73-87 Tachometer Repair

[crazy4offroad]

OK when you know you're over your head sometimes you just gotta admit it. I can do a lot of things for an electrical problem but there are some things beyond my knowledge, which is why I started this thread. Here I will post pics of my tach and see if I can arrive at a working tach by the end, and hopefully this thread will be chocked full of info for others in the future.

First thing I noticed upon inspection was a burnt circuit on the circuit board. Always look for the obvious first, sometimes it can be something simple. I trimmed back the burnt circuit to the positive (+) post, then soldered in a jumper wire from the positive post to where it was supposed to go...

Re-assembled and tested, and it did not fix it. Instead of reading 1500 RPMs with no response to throttle, it now read 2500 RPMs with no response to throttle. The next resistor in line from the (+) shows zero ohms of resistance so I'm partly wondering if it may be bad, too early to tell though. It could be a diode instead of a resistor, which I believe works like a check valve to prevent back feeding of voltage.

Next came research lol. After many google searches with different terms, the best thing I could come up with was this page on "GM Tach Repair"...
http://www.howtoalmanac.com/kevin/projects/automotive/tachfix.htm
Jumping the gun, I went ahead and ordered the 500k ohm 12-turn potentiometer described in the write-up, when I should have paid attention to the graphic at the top of the page that shows the tach still responds to throttle. I have doubts the method outlined in that write-up will fix this tach. Down the page he makes this statement...

"The tachometer system uses a National Semiconductor LM181 with a GM part number on it. if your tach is completely dead you can replace this chip with the NTE01670 as the LM181 is no longer manufactured. this is the other chip that is completely black. not to be confused with the resistor chip. if your tach works at all the LM181 is fine."

Which is kind of leading me to believe this could be what's wrong with mine. This lead me to finding info on the LM181 air core meter driver (closest I could find is the LM1819) and I found this PDF tech sheet on it...
http://www.ti.com/lit/ds/symlink/lm1819.pdf

Since it is obsolete it also lead me to find this PDF tech sheet on the NTE01670 integrated circuit air core meter driver...
http://www.nteinc.com/specs/1600to1699/pdf/nte1670.pdf

So now I have a couple schematics to do some testing. I have a computer power supply I modified to use as a 12v 18 amp power source for testing. It has built-in power protection and when I hooked it up to the (+) and (-) of the tach the circuit protection tripped in the PSU. Now I'm wondering if something is shorting out on the tach's circuit board.

Near as I can tell, the write-up at the howtoalmanac.com website above outlining the resistor chip, this is the chip on my board...

The pin described as having 2 leads going to it (#10) is circled in red below, and #4 circled in blue...

IF it turns out the potentiometer is required I am thinking these are the 2 pins to work with. The flipside of the chip looks like this, remember it is reversed and the circles are correct...

The thing that concerns me about this chip is the solder joints on two sets of pins seem to be touching, and I'm not sure if they are supposed to, circled in red below...

So now I have some testing to do. I will try to verify the LM181 is good, and get an ohm reading for pins 4 and 10 on the resistor chip, and post back the progress later...

 

[crazy4offroad]

After a lot of reading and testing I am coming to the conclusion the air core meter driver chip (the black chip) is bad, since the tach is unresponsive to throttle. However is is discontinued and I can't find a cross reference for a replacement. The write up about the Camaro tach describes the chip as an LM1819 and the pics I've seen show the number LM1819 right on the chip, however mine does not say that. If someone could give me a hand finding out what chip I need or the replacement for it, would be great! Here's a closeup pic of it...

The insignia at the beginning I am guessing is "NS" for National Semiconductor but every search I've done for the numbers after that have lead nowhere. I would hate to order the LM1819 and it not be the same thing as what I need.

 

[konakustoms]

69.00 for the chip but cant get the site to post......Must be our site blocking me. Let me do it a different way....give me a few min..

 

[crazy4offroad]

Post the search terms you used on NS (or Digi-key if you used them, or wherever lol). Wow $69 is very expensive compared to the Hong Kong ebay LM1819 chips, averaging $15.

 

[konakustoms]

I just did a Firefox search on it and it poped up on a couple different sites

 

[bucket]

Dang, a $69 hunch. I'm starting to see why it can be so expensive to have these tachs repaired.

 

[Roadhazardguy]

Hey I just joined so I could see your pictures because im trying to fix the one for my 78. Mine looks a little different than yours and mine does respond to throttle but it reads way too high. In your case I would buy a just buy a replacement circuit board from http://willcoxcorvette.com/product_info.php?cPath=1810&products_id=26788 , they make replacement circuit boards for corvettes and our trucks, when you order you have to put in the comment box to note that your ordering for a chevy truck. I have a thread about some different tachs ive found on another site too http://67-72chevytrucks.com/vboard/showthread.php?t=503733

I got ahold of Kevin that made the "How To Almanac" after I had ordered some potentiometers : http://www.ebay.com/itm/220870376302?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 and he said these were fine.

I think im going to send him another email so he can verify the correct trace to cut and solder to as id like to be sure before I mess something up.

 

[crazy4offroad]

Thanks for the info, I'm pretty much in the same boat, scared to dedicate to a cut and end up being wrong. I sat and ran all kinds of tests on the whole board, not really knowing what to look for but drew a basic map of the board and wrote down all the readings. I would hate to cut something and do the potentiometer mod if it is unnecessary. I'm thinking a couple resistors immediately after the (+) connector got torched, as they show no resistance just straight through current. If I knew what it was supposed to be, I would know what to fix. As of now I'm going with the Camaro tach/speedo setup. Higher redline, higher speedo and hopefully not too much trouble to mod in.

 

[Roadhazardguy]

With Kevin's help I got my tach fixed and now its working as it should. I sent him pictures of everything and he said he was going to add it to the How To Almanac.

 

[crazy4offroad]

Glad to read you got yours fixed, but even yours looks different than mine, I cant figure out why I got the bastard of all tachometers lol. I guess I may as well send it off when I go to fix it, meanwhile I will be trying the Camaro tach. Thanks for the update Tony!

 

[Roadhazardguy]

No problem Curt! Here's that link again for the replacement circuit board: http://willcoxcorvette.com/product_info.php?cPath=1810&products_id=26788 It didn't work the first time i posted it, they are $58 and when you order you have to put in the comment box to note that your ordering for a chevy truck. I don't know why GM changed the tach around on our body style trucks.

 

[crazy4offroad]

Cool, thanks! I wonder if they can do one with a higher redline, or how you calibrate it to match your particular rpm spread? I know they have 5, 6 and 7k rpm tachs for our trucks. Guess I'll have to contact them and see!

 

[Roadhazardguy]

They probably only make them for the stock 4 or 5K tachs, then you could fine tune it with the POT

 

[Dr. Paradox]

the number on that chip is the GM special part number that they had the manufacturer put on the LM181's for them

-Kevin
(howtoalmanac)

let me look through the pictures you have here and I might be able to get you an answer as to why your tach doesn't work.

I am also refining the website as a couple things weren't clear I guess

I'll put up a page with Tony's tach and pictures and we'll see what we can do

 

[Dr. Paradox]

ok, here's why your board is different from tony's and mine. your board has traces on the top and bottom. that's why it seems like nothing connects on the bottom.
it looks like the two sets of pins that are solder bridged are supposed to be that way.

can you put up some pictures of the top of the board so I can get an idea what's going on?

-Kevin