Electrical Stock Tach Does Not Work

mitymerc · Jul 15, 2017

My '87 5.0 LX has been parked in the garage since 1999. My son is almost ready to get his permit. We have it almost ready to go back on the road, but the tach does not work anymore. I have cleaned the pins, checked continuity on the instrument cluster traces, checked that it is getting power, checked the signal @ the connector with a test light, it gets brighter as the revs are brought up.

I imagine the tach itself is toast. I have not done any board level troubleshooting in decades. I checked point to point for continuity on the board, also none of the resistors read open.

Any suggestions?

Thanks in advance.

General karthief · Jul 16, 2017

@jrichker?

mitymerc · Jul 16, 2017

karthief said:
@jrichker?

No...

General karthief · Jul 16, 2017

mitymerc said:
No...

I was getting the attention of our resident motorhead, he has answers to questions that have not even been asked yet.
I can usually tell what something is but he can show you what it does (in detail), how it's built and sometimes who built it.
There are several others here that will chime in also.

mitymerc · Jul 16, 2017

Awesome, Thanks

jrichker · Jul 16, 2017

The 4 cylinder, 6 cylinder and V8 tachs are not interchangeable.

The tach isn't easy to check unless you have access to an oscilloscope or a good frequency counter and an audio frequency signal generator. I have done a calibration check on 5.0 Fox body tach while it was sitting on the bench and I am familiar with how to do it.

If you have a 12 volt AC power supply, you might be able to cheat a bit and use it as a fixed frequency signal generator. I don't have at tech note on how to do it, but it should be simple once the tach has been removed from the instrument cluster.

General karthief · Jul 16, 2017

Are the V8 tachs interchangeable between the years say 88 through 91? I have several clusters I could pull tachs out of.
Maybe he could try a different one.

mitymerc · Jul 16, 2017

I believe I read somewhere that 87-89 are the same.
12V AC?
I haven't seen a 'scope in years.
I Believe it is getting a signal, sig term to gnd w/a test light lights up & gets brighter w/increased revs.

AC? If I meter the sig to gnd with tach reinstalled will that prove anything?

jrichker · Jul 16, 2017

mitymerc said:
I believe I read somewhere that 87-89 are the same.
12V AC?
I haven't seen a 'scope in years.
I Believe it is getting a signal, sig term to gnd w/a test light lights up & gets brighter w/increased revs.

AC? If I meter the sig to gnd with tach reinstalled will that prove anything?

The signal that works the tach is an AC waveform that is in the rage of 10-14 volts peak to peak. The 12 volt AC output from a transformer is just good enough to make the tach work. It should indicate a little less than 1000 RPM with 1 transformer lead tied to the ground side of the circuit and the other on the S center pin on the back of the tach. You'll need 12 volts from the battery applied to the tach battery pin and ground applied to the tach ground pin when you do the test.

mitymerc · Jul 17, 2017

IIRC a Lionel transformer outputs AC.
Probably cannot play w/car till the weekend.

mitymerc · Jul 18, 2017

Got off work tonight & assembled a test rig. My long unused wavetek meter also died from lack of use. New battery no help.
AC transformer was set to approx 12V w/elcheepo free HF meter. Booster pak provided 12V DC. No response from tach.
Touched up a couple suspect looking pins.
Again, no help....

General karthief · Jul 18, 2017

Pm me the part number on the tach and the build date on you cluster an I'll see If I have one

mitymerc · Jul 18, 2017

Cool!, where would I find the build date on the cluster?

General karthief · Jul 18, 2017

Give me the part numbers, I can figure it out from there

mitymerc · Jul 18, 2017

I'm on my phone & don't see where to send a PM...
Also, it won't allow me to attach a pic unless it is hosted somewhere.
Tach = E7ZF-17B316-BB 3513-1
Stepper motor = B-E7ZF-17360-BA
Car was purchased in '87 w/14 miles on it & stored in the garage in '99.

General karthief · Jul 18, 2017

I will get back to you on friday when I'm back in town

mitymerc · Jul 20, 2017

Safe trip.

General karthief · Jul 24, 2017

I pm'd you

mitymerc · Jul 25, 2017

Did you get my reply?

92wvstang · Jul 30, 2017

Sorry for the highjack, but l'm now having the same problem. I hooked up a set of jumper cables backwards on my 92 and now my tach quit and my wipers will not shut off. Is there something l could have burnt out or is it just a coincidence.

Electrical Stock Tach Does Not Work

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