Guys,

My tach quit a while back. I replaced the alternator with a used one but there was no change in the tach, still totally dead.

In doing some research in the RAVE, I read that it is a varying frequency based upon the engine speed.

Is there a simple way to test the output of the alternator?

Any help appreciated.

 

I'm on my third alternator, I believe the tach signal is on the small wire that plugs in. Since alternators don't have much in them except diodes and a regulator, that wire probably comes out before the regulator and gives some sort of pulsed 12 volts, the tach counts the pulses.

So there should be some voltage there, but your meter may distort the value based on the waveform. If it is zero, then time for another alternator, unless your wiring harness is suspect.

Alternator Theory v15 has a very extensive coverage of alternators, just on those pesky Jeeps.

Would think that if you have volatge with the wire off, and then it drops to zip with the wire connected you might have some sort of ground in the harness.

 

With a Digital meter I believe you are right but an analog meter would display the modulation.

*eep* are pesky... they're everywhere

 

When I get back to the shade tree I'll put a meter on mine and report back.

Here are the readings from the "flying lead" that plugs on the back of the alternator and negative battery, lead connected, engine cranked (1000 rpm and dropping):

On DC scale - around 6.5 volts, no change with rpm change

On AC scale - around 7.6 volts, very tiny change with speed change (so there is an AC signal "floating" on top of a DC voltage)

On Hz scale - 270 @ about 900 rpm, saw it go to over 400 Hz when I revved the engine somewhat. Hz (Hertz) is old skool cycles-per-second. So that is the pulses the tach is counting. Basic voltmeters may not have a Hz scale. IMHO if the AC part is missing, the tach will have nothing to read.

So there should be voltages present on that tab of the alternator. If the alternator has an internal resistor in series with this conection, a harness short to ground would kill the tach, but not blow a fuse becuase of the current limitting function of the resistor.

 

i think you will find that the tacho works of the -ve side of the coil, and for some reason the tacho gets i power from the alt, i seriously doubt the tacho signal itself is generated from the alt

well bugger me, read the rave and it dows sense from the alt, i take it all back!!!


Cheers

Pedronz

 

Proof that you shouldn't read the RAVE!!!

 

An easy way to verify tach signal is with an OBD2 scan tool that has live data stream. If the scan tool receives an engine speed signal and gives you a value, then the instrument cluster tach has left the building.

 

I did use the loaner code reader at AA. It did indicate the engine RPMs.

Has anyone tried an aftermarket tach?

I was considering getting the code reader currently on sale at a new Autozone nearby. It os only 50 bucks and says it reads live data, so i could just leave it plugged in and use it I guess.

I sent a PM to the dude that was parting out his 98 askong about his instrument cluster but I would hate to put in the whole thing with his having almost 200k showing and my 97 having just under 120K.

I guess it could also be a bad wire inbetween the alternator and the inmstrument cluster. Need to do more investigation before I jump to a conclusion. Wish I had a freq meter or a good old analog. Don't remember if the Simpson 260 had the ability to read frequencies or not.

For it to be frequency, it would have to have an AC component. The faster the alternator spins the higher the frequency that would be produced.

 

Hold on a minute. It may be just a wire. I don't agree with priceg because of the following test.

I just took my "flying lead" off the alternator, and my tach stopped, but my scanner still tracked rpm. That means the source of the ECU rpm is somewhere else, not the tach wire from the alternator. Probbaly because the ECU would like the rpm sensor to be real accurate, not subject to change by substitution of a different pulley.

If the tach is not plugged in, if the connector has the female lug pushed back too far, or just loose from heat stress, if the wire is skinned and grounded - all these could stop the tach.

The circuitry inside the alternator sends pulses from the diodes to the tach, so it can count them. When the diodes go out in the alternator the tach dies and you are not charging the battery,

I took these readings just now after a 30 minute drive:

AC on, idle at 695-715, outdoor temp 95 F, coolant 201 F per scanner

Alternator housing 195 F per IR thermometer.

Battery volts 13.82

Brown/yellow wire from alternator - 13.5 volts

Main stud on alternator 13.94 volts

Drop across 100 amp fuse in fuse box 0.0204 volts

Tach "flying lead" prong 6.93 Volts DC, 7.68 volts AC, 230 Hertz

Meter = Fluke 189 ($450 - $575 depending on vendor). Many smaller digital meter do have a separate AC and DC setting, but usually not Hertz.

Suggest you read AC volts on alternator to tach prong, with and without wire connected.

The Simpson 260 does have an AC input, which is basically a capacitor in series, to block the DC. But no freq meter function. IMHO if no AC voltage present then nothing for tach to count. You areorrect on the freq change, I saw it shift when I tugged on the throttle cable.

 

Since it did not change when changing alternators and battery does read something like 13.9 with engine running, I do suspect it is a wiring issue.

Looking at the wiring harness, that lead goes into the harness really close to the alternator then somehow routes to the instrument cluster. It is just such a PIA dealing with the wiring harness, but I pretty well have to. Anyone have any experience with this lead?

Guess I need to take a detailed look at the Electrical portion of the RAVE.

If the tach has died, I wonder how big a pain it would be to put in a nice big old fashioned Tach. Guess I could do a search on tachometers. Might find a nice one I like.