Volvo Penta 2000 series Tachometer - No reading at low revs

[Joeninety]

The tachometer on my Volvo Penta 2002 works, but drops to zero when revs fall below about 1000.
Curiously, when the alternator is not outputting (another story), or is disconnected, the Tachometer runs normally and smoothly with no cut out even when idling.

Can anyone shed any light on this anomaly? The sensor is located on the engine close to the flywheel and has two identical grey wires. The Instrument panel is the basic variety (ie warning lights for oil and temp but no gauges for these).

Thanks

 

[Slowtack]

Sometimes mine needs a tap on the glass to get it going

 

[pvb]

I'm bewildered by your mention of a tacho sensor near the flywheel - all the 2000-series documentation I've ever seen has the tacho driven from the W terminal on the alternator. Could it be that your tacho has been retro-fitted?

 

[Joeninety]

Thanks, I'll try that. But I suspect it's not simply a mechanical problem or poor connection in the Gauge itself, as it functions perfectly when the alternator is not outputting (such as when the alternator is disconnected).

 

[Joeninety]

Yes PVB, I believe it is original - it certainly has that look, and the two grey wires look identical to the two grey wires which lead to the back of the tacho gauge itself. The year is approx 1985-6.

I agree it is not as shown in the wiring diagram in the owners/workshop manual. Interestingly, two grey wires are shown in the diagram to leave the tacho (marked G and W respectively) and take up positions 12 and 13 in the loom, but in the diagram only the wire at position 13 continues at the engine end, for the W position on the Alternator. This is not so on mine: both the G and W wire from the tacho attach to the sender near the flywheel.

 

[VicS]

I'm bewildered by your mention of a tacho sensor near the flywheel - all the 2000-series documentation I've ever seen has the tacho driven from the W terminal on the alternator. Could it be that your tacho has been retro-fitted?

The Op seemed to be quite clear about it , but you are right the tach on a 2002 is normally driven by a signal from the alternator W terminal .

However a little investigation reveals the existence of a "cog type" tacho kit.

Presumably this is what the OP has. When or why such is/was fitted I know not! When no tacho is fitted as standard ???

Surprised though that its a sensor on the flywheel. There seems to be a place on the timing gear cover at the front of the engine where a sensor might fit and "see" the camshaft gear

 

[earlybird]

From VicS's suggestion, from the diagram, the sensor looks possibly to be a magnetic or inductive type. I once made a tacho. using such an RS sensor for a GM Yanmar, which I calibrated using a gearwheel mounted in a lathe driven at different speeds.
The device operation proved very sensitive to the gap between sensor and gear-tooth.
If the OP's tacho. is this type, I suggest he checks the clearance to see if it's adjustable, if so then try closing it slightly.
On my Yanmar, the sensor looked at the ring gear. It was cheap and worked very well.

 

[VicS]

From VicS's suggestion, from the diagram, the sensor looks possibly to be a magnetic or inductive type. I once made a tacho. using such an RS sensor for a GM Yanmar, which I calibrated using a gearwheel mounted in a lathe driven at different speeds.
The device operation proved very sensitive to the gap between sensor and gear-tooth.
If the OP's tacho. is this type, I suggest he checks the clearance to see if it's adjustable, if so then try closing it slightly.
On my Yanmar, the sensor looked at the ring gear. It was cheap and worked very well.

FWIW the sensor is the same part that is used on the ubiquitous MD11C and numerous other engines with cog sensing

We are not getting close to explaining, let alone solving, the OPs problem ............ I have no suggestions to make that fit the symptoms other than the usualone to check all connections and based on what you say make sure the sensor is screwed in properly.

 

[earlybird]

The point I was making was that the fairly standard frequency counting chip and sensor that I used had a definite trigger point, below which no signal was generated. Closing the clearance by only a few thou would result in the circuit suddenly starting up, there was no half-way house. Gear tooth speed also just might have some influence, giving rise to the OP's observation.
Hence my suggestion that adjusting the sensor might produce a result. It could have moved slightly over time.

 

[VicS]

Hence my suggestion that adjusting the sensor might produce a result. It could have moved slightly over time.

From what I remember of the MD11C , supported by the parts drawing for the 2002 , there is no provision for "adjustment". It screws in and tightens onto a gasket washer.
Hence my suggestion to check that it is screwed in properly but it still would not explain why it works perfectly when there is no output from the alternator but misbehaves when the alternator is operating correctly.

 

[earlybird]

it still would not explain why it works perfectly when there is no output from the alternator but misbehaves when the alternator is operating correctly.

Fair comment, TBH, I wondered if that was just one of these coincidences that the gods send to confuse us. The type of tacho. we're assuming here shouldn't have anything to do with the alternator other than variation of the supply voltage, and working at a lower voltage seems illogical!

 

[Joeninety]

Thanks all. I took a photo of the sensor but do not know how to attach it. It looks like that shown in the diagram posted by VicS. It's location is on the front of the engine, just above the oil filler cap, between the engine fuel filter and the first fuel pump. (Was flywheel the wrong term? I meant the large pulley which runs the belt).

I guess I'll have to live with it. The instrument panel was not working well when I acquired the boat at the beginning of the year, so I have refurbished it including digging out and replacing corroded bulbs, cleaning terminals, new alarm and re made the loom socket and plug connection at the panel end. I did not replace the cable, just shortened it a bit, and it may be that some wires have an excessive resistance due to corrosion.

I'll see what voltage the sensor puts out at idle and higher revs and compare this to voltages further upstream. Still doesn't make sense though. I was hoping there was a simple explanation!

 

[VicS]

Nothing to add except yes that is not the flywheel. I think it's the camshaft drive gear wheel it's looking at, The flywheel is at the back of the engine and carries the ring gear that the starter motor engages with.

Is it located where the diagram below of the timing gears and cover shows plug #3 ?

 

[Joeninety]

Yes, I believe that's the position. It's a bit difficult with so many recognizable features removed. I've attempted to load a photo but can't tell if it will be there until I've done the post.

 

[reeac]

I just wonder whether the sensor is being masked by an accumulation of dirt which along with a reduced signal at low revs. is stopping it working. Had such a problem with the crankshaft position sensor on a car once.

 

[VicS]

Yes, I believe that's the position. It's a bit difficult with so many recognizable features removed. I've attempted to load a photo but can't tell if it will be there until I've done the post.

Thats it. Exactly where I said

 

[Joeninety]

I'll remove and inspect/clean the working end of the sensor sometime in the coming week. The fact that the tacho works perfectly when the alternator is not working still is puzzling.

 

[VicS]

I'll remove and inspect/clean the working end of the sensor sometime in the coming week. The fact that the tacho works perfectly when the alternator is not working still is puzzling.

I think I would be looking at the power supply to the tacho although not sure what I might be looking for.