Testing a tach sensor and/or gage & harness?

[lotzagoodstuff]

Well my tach is finally irritating me enough to try to fix it. It used to be really lazy until it warmed up, and now it seems like it is never reading full scale no matter how long I run the truck.

I'm ok with putting a good used sensor in my truck, or maybe even putting down the sheckles for a new one but I also want to not be a "parts swapper" and find out there's something wrong with my harness or gage.

Anybody know what the resistance of the sensor should be? I've had it out and cleaned it before and it seemed to get better for a short time, which makes me assume that the sensor is going bad, but you know what they say about assuming.....

I did a quick search, but didn't see anything specific to voltage/impedance values. As always, any sensor/wiring input would be much appreciated.

 

[rhkcommander]

Resistance isn't really important.... it generates a AC power signal though. its a magnet that reads the teeth so for each tooth it generates a pulse. If the sensor is bad it wont get a signal going.

If you want to check the wiring, probe the wiring and there should be virtually no resistance, unless there is corrosion or something. That could help determine if its a bad connection to/from the sensor. Sounds like it might be a bad connection or the wires to the sensor are damaged. Most people add glues near the plug to protect the wiring from engine vibrations.

I found that my cluster is probably the problem in my case because I can see the gauge being moved ever so slightly with the turning the lights on and off. I'm guessing the ground to the cluster is bad but haven't verified yet. I'm gonna try to run a dedicated ground wire and maybe power wire to the tach gauge itself

 

[mobilemech]

the tach sensor is a AC pulse, you measure it with a multimeter set to AC volts, unplug the sensor and start the truck, with it running touch the leads to the sensor wires, you should get a reading around 2.5 V AC if the sensor is good. most of the tach issues are ground related. the clusters have circut issues in them also.

 

[lotzagoodstuff]

.84 volts ac at idle

So my harness has no resistance on the tach side, and the sensor read .84 volts, which seems pretty low from your descriptions. The tach read slightly about zero, like 50-75 RPM, and it moved with revving, but still seemed lazy.

So I pulled the vacuum pump and removed the sensor, clean, inspected and reinstalled it. About the same results, so can I conclude that the sensor is bad?

Anybody got a good used tach sensor or a new in box they want to part with? Looks like they are $75-100 for a new Ford piece.

Thanks

 

[mobilemech]

From what your results are, I would say the tach sensor is bad. The sensor from a 6.9 or a 7.3 will all work.

 

[Optikalillushun]

Great question, great answer and good thread to bookmark.

 

[lotzagoodstuff]

As soon as I get a replacement, I will document the before and after voltage. I'm pretty sure that my tach worked pretty good a few years ago as I turned up my idle to around 900 RPM at idle to cover up my rollover noise from my single mass flywheel.

Stay tuned: gotta find another sensor, hopefully reasonable, because my truck's not an auto and I'm pretty sure tachometer accuracy isn't work $102

Thanks again for all the help

 

[typ4]

I might have one, at least swap and test, .

 

[seawalkersee]

Yeah...that's kind of where I am. I have no one close with whom I can swap/test. As you can see in my vids, it does not work.

SWS

 

[lotzagoodstuff]

I might have one, at least swap and test, .

Thanks Russ, I'm going to be in town this next week and I need an convenient excuse to come down to your place.

 

[chevytaHOE5674]

I'm curious to see what the before and after voltage is from the sensor. My sensor is putting out less than 1v and think that might be my tach's problem. So knowing a good sensors voltage will allow me to pinpoint my troubles.

 

[mobilemech]

there was a typo in the voltage number, good sensor at idle is .25vAC. voltage at 2000 rpm is about 1.5vAC. at 3000rpm it is about 2.5vAC. this is the results of a proper working tach sensor, sorry for the typo and error in info. when testing the voltage at idle, the reading should be constant and smooth and increase constant and smoothly with RPM's

 

[lotzagoodstuff]

there was a typo in the voltage number, good sensor at idle is .25vAC. voltage at 2000 rpm is about 1.5vAC. at 3000rpm it is about 2.5vAC. this is the results of a proper working tach sensor, sorry for the typo and error in info. when testing the voltage at idle, the reading should be constant and smooth and increase constant and smoothly with RPM's

Well I have .84 at idle, which sounds really high, but my tach reading is very low, which now makes me question both components

Time for a swap and test I guess.

Stay tuned.

 

[riotwarrior]

Well I have .84 at idle, which sounds really high, but my tach reading is very low, which now makes me question both components

Time for a swap and test I guess.

Stay tuned.

Maybe test at the suggested RPM and see if the unit of Voltage changes at all? just sayin is all....

I have a known working tach sensor and guage cluster I can allow out for testing....not for sale cause I gotta have them for my build when it's swap time.

 

[chevytaHOE5674]

Well I have .84 at idle, which sounds really high, but my tach reading is very low, which now makes me question both components

Time for a swap and test I guess.

Stay tuned.

I have around .86v at idle. So i am very curious to see what you come up with to fix the issue.