LM3914N IC Problems (A/F Ratio Meter)

SgtWookie

Joined Jul 17, 2007
22,230
I purchased the .22uf cap. I'm about to solder it to the circuit. Just want to make sure I get this correct. Power wire stays as is to the LM340 input; add the .22uf to LM340 input;and connect the ground to both the .22uf cap and GRD Term of the LM340?
Yes.
If the capacitor is polarized, make sure that you install it correctly.
Tantalum capacitors have the + lead indicated.
Aluminum electrolytics have the - lead indicated.

Then apply power, and measure the output voltage of the LM340. If it's still around 3v, you'll need another regulator.
 

Thread Starter

motown07

Joined Nov 3, 2009
17
I changed the capacitors and it worked. as soon as the o2 sensor would warm up the meter would die out. I got so frustrated I took all the main components of the board. It wasn't working like was supposed to. It shouldn't light up w/ 0v. As soon as the o2 sensor sent any voltage the the circuit the lights would turn off. It was working backwards.

Back to the drawing board. Bought a new board and found different diagram. Hopefully this is it. New diagrahm http://74.54.120.132/dbbp/tech-tips/mm.html

if anyone sees a potential problem w/ this diagrahm feel free to post a response.
 
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Thread Starter

motown07

Joined Nov 3, 2009
17
So I tried the new circuit and it works. Yeah finally.

One thing. Whenever I turn the headlights the circuit loses power and only 1 led stays lit. Do you think that if I add a 2.2uf cap between the power source and the ground it will stop this problem?
 

SgtWookie

Joined Jul 17, 2007
22,230
So I tried the new circuit and it works. Yeah finally.
Cool. :cool:

One thing. Whenever I turn the headlights the circuit loses power and only 1 led stays lit.
That's not good. :(
Do you think that if I add a 2.2uf cap between the power source and the ground it will stop this problem?
If you don't have a cap across the input of your regulator to ground, you need one.

Also, check your ground strap from the engine block to the firewall. If it's corroded, loose, frayed or missing, that's a problem.

Additionally, check your "ALT" idiot light on the dash with the engine running in the dark. Turn the headlights on; dash lights dimmed way down. If there is any light coming from the ALT light, even dimly, your alternator very likely has either a bad diode trio, or a bad rectifier bridge. These parts are very common to go bad on GM alternators across many years/makes/models.

The diode trio is a 3-phase 1/2 wave rectifier that feeds current to the integral regulator. If one of the diodes in the trio goes bad, your alternator will only be generating 2/3 of it's capacity. It will also cause the voltage in your car's electrical system to fluctuate wildly from 12v to 14 or 15v, which will mess up your sensor.

Same thing with the rectifier bridge; but the rectifier bridge is on the high-power side of the alternator.

These alternators are really easy to take apart, swap out the dead bridges/diode trios and slap back together. The pain is getting them out of the car in the first place without snapping anything off or busting your knuckles on something.
 

Thread Starter

motown07

Joined Nov 3, 2009
17
This new circuit doesn't have a regulator. I've looked at the diagram a few times and I can't figure out how it works, but it does.

I did notice that the voltage does increase and decrease from 12 to 15 volts.

I'm going to take it in to get it checked at the Autozone. If it's bad I'll give it the good old college try and attempt to fix it myself.

So if I continue to run the A/F meter, the Alternator will eventually mess up the O2 sensor?
 

SgtWookie

Joined Jul 17, 2007
22,230
This new circuit doesn't have a regulator. I've looked at the diagram a few times and I can't figure out how it works, but it does.
I completely missed your post of a couple days ago about you finding a new diagram. I've been busy, sorry. Don't have time or energy right now to review it - but an off-the-cuff guess says that the new schematic uses the regulator that's built into the LM3914.

I did notice that the voltage does increase and decrease from 12 to 15 volts.
You have something strange going on then. Measure from your negative battery terminal to the frame; see if you measure any voltage - with and without the headlights on. If you do, your ground strap is bad/missing. If you don't, your regulator or perhaps the rotor brushes are going south.

