Alternator issue?

[Steve P]

My 2.5 Vitesse has a Saloon gearbox with o/d. Due to space around the chassis rails I converted to electronic Speedo and Tacho when I put the engine in (bought from Speedy cables in Wales).

The tacho works from the coil  and the speedo has a sensor that counts prop rotations, recently when driving the speedo starts bouncing around intermittently, on two occasions recently the tacho has gone to zero for about a minute before returning. On Sunday on a run down the coast, it went to zero before coming back, it now seriously under reads revs never going over 2K.

I was fitting a new Canleys ali o/d gear knob today (nice bit of kit) and I thought I would test the battery voltage. It was 13.04 on tick over and rose to 13.70 when increasing the revs, I`m guessing the revs as the tacho is under reading as previously mentioned but it ticks over at about 900.

Could this be a sign of the regulator dying?, I wondered if it may have spiked damaging the speedo and tacho?. Alternator is a Lucas one about ten years old.

Opinions appreciated.

Steve

[johny]

You could try running the car with the alternator unplugged to see if just using battery output makes any difference?

[Wagger]

Look for a poor connection if what johnny advises does not reveal anything. Both are operated by impulses.  You really need an oscilloscope or a frequency counter to fault find this.

Edited September 20 by Wagger

[SpitFire6]

On 20/09/2024 at 12:49, johny said:

You could try running the car with the alternator unplugged to see if just using battery output makes any difference?

Rotating the alternator with the cables disconnected is not recommended.

[johny]

Are you sure youre not thinking of the battery which yes mustnt be disconnected from a running engine? However for the alternator heres what the manual says:

[Wagger]

If your measuring device is accurate, 13.7 volts is a bit low but within the spec across the whole operating temperature range. The best test for an alternator is to measure the battery voltage with all lights and heater blower on at 2000 to 3000 rpm. It should hold up at 13.5 volts minimum over a period of five minutes.

Alternators usually have a three phase bridge rectifier using six large diodes. if one or more fail, the voltage does drop off. If they short, then it dies very quickly.

Your erratic tacho and speedo could be caused by spikes from the alternator. Disconnect it before starting and try running without it as a test.

Speedo could be the prop wobbling due to the rubber mounts. Where is the sensor placed? Distance may be critical.

Edited Sunday at 11:30 by Wagger

[Steve P]

Voltage was tested with my work Avo meter, not a fluke one just generic.

The prop is a sensor mounted on a home made bracket on the back of the overdrive, it's adjusted to less than 3mm from the prop bolts as they pass, I guess I could check the o/d mount for movement as the modern ones do fall apart but I haven't noticed movement from the gearstick.

I was worried the alternator has spiked the tacho causing it to fail, it works but under reads and won,t go over 2k, tickover is zero on the tacho.

I thought an alternator supplied a constant voltage without variation of revs?

Speedy cables will test them if I send them back to Wales.

Steve

[Wagger]

If you are brave enough, drive with the tunnel cover off and have a passenger watch the prop at speed. Even new rubber mounts will wobble if anything is out of balance, and torque reaction will move it foot down then foot off.

Yes, alternators do produce constant voltage with revs but only from about 1000 rpm upwards. It depends upon loading and the size of your pulleys.

My son has a 2.5Pi saloon fitted with a ZF 4hp22 auto box and a diff that uses a sensor to count he crown wheel teeth. He is fighting hard to achieve accurate speedo readings.

Mounting you sensor near the diff may be better as that is more firmly mounted than the rear of the engine, (I believe).

[Steve P]

It's been on there about 10 years and it only started playing up recently,and it is definitely accurate as far as speed goes, or was.

S

[SpitFire6]

On 21/09/2024 at 15:14, johny said:

Are you sure youre not thinking of the battery which yes mustnt be disconnected from a running engine? However for the alternator heres what the manual says:

Hi Johny,
it looks like its saying do not run the engine with all the wires disconnected. In the above case, the battery is still connected?

Here is a type of ACR where disconnecting the plug will cause no problem as the field current will be zero and no harm can occur.
Cheers,

Iain.

