max operating temp: 125°C
Storage temp: -40°C to 125°C
Pins: 1 (output), 2 (ground), 3 (input)

Direct replacement stabilizer:
Manufacturer: Fairchild Semiconductor
Type: KA7810A
Housing: TO220
Specs: 10V output, 250mA current limit, 12.5V to 25V input
Output tolerance: ±2%
Operating temp: 0° to 125°C
Storage temp: -65°C to 150°C
Pins: 1 (input), 2 (ground), 3 (output)

dhaavers wrote:
12.8 minimum sounds to me like borderline trouble...although I suppose in operation
the VR sees charging voltage from alternator, not just standing battery voltage...???


Once the engine is running, stabilizer should see charging voltage.

These stabilizers have been replaced in countless Cabriolets (most, if not all, VW's of the '80s use the same stabilizer) over the last decade; haven't heard of any issues using the direct replacement KA7810. If you buy the replacement from VW (or Van Cafe) you will get KA7810 with heat sink spacer.

dhaavers wrote:
Considering the <slight> differences in specs of the three options posted here, is there
a good/better/best recommendation? ...or am I nervously overthinking the operation
of a very simple little gizmo? (Wouldn't be the first time for that... Wink )


If you want the best replacement, it would be this one: http://www.ebay.com/itm/Voltage-Regulator-Repair-K...mp;vxp=mtr . As pablum mentioned, the newer TO220 replacement is 180° off from the original TO202. The original stabilizer's mounting tab was mounted directly to the instrument cluster's heatsink; to mount the new version properly, it will need a metal spacer (available at hardware stores if you wish to save dinero). The eBay seller is providing not only a spacer, but a new (additional) heatsink as well.

Or, buy the original equipment while you can: https://www.ecstuning.com/b-genuine-volkswagen-audi-parts/voltage-regulator-priced-each/171919803/ .

Vanderbuilt wrote:
So I'm not sure if the LM7810CT is a direct replacement for the original foil "board". Also, I believe DigiKey carries a LM7810ACT but I didn't like the temperature ratings so I stuck with the LM7810CT.


Manufacturer: Fairchild Semiconductor
Type: LM7810ACT
Housing: TO220
Specs: 10V output, 250mA current limit, 12.5V to 25V input
Output tolerance: ±2%
Operating temp: 0° to 125°C
Storage temp: -65°C to 150°C
Pins: 1 (input), 2 (ground), 3 (output)

Manufacturer: Fairchild Semiconductor
Type: LM7810CT
Housing: TO220
Specs: 10V output, 250mA current limit, 12.5V to 25V input
Output tolerance: ±4%
Operating temp: -40° to 125°C
Storage temp: -65°C to 150°C
Pins: 1 (input), 2 (ground), 3 (output)

Operating temperature range and output tolerance are higher for CT than the ACT.
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pablum
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PostPosted: Thu Jan 26, 2017 3:33 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

Quote:
Specs on your VR from Mouser say minimum voltage 12.8v/100mv "line regulation"


I'll take a stab at this:

The VW spec requirement for the 'stabilizer' is 10V +/- 500mV (according to Kam's image),
and the 'line regulation' 100mV is the part's output voltage accuracy at the nominal input voltage of 12.5V.
So that's not a minimum input voltage, but the input voltage at which they tested variance in the output voltage.

Low voltage drop-out is an extra feature that didn't even exist back in the dark (polyester) ages.
I think that'll just shut down the output altogether when input is below (in this case) 2V.
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Vanderbuilt
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PostPosted: Thu Jan 26, 2017 5:38 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

Since it looks like I have some guru's here. I'm curious if you think that my choice of the LM7810CT will be acceptable. I created my own circuit board and can swap it with a KA7810A if I need to. I also have yet to install it so would like to hear if this will be a problem before I do. Don't really need to ruin anything right now. So here's my monstrosity, which I pretty much did similar to a (now forgotten) member on the club 80-90 forum. Anyway, input appreciated.