I'm going to take it in to get it checked at the Autozone. If it's bad I'll give it the good old college try and attempt to fix it myself.
It's really not hard to do. Scribe a line (or use a Sharpie) between the alternator halves before taking them apart so that you get them back together the same way - otherwise it won't fit.

So if I continue to run the A/F meter, the Alternator will eventually mess up the O2 sensor?
No, but you'll have wacky things going on with your car's electrical system.
 

punisher454

Joined Jun 29, 2009
16
I have some real world experience with narrowband O2 sensors on a carbed engine. I used to have an edelbrock narrowband unit, basically a series of led's that light up as the voltage increases.

A NB setup is no substitute for a wideband unit, but it can be helpful. Keep in mind that an O2 sensor works almost just as good as an EGT sensor as it does as an O2 sensor, they are VERY sensitive to temperature.
At normal operating temperature most narrowband O2's will put out about 450mV at around 14.7:1 with gasoline. The big problem is that the sensor output is on a really terrible curve above and below the mid point. You wont get very useful voltage levels outside of the 13.5-15.5 range, and that may be a bit optimistic. Don't believe the numbers associated with the LED's on units like the edelbrock or halmeter they are complete B.S.!
Anyhow, with a carb you can get good steady readings and will know when you are rich, lean and in between, but you wont know how much. Use it as a reference point only, and don't worry about converting actual values of the output to real world AFR numbers.

Now the much better way is to buy a "JAW" wideband kit. Do a google search, it works good and is real inexpensive. it uses the LSU 4.2 sensor(most popular wideband) which can be had for about sixty bucks.

Be aware that building your own wideband driver circuit is a real nightmare, they have very specific driving requirements for the ion pump, heater and Nerst cell. Right now the "JAW" option would be REALLY hard to beat by designing your own circuit. And now there is an even lower cost version of the "JAW" called the "NAW", check it out.

Marvin
 
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SgtWookie

Joined Jul 17, 2007
22,230
How did things go with the electrical system testing?

Were you missing the ground strap from the block to the firewall?
Or maybe from the battery ground terminal to the frame?
Any alternator problems?

The link you posted to the new A/F tester - there wasn't a schematic, so I started to create one from the board image. Looks like it has a maximum range of 1.25v. It's really a non-standard configuration for the LM3914, but I haven't had time to really go over it.
 

Thread Starter

motown07

Joined Nov 3, 2009
17
The ground strap from the battery to the body was fine. I also re-checked the voltage when the car is on. It only drops .5 volts when I turn the lights on or hit the brakes.

A weird thing happens when I check the a/f meter. Whenever I have my hand close to the LEDs w/o touching them, the led bar graph will increase by 1 led. Any clue why this might happen?
 

SgtWookie

Joined Jul 17, 2007
22,230
Sounds like you have an electrical noise problem.

Did you add a capacitor across the +12v and ground supply lines?

Perhaps the 0.047uF cap on the LM3914 board is not good, or the wrong size. If you bought an assortment of ceramic caps from Radio Shack and used one that had a 47 on it, that's 47pF not 0.047uF. 47pF is 0.000,047uF; about 1,000 times too small.
 

Thread Starter

motown07

Joined Nov 3, 2009
17
No its a .047uf. I know this cause I originally purchased a .47uf and didn't realize it until i got home. I was so mad.

The is no cap between 12+v and ground. Any suggestions as to a size? Maybe a 2.2uf cap?
 

Thread Starter

motown07

Joined Nov 3, 2009
17
I still have the .22uf that I used on the old a/f meter. Can that be used? How do you determine which cap would be appropriate for a particular application?
 

SgtWookie

Joined Jul 17, 2007
22,230
I still have the .22uf that I used on the old a/f meter. Can that be used?
It'll be a step in the right direction. You can always add another later, if need be.

How do you determine which cap would be appropriate for a particular application?
Looking in the datasheet for a part is always a good place to start. If you look at a datasheet for an LM3914, you'll see a 2.2uF capacitor across the ground and Vled supply wires. It's shown with dotted lines, which means it's optional. But really, it's only optional if your power/ground wires are less than about 6" long from the IC to the battery.
 
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