Stock 4k tach problems

[travis401]

So I bought a cluster off someone on here with the 4k diesel tach with integrated fuel gauge. Got everything hooked up and the tach doesn't work right. It zeros out when I turn the ignition to on, but when I start the truck, the tach only reads about 150-200 rpm and when I rev it, the needle does move up, but just barely. I know there's a post on here to fix non working 5k tachs by cutting a couple of traces on the circuit board and soldering in a resistor, but the 4k diesel tach is a different board and the traces to cut, aren't there. Ive already tested the signal coming from the distributor with my Autometer tach and it reads fine, so I know the problem is in the tach itself.

[RandyP]

Not to sound stupid, but just to know for sure. Are you installing this in a gasoline truck or a diesel truck? And is it for an 8 cyl diesel or a 6 cyl diesel?

[travis401]

Its going in a gas v8. The v6 tach be made to work in a v8 by adjusting the potentiometer used to make the non working 5k gas tach work again. I just need to know where that pot should go on the diesel tach board.

[motornut]

no chance it is a simple snip here eh? like aftermarkets?
what would they be for?

i've seen these online but don't know how they work
http://www.dakotadigital.com/index.c...127/prd127.htm

[D13]

Factory diesel tachs work by reading alternator phase pulses. No way is it going to read any type of iginition coil correctly.

[travis401]

Originally Posted by motornut

no chance it is a simple snip here eh? like aftermarkets?
what would they be for?

i've seen these online but don't know how they work
http://www.dakotadigital.com/index.c...127/prd127.htm

from what ive gathered, that spot was for a little clip on to calibrate the tach for the different diesel options.

[superwade57]

Originally Posted by D13

Factory diesel tachs work by reading alternator phase pulses. No way is it going to read any type of iginition coil correctly.

Ill ask the question.... is there a way to make that work on a gas v8?

[ray_mcavoy]

Originally Posted by D13

Factory diesel tachs work by reading alternator phase pulses. No way is it going to read any type of iginition coil correctly.

Actually, these ones take a reading off a magnetic pickup coil that senses the flywheel teeth:



And those slots that motornut pointed out were used for calibration purposes:





I don't know where a person would find those factory white or orange jumper pins. And even then I don't think they'd help with calibrating one of these tachs to work with a gas engine.

From the pics of the circuit board, it does appear to be based off the same basic design as the gas engine tachs. Those use a GM house numbered chip that's basically a National Semiconductor LM1819.

If you download the datasheet for the LM1819 chip at http://www.alldatasheet.com/datashee...SC/LM1819.html and take a look at the "typical application" schematic on the first page you'll notice that it's pretty similar to what GM used. Resistor R2 that's connected between pins 5 and 8 is what is responsible for needle deflection.

From the factory, R2 is one of the many resistors that are located in that white ceramic chip on the circuit board. As you've read, the fix for 5000 rpm tachs that don't read correctly is to cut one of the traces leading to R2 and solder in a potentiometer as a replacement. Even though the traces have a different physical layout on this circuit board, I think you can still do the same thing if you're willing to take a chance on it.

From what I can see in your pic, it looks like there is a trace on the foil side of the board connecting pin 5 of the LM1819 (black chip) to pin 6 of the resistor network (white chip). And a trace on the component side of the board connecting pin 8 of the LM1819 to pin 3 of the resistor network. That means that the calibration resistor R2 most likely uses pins 3 and 6 of the white chip. Cutting either one of those traces should take the old R2 out of the circuit and allow you to replace it with a potentiometer.

For example, Cut the trace on the top (component) side of the board that connects the LM1819 pin 8 to the resistor network pin 3. Then connect the potentiometer between pins 5 and 8 of the LM1819. I can't guarantee that this will allow that tach to work with a gas engine but I think there's a good chance it will work.

[travis401]

Thanks, ill give it a shot when I can...worst case scenario is I completely destroy the tach right?

[ray_mcavoy]

Originally Posted by travis401

Thanks, ill give it a shot when I can...worst case scenario is I completely destroy the tach right?

Replacing the calibration resistor is a fairly low risk procedure and shouldn't cause complete destruction. Just be careful when soldering in the potentiometer so you don't overheat any components on the board and/or accidentally create any solder bridges that short circuit to nearby pins/traces. If physically locating the new potentiometer to where it's leads will reach the necessary points on the board is a problem, you can solder some light gauge insulated wires onto the pot and mount it wherever is convenient.

The pin numbers I mentioned in my first reply are based on your tach using a chip that is pinout compatible with the LM1819. From what I can see in your photos, that appears to be the case. But to be on the safe side, you might want to double check by carefully comparing your circuit board to the "typical application" schematic from page 1 of the LM1819 datasheet. Then you can be sure you are dealing with a compatible chip and correctly locate the trace(s) that need to be cut to take the old calibration resistor out of the circuit and locate the connection points for the new potentiometer.

[83GMCK2500]

Resurrecting this thread, because I just encountered a nearly identical problem. I picked up a late style 4K tach/fuel combo cluster out of a medium duty with an 8.2L diesel in it. I could have sworn I disconnected the wiring from the alternator, unable to find the harness for it at the moment to confirm.

I'm installing it in a 1987 V10 6.2 diesel truck. I just installed a 12SI alternator with an R-terminal and the tach zeros with power, shows ~125RPM at idle, will rev up to ~600 if I rev the engine up a ways. Was there a way to drive one of these 4K tachs with an alternator pulse?

[83GMCK2500]

Bump. I dug up this old thread because the problem seemed unresolved in this thread, why make another post for exact same topic? I'm still looking for ideas/answers.

[nonstop]

What I did - grabbed a magnetic pick up from a medium diesel truck with the tach pick up off the flywheel. I then made a bracket to mount it to pick up off my flywheel. One wire off the sender goes to tach pick up and the other to ground. My tach was off - different amount of teeth on the flywheels. I bought a convertor, mounted it in line, and adjusted it to read as close as possible.

[Corts60]

I have a gas 5k tachometer in my diesel truck and I run it off of the R terminal on my alternator. The trick is, you need Dakota Digitals tach interface to make it work.

[83GMCK2500]

Originally Posted by Corts60

I have a gas 5k tachometer in my diesel truck and I run it off of the R terminal on my alternator. The trick is, you need Dakota Digitals tach interface to make it work.

THANK YOU! I wasn't sure if that would do it or not and I've already got the new alternator with R-terminal installed.

Do you use the one with the 4 switches or the one with the digital display and two buttons? (I think they're DSL-1 & DSL-1E, respectively.)