Intermittent tachometer problem 1984 2L (1 Viewer)

[RufusTheDufus]

I've got an 84 Hilux 4wd with the 2L. Factory tach in the dash. I've got an intermittently functional tachometer. When I start the truck it will go all the way full-scale to the right (6K+ RPM). At some point it will simply start working. It will work for a few minutes to an hour and then stop for a little while, take a break and then get back to working again.

I've tried everything short of replacing the tach sensor in the injection pump. I've checked for loose/bad wiring between the sensor and the plug on the back of the gauge cluster. I've taken apart the gauge cluster and looked for bad/corroded contacts. Even visually inspected the circuit board on the back of the tack for bad components and solder joints.

Do the tach sensors in the injection pumps fail intermittently? I'd expect it to work or die completely. I assume they are just a magnetic pickup with no internal components to wear out.

My next step is to look at the waveform coming from the sensor with an oscilloscope unless someone here has a better idea.

 

[MatS]

Silly scope will be the best way to isolate your issue, if you have a plug on the sensor you can test it there and test it on the plug at the cluster and see if you got a difference. I'd be tracing the wiring and make sure you're not grounding out somewhere on the firewall, being intermittent and all. I just put dielectric grease in all my cluster connections last week and when the dash was out made sure all my grommets in the firewall are intact.

 

[RufusTheDufus]

Haven't had a chance to drag the silly scope down to the garage yet from the office. But I've made a little progress. Well, maybe not progress but at least the needle looks like it's working now. No idea how accurate it might be.

There's a circuit board on the back of the tach. This board appears to be a pulse to voltage converter. There's a trimpot on the edge of the board. I used a small screwdriver to cycle it through its full range a few times with the tach out of the gauge cluster. I think I got close to its original location/setting but I don't know. I didn't measure the resistance it was set at. I reassembled the gauge cluster and installed it in the dash. It's working now. I know it's not accurate because it's reporting idle speed as 200RPM. But at least it's moving in a reasonable range.

 

[bj40green]

Is there a calibration reference stamped somewhere? Something like: 50Hz is 3000rpm.

Rudi

 

[Stephen835259]

Any resolution to this by chance? I have an 1986 HJ61 with an intermittent tachometer right now. Even replaced the sender and still have the same issue.

 

[RufusTheDufus]

Nope. Gave up on it. It works a bit when it feels like it which is almost never these days.

 

[Stephen835259]

Hahaha. Yea I'm about to the same point too.

 

[fitzmooney]

On the HJ60 there is a multi pin plug over the flywheel housing that that the sensor wires go though that is prone to corrode.
The same would apply to the HJ61.

 

[Stephen835259]

Yea actually replaced the sender unit on the bell housing. Still no luck.

 

[fitzmooney]

The sensor has a 2 pin plug where it connects to the loom. The 2 wires then go into a multi pin plug near the starter.
This is the one that gets corroded.

 

[Stephen835259]

Sweet I'll check it out today