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dhaavers
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PostPosted: Thu Jan 26, 2017 5:55 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

pablum wrote:
[Low voltage drop-out is an extra feature that didn't even exist back in the dark (polyester) ages.
I think that'll just shut down the output altogether when input is below (in this case) 2V.

Whoa...I took "voltage dropout" to be equivalent to "line regulation", as in "0.5v voltage dropout" = "500mv line regulation"...???

And to restate my original question: still curious how you would rate the three posted (>$2 VR's) from good-to-better-to-best...???

- Dave

PS: If I can get a "good" one for $4 shipped (instead of $22 or $40) I'll do that. I've said it before...I'm CHEAP... Wink
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pablum
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PostPosted: Thu Jan 26, 2017 6:17 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

I haven't looked closely enough to distinguish between them, but I think they are essentially identical.

You want at least 2V difference between Vin and Vout, and as you approach that the variance will go up (I think they call 12.8V the limit because they want +/- 50mV variance. At 12.5V the datasheet says it jumps to 100mV) and the current output drops pretty dramatically below that.

It's not digital, it's analog, so the limits are a bit plastic according to the application.

P.P.S. If memory serves, kam's mouser link was listed at $0.80, so you are golden.
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wcdennis
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PostPosted: Thu Jan 26, 2017 7:58 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

The voltage regulator powers the gas and temp gauges. It keeps the needles from drifting around with changes in your electrical system. If you swap it for one with a different output, it would only cause the gauges to read a little higher or lower.
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kamzcab86
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PostPosted: Thu Jan 26, 2017 9:42 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

Vanderbuilt wrote:
I'm curious if you think that my choice of the LM7810CT will be acceptable.


Since you built your own, hard to say. If that's what GoWesty is using in theirs, it should be okay in yours. If you see the gauges acting a bit funny, swap it out for the KA7810A or LM7810ACT.

pablum wrote:
P.P.S. If memory serves, kam's mouser link was listed at $0.80, so you are golden.


Actually, $0.66... so, better than golden. Wink Very Happy
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PostPosted: Thu Jan 26, 2017 9:49 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

For the record, I posted that link to the voltage regulator after a very cursory search at the site Pablum recommended. I don't want anyone to get the erroneous notion that I actually know what I'm talking about here.
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PostPosted: Sat Jan 28, 2017 1:57 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

Thanks for this post. I changed out my voltage regulator from the one in my spare cluster and now it shows full tank again 😘. Before the change a full tank barely read over 1/2 tank. I was also having periodic temp light flicker, so far so good.

Also added a volt meter to the old clock location while I was there.


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WiboBusMan
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PostPosted: Sun Jan 29, 2017 2:31 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

Given that both gauges are reading low, I too would suspect the voltage stabilizer, but it may just be the connections - corrosion builds up over 30+ years.

I changed out my 1984's fuel sender, only to discover in the end it was corroded connections to the gauge ( the tank needed a bit of tlc in any event, but I could have done without the trouble)

So given you've got it all apart, you may as well clean the connections too.

Regards

W
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PostPosted: Mon Jan 30, 2017 8:35 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

I finally got the chance to pull the instrument cluster today and check the workings of the VR and yes it was bad. Replaced it with a LM7810CT and now have working gauges. Without the engine running (input voltage about 11.5V) the output of the regulator is pretty low ~8V so the gauges don't read accurately but with the engine running they seem to read just fine.
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PostPosted: Mon Jan 30, 2017 8:48 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

I don't know what a low voltage is for a car battery. Is that normal voltage?
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PostPosted: Mon Jan 30, 2017 8:53 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

pablum wrote:
I don't know what a low voltage is for a car battery. Is that normal voltage?


It is probably about 1/2 volt lower than I would expect to see.
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PostPosted: Mon Jan 30, 2017 9:40 pm    Post subject: Re: Fuel and Temperature Gauge Malfunction Reply with quote

The lm2910 part wasn't any better, based on what the datasheet had to say. They don't test below 13v on that one.

Even the adjustable VRs lose current flow rapidly when the voltage differential is small.
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