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mx71

Tach Mystery

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Ok, I can't figure out the tach. I did order a new resistor in case that's it, but I don't know how else to get it to work. Is something supposed to be actually hooked up to the coil? The resistor is just plugged into the connection. The white/blue wire has 12 volts and the black/blue wire has no power. Why is it by the coil if it isn't connected to anything? What should I be looking for? I've been scouring this forum, the internet, and the service manual and it isn't super clear.

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Are you just adding a tach? or the tach stopped working?

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I bought the truck with the tach not working. It's an 85 Deluxe King Cab. What goes to the negative on the coil, and which coil?

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I have an 82 my wire was blue. Follow your wiring diagram there should be a eye on the wire and it just went on the negative post of the coil. I believe intake, the one that doesn't cut off under load.

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There may be other wires attached to the coil negative (-) terminal, but the White/Blue stripe one goes through the 2.2K ohm resister and then becomes a Black/Blue stripe to the tach in the dash. The tach also has a Black ground wire and a Yellow/Red wire, connected to the illumination system (dash lights.)

 

The intake coil is usually the source for the tach signal but it will work just as well on the exhaust.

 

 

If you bought it not working maybe it's broken.

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Clearly it's broken. I'm just trying to troubleshoot to get it working. The diagram isn't clear that what color the wire is. So the wires with the resistor don't connect to the coil at all, or are they connected somewhere else? I couldn't find a new resistor, so I just ordered a general electronics resistor to test it. (2.2k ohm, 1w)

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Hey I just went through this in my truck.

How are you measuring things?

 

Look for the mentioned 2.2k resistor by the coils. It will be in a thumb-sized black rubber connector on the wire. If you pull it you can check the continuity between the resistor, coil and the tach connector without worrying about wire colors.

 

Since you have 12V, it seems like your fuse is fine. Make sure you have ground on one of the other pins (not both) and the third will be the signal to the gauge. If you have access to an oscilloscope you can check to see if the connector in the dash is getting any kind of signal.

A cheap DMM might also be able to register some kind of AC response.

 

Also, you can measure the tach meter. If you remove the gauge from the cluster, you should see some tiny, silver wires connecting the control circuit to the little springs that move the needle.

If you measure these VERY CAREFULLY you should get a resistance of some kind, probably around ~100Ω, mine was 70Ω.

The bad one died as 'open circuit' if I remember correctly, and the small voltage from my cheap DMM was enough to nudge the working needle.

 

Check this:

http://community.ratsun.net/topic/71865-84-datsun-720-gauge-hackery/

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Clearly it's broken. I'm just trying to troubleshoot to get it working. The diagram isn't clear that what color the wire is. So the wires with the resistor don't connect to the coil at all, or are they connected somewhere else? I couldn't find a new resistor, so I just ordered a general electronics resistor to test it. (2.2k ohm, 1w)

 

 

Well I thought I was clear where the tach wires are connected.... From intake coil negative terminal, to resistor, to tach. If the resistor is bad or the wires not connected these are the things, other than the gauge itself, that could cause it to not work.

 

There may be other wires attached to the coil negative (-) terminal, but the White/Blue stripe one goes through the 2.2K ohm resister and then becomes a Black/Blue stripe to the tach in the dash. The tach also has a Black ground wire and a Yellow/Red wire, connected to the illumination system (dash lights.)

 

The intake coil is usually the source for the tach signal but it will work just as well on the exhaust.

 

 

If you bought it not working maybe it's broken.

By broken I mean the gauge is at fault. The tach is quite complex, and so, is the most likely thing to fail. Two wires and a resistor are less likely.

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I haven't yet gotten to the point of opening the cluster, but when I do, I'll try this. Thank you!

 

Hey I just went through this in my truck.

How are you measuring things?

 

Look for the mentioned 2.2k resistor by the coils. It will be in a thumb-sized black rubber connector on the wire. If you pull it you can check the continuity between the resistor, coil and the tach connector without worrying about wire colors.

 

Since you have 12V, it seems like your fuse is fine. Make sure you have ground on one of the other pins (not both) and the third will be the signal to the gauge. If you have access to an oscilloscope you can check to see if the connector in the dash is getting any kind of signal.

A cheap DMM might also be able to register some kind of AC response.

 

Also, you can measure the tach meter. If you remove the gauge from the cluster, you should see some tiny, silver wires connecting the control circuit to the little springs that move the needle.

If you measure these VERY CAREFULLY you should get a resistance of some kind, probably around ~100Ω, mine was 70Ω.

The bad one died as 'open circuit' if I remember correctly, and the small voltage from my cheap DMM was enough to nudge the working needle.

 

Check this:

http://community.ratsun.net/topic/71865-84-datsun-720-gauge-hackery/

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Got it, thank you!

 

 
 
Well I thought I was clear where the tach wires are connected.... From intake coil negative terminal, to resistor, to tach. If the resistor is bad or the wires not connected these are the things, other than the gauge itself, that could cause it to not work.
 

By broken I mean the gauge is at fault. The tach is quite complex, and so, is the most likely thing to fail. Two wires and a resistor are less likely.

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Well, it wasn't the resistor. Not sure how else how to check.

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I think you're ignoring the probably cause, which is the tach gauge itself.

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I guess I was wishful thinking. Is there a way to test it? Maybe check for a signal at the connector? I don't have an oscilloscope.

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A signal at the connector merely says the wiring is good.

 

You could remove the tach and wire it directly to your truck's coil. You would still need the 2.2K resistor in line, but if it did work then you have a wiring problem.

 

 

The 'signal' is an on and off ground. If you connect a test lamp to the coil negative and the other end to the battery it might flicker with the engine running. On the other side of the 2.2K ohm resistor I doubt there would be enough current flow to light it.

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Hmm. Bummer. It's the last thing left that doesn't work on the truck, but I'm not sure it's worth spending any more money on,

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Keep eyes open in the wrecking yards. The ST model will have them standard, optional on the others. They are at least, plug 'n play 

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They are relatively easy to come by. Post an ad in the classifieds. I've sold them here before.

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Possibly. The only irritating thing is it's now the ONLY thing not working. I wouldn't care so much if other stuff was broken.

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Ok, so far I've tried a new resistor and new tach, neither worked. I know it could be the coil. How do I test that?

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If the engine runs the coil is good.

 

 

Maybe the dash wiring that the tach plugs into? Check for continuity between the coil and one side of the resister. Then from the other side of the resister to the tach.

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The white/blue wire has continuity to the intake coil. When running, the white/blue gets 12 volts. and the black gets 9 volts. All ok so far?

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What about from the other side of the resistor to the tach?

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I was hoping not to have to get that one out, but I'll check it next time I mess with the truck. Thanks.

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Mystery solved. Tach was bad. It's a bummer because upon removal it's in visually perfect condition. Think it's worth anything on eBay not working?

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