Tachometer Troubleshooting

[parkergn]

Been trying to hookup an aftermarket tachometer to my 6.9 E350 with C6. Don’t necessary need one, but thought it would be nice.

Anyways, after my first used sensor was no good, I received a new one (Standard Motor) and wired it to my 3” diesel tachometer from eBay. But after some frustrating attempts, the tachometer will not respond to my signal.

I have good resistance about 2500ohms. And an AC voltage of about 0.15V This voltage will increase very precisely as I increase RPM’s.

So my thoughts are the sensor is good. OR I’m not getting enough voltage at idle. Another thread had the sensor volts at about 0.25V. However I would think the tachometer would at least fluctuate at higher RPM’s when the voltages increase above idle… yet nothing.

The Tachometer does react to 12v. The needles drops to 0 from about 400 when it has no power.

I bought this tachometer from eBay last year and payed probably too much for it… can’t return. What are my options for a 3” tachometer?

Thanks!

 

[Kizer]

The needles drops to 0 from about 400 when it has no power.

Could this be an issue where the tach. and the sensor require different resistance values?
Perhaps a polarity issue?

 

[ROCK HARVEY]

I bought this one but I can’t comment on quality because I haven’t installed it yet:

https://a.co/d/66KSFuk

For a tach signal, I’ve heard you can use the stator output on the alternator. Maybe you could try that before switching to a different tachometer.

 

[david85]

The sensor may not be outputting a high enough voltage for this particular tachometer (0.15VAC does seem low). I wouldn't necessarily expect a low reading for a low voltage. Variable RPM signal is from variable frequency, not variable voltage. If the voltage is below the minimum voltage, I would expect no response at all on the tach, no matter where the frequency is. Chances are a frequency output too high will also result in no reading at some point (dropping the tach back to zero).

There are few different ways tachometers usually get their frequency output. Gas engines usually have 0.5 pulses/revolution/cylinder (for a Four-Stroke engine, 2-strokes are 1 pulse/Rev/cyl). So a V8 would be 4 PPR, a V6 would be 3PPR, and so on. Diesels in our trucks are different however. They take a much higher number of pulses per revolution, because they count the teeth on the injection pump drive gear. A third style uses the AC output from the alternator. This alternator option usually requires calibration of the tachometer, since pulley sizes vary from one engine application to another. And or course, modern engines use the ECU...whatever that is. So the question is, what is your used tach intended to read? A relatively low frequency similar to an old coil ignition gasser? Or a relatively high frequency count like the magnetic pickup sensor our IDIs use?

I don't know if this helps, but I did a tachometer adapter circuit in a diesel ranger many years ago. Similar vintage and possibly similar tachometer. I had access to an electronics lab at the time, so it made things a little easier to figure out. I purchased a mangnetic pickup sensor that worked, but the output voltage was too low to trigger the tachometer. To solve this, I used a transistor circuit to boost the signal high enough to drive the tachometer. Then I figured out how to calibrate the tach. It worked for many years without issues and probably still does (truck is in storage until I can restore it).

V6 vs 4cyl tachometer wiring (for 1988 OEM tach)

Hi guys, tried doing a search and couldn't come up with a difinative answer. Can you use a tachometer that was originally in a V6 equipped ranger (late 1987) on a 4 cyl engine? I'm guessing the V6 wiring will expect a 3 pulse per revolution signal, while the 4 popper wants to see 2PPM. I know...

www.therangerstation.com

 

[XOLATEM]

Jumping in with a coupla thoughts...

@david85 , that was great input !

@ROCK HARVEY , I always appreciate your thoughts.

@Kizer , good input as well...

On the electricals...I have nothing to add at the moment...still groggy from the week...but on the tach that was shown...

At least it was made in the USA...

And...it has a Ford part number, which makes its original application easier to track down...

A quick search comes up 'Freightliner'...so...just look up how it was driven and what kind of engine it had and we are closer to the answer...

1984 Freightliner.

So...at the moment we have at least three possibilities...

Cable driven,

Alternator driven,

Sensor driven.

Find a Freightliner shop manual and see what drives the instrument.

To the OP...try to not have buyers remorse on the tach...if you find that you don't want it for any reason...offer it up on the internet...

Someone will need it....

Just my .02

 

[divemaster5734]

I have a 7.3 E350 C6. After sitting for years got it running and discovered it was pulling very high amps with new batteries, broke a couple V belts and went down the wrong rabbit hole before getting to the correct diagnosis.
Then realized it is wired differently.
The harness had a stakon for the alternator stator that I'd attached.
Once I found a diagram I disconnected that wire I stopped getting the high amps and the tach worked.

You must be registered for see images attach

The I wire at the regulator is grounded through a noise capacitor and the stator wire is not connected at the alternator.
The regulator controls the field output.
Turns out the regulator was bad, not sure if I smoked it by hooking up the stator.
Not sure if this helps, reading the posts above makes my head spin...

 

[parkergn]

Thanks for everyones input so far! Much to think about.

@david85 - yes it should be for a diesel engine, however not sure if it's calibrated for Alternator signal or MAG signal. Looking on the Beene website, they differentiate the two types of diesel tachs. Found a manual - Magnet Pickup Tach. Setup and they state the tach signal is lower then the alternator signal.

They also state to have the ground signal wire directly onto the tach. NOT the engine. I'll try this since my signal ground wire is on the body.

My tachometer has 4 settings for the different engines.
- DO NOT USE
- CUMMINS ENGINE
- 3406 CAT
- 6V92 & 8V92 DDA

Seems like most of these engines get a tach signal of the flywheel / camshaft?

 

[david85]

I looked briefly online at tach sensors for those engines and they seem to all be mag sensors. Not much shows up for alternator drive, but it looks like those should be higher than 0.15V anyway.

Ideally, I would hook up the tach to 12V power, then start feeding it an AC or Pulsed DC signal from a function generator. Start around 5V and 50Hz and then ramp up from there until you see a response from the needle.

Alternatively, if you had a 6V power supply (or battery) you could try to use that instead of a function generator. You can do this by connecting the 6V ground to the 12V ground and then try quickly tapping that 6V wire to the signal input of the tachometer. If it works, it means the tach is probably good, but your signal voltage is too low to drive it at only 0.15V. I'll admit this is a very crude test though.

 

[parkergn]

Great thanks @david85 yes I’ll have to try some more tests like you suggested to confirm if my tachometer is functioning properly.

Strange I’m getting such a low voltage off my new sensor, I’m thinking maybe it’s not close enough to the cam gear, gonna try to torque down the sensor more and see if that changes at all. Maybe remove the o-ring on the sensor if I need to get closer.