[johny]

oh dear its getting so complicated. I suppose the manual should have explained it more clearly but its saying to avoid damage run the alternator EITHER with everything connected OR run the alternator with all the alternator connectors unplugged👍

[SpitFire6]

10 minutes ago, johny said:

oh dear its getting so complicated. I suppose the manual should have explained it more clearly but its saying to avoid damage run the alternator EITHER with everything connected OR run the alternator with all the alternator connectors unplugged👍

Oh dear. Read it again.
Why don't you try on your alternator please?
Pretty please. Disconnect all cables and run at over a few thousand RPM. If the field is energised it will not end well. Ten minutes should be enough.
Cheers,
Iain.

[johny]

Well its clear enough to me what the manual says....

[SpitFire6]

23 minutes ago, johny said:

Well its clear enough to me what the manual says....

Ok.
You win.

 

[SpitFire6]

On 20/09/2024 at 12:40, Steve P said:

Snip<><><>
The tacho works from the coil  and the speedo has a sensor that counts prop rotations, recently when driving the speedo starts bouncing around intermittently, on two occasions recently the tacho has gone to zero for about a minute before returning. On Sunday on a run down the coast, it went to zero before coming back, it now seriously under reads revs never going over 2K.

I was fitting a new Canleys ali o/d gear knob today (nice bit of kit) and I thought I would test the battery voltage. It was 13.04 on tick over and rose to 13.70 when increasing the revs, I`m guessing the revs as the tacho is under reading as previously mentioned but it ticks over at about 900.

Could this be a sign of the regulator dying?, I wondered if it may have spiked damaging the speedo and tacho?. Alternator is a Lucas one about ten years old.
Snip<><><>

Hi,
You have not mentioned the ignition light so I assume it's working normally. If your voltages are true with respect to ground I would expect your electronic equipment to function normally and be factual. If you used an analogue AVO, I would believe what the probes reported as an AVO multimeter only needs internal power for resistance measurement.
The output voltage is abnormal.
I can think of any failure mode of the alternator that would cause the voltage to spike, as the non-charging battery would filter such.
Your sensor distance does not change as clearance can only change if the sensor bracket flexes.

What is the alternator/charger model?
Does the tacho have switch settings to make?
Measure the voltage between B+ and D+/Ign on the back of the running alternator. Should be lower than 200mV and closer to 50mV.
Put AVO on AC, should read less than 100mV.
Verify your measurement methods & measurement points are correct.

I would ask the manufacturer to test & go from there.

Cheers,
Iain.
 

Edited Monday at 20:27 by SpitFire6
What is the alternator/charger model?

[Steve P]

20 hours ago, SpitFire6 said:

You have not mentioned the ignition light so I assume it's working normally.

Yes the ignition lamp is behaving normally, although it reminded me that when I installed the electronic speedo etc, it came with no lamps so I made a bracket to go under the dash and installed 10mm Led`s for the ignition, oil and high beam lamps, the ignition light went out but needed a higher blip of the throttle to make it go out, fine once moving. I recently changed the lamps back to filament lamps as It looks neater.

I will do the other checks this weekend. From memory the alternator is an 18ACR I think.

Steve

Edited Tuesday at 17:20 by Steve P

[SpitFire6]

1 hour ago, Steve P said:

Yes the ignition lamp is behaving normally, although it reminded me that when I installed the electronic speedo etc, it came with no lamps so I made a bracket to go under the dash and installed 10mm Led`s for the ignition, oil and high beam lamps, the ignition light went out but needed a higher blip of the throttle to make it go out, fine once moving. I recently changed the lamps back to filament lamps as It looks neater.

I will do the other checks this weekend.

Steve

Hi Steve,

it is quite normal for an alternator to behave as you describe when fitted with an LED "IND light/lamp". A lot of people like this as it removes load on the engine until you blip.
Once the regulator kicks in, all is as normal.
Await the weekend.
Cheers,
Iain.

Edited Tuesday at 18:47 by SpitFire6
added IND

[Pete Lewis]

go back to early alternator introduction and on some of the diesels were low idle and small pullies  made cut in speeds were often a bit abnormal 

the solution was to fit a much higher wattage warning lamp bulb which brought the cut in speed down 

cheap solution in the late 60s /70s 

Pete

[SpitFire6]

Hi,
The lowest cut in speed would entail a high-power bulb or low-resistance resistor. Too low resistance or high wattage lamp/bulb risks the ignition circuit being powered by the alternator instead of the battery and the ignition circuit continuing to be live when the key is switched off! Having the alternator D+ supplied from an ignition souced relay would be best if the lowest boot-strap was required. Most moderns kick in some time after starting.
Cheers,
Iain