328 Flywheel RPM sensor Question (technical)

Hi everyone!

Just a quick but pretty technical question regarding Microplex diagnostics: my 328 RPM (tachimetric) sensor is developing 1.6v AC at idle. Manual states ">2.0v" is normal. Voltage seems to decrease with increasing engine speed; unplugging connector instantly stops engine. Mounting pad is spotless. Swapped in new Marelli sensor which gave same reading. All other diagnostics on Microplex procedure read in normal range. Using good-quality DMM, but do not have a scope to look at waveform.

Symptom is intermittent jump in timing advance at idle.

What does this mean? The ECU is seeing a "weak" or "engine running at higher RPM" signal? Are these adjustable in any manner? Any thoughts?

Thanks of course,

Carl

 

Your dmm is probably reading rms, check the voltage with a oscilloscope

 

Have you taken a look at the condition of the wires. I had a misfire problem on my 348 that turned out to be a bad wire, under the rubber boot, for the connector to one of the crank position sensors. It could be that you only have a few strands sending the signal instead of the whole wire. The car ran fine at the lower rpm's but as the engine speed rose the misfires started. Once I fixxed the wire the problem was solved. So try having a look under the rubber boots on the connectors.

 

Carl,
My 89' also reads 1.6V, but car seems runs fine. The resaon I know this is because I was trying to track down a Tach problem where my Tach always reads low, it seems ok at idle but when the engine is reved, it won't go over about 3000rpm. I send it out to Palo Alto Speedometer service and they said it was ok. So I have still not resolved my problem. Maybe it's related to your problem, Does your Tach work correctly?? I even tried changing out the Microplex, but it made no change.

Chris

 

I am pleasantly surprised seeing this number of answers to such an obscure question on a Sunday morning...!

To follow-up:

*Unfortunately do not have an oscilliscope. Forgive my ignorance, but what does "rms" abbreviation denote?

* Ernie, the wiring to the sensors is a separate harness that begins under the fenderwell and ends with both connectors. I have removed this & cleaned terminals in the past, but confess haven't actually looked at wires. Will add this to list. The 1.6v is measured however either directly out of sensor or at Microplex, so I suspect the signal is getting through?

* Chris, I'm sure you know this, but the tach signal originates at the microplex and is sent to dash unit. Is your ground (at dash) good? How about ground at (-) battery cable? (less likely though). My tach reads OK. Have also exchanged microplex (twice now) to no effect.

Let me re-phrase this question then: at constant fuel mixture (O-2 sensor disconnected) I still see a bit of jump in timing (watching flywheel with strobe). Besides incorrect mixture, what else will effect this?

Thanks as always,

Carl

 

RMS = "root mean square". This is the square root of the of the mean of the current or voltage. For a square wave (like a hall sensor) this is actually the same as the average voltage or current. For a sinusoidal wave it's a little more complex.

This is the reason why a 'scope is the best tool for diagnosing sensor issues -- you get to see the waveform, the quality of the wave and the peak values. Using a DMM is a guide at best.

 

The sensor puts out a series of pulses. (My best guess is about 144 pulse/rev) The diagnostics that say to look for 2.0 VAC are probably expecting an averaging voltmeter to be used.

If memory serves, the recommended "remedy" for a low voltage reading is to check the gap between the sensor and the flywheel. If the gap is too large, the voltage will be too low.

 

Actually, Fluke (and others) make a complete line of "true rms" multimeters. It is what I use.

Dave

 

True, if what you want to measure is RMS. Any common-or-garden DMM will have an AC or DC selector. Switch to AC and you get RMS.

However, the point is that for a more comprehensive sensor diagnosis it is best to use a scope. That's why WSM's show you what the waveform should look like for various sensors.

"This is the reason why a 'scope is the best tool for diagnosing sensor issues -- you get to see the waveform, the quality of the wave and the peak values. Using a DMM is a guide at best."

 

Thanks everyone! Does sound like a scope (or a trip!) is the next best step. I don't think it's technically possible to adjsut sensor position/air gap on this, although I think DGS you're correct increasing air gap = decreasing signal voltage.

Any other thoughts on intermittent timing advance at idle?

Have a good day,

Carl

 

Hi Carl

if you tach does not indicate correctly, first of all, I recommend you make a simple test: disassembles the battery and recharge, reviews the front electrical ground connector, and the tips of connection of the battery, verifies that it is clean and strong.
Whenever I do this, my rev counter returns to work well, I have read other cases in which the problem has been solved this way

greetings,

Alfredo.