Ammeter Reading

[Mr Tiger]

Up until today the ammeter on my 2500 has shown a reading of around 0 when driving with all electrics off and a slight positive reading with the radiator fan or lights on.

Today, with all electrics off it's reading around +15 at tickover and up to around +40 when driving.

I don't understand ammeter readings. Either something has gone wrong (most likely) or it's suddenly started working correctly (least likely).

Any ideas?

Thanks,

Chris

[Dollyman1850]

2 possibilities..

1. Something shorting at the ammeter connections to earth behind your dash.

2. Your battery is knackered and as such you are showing a high current draw when running since your alternator is trying to charge a knackered battery.

Run a multimeter across the battery terminals to check condition of battery.

I would suspect the latter since you would probably be smelling smoke by now if something was up at the gauge end.

if you have an alternator I would be thinking of a volt meter rather than ammeter.
N.

[Mr Tiger]

Thanks for your reply.

I've just checked the battery and it's showing a healthy 13.5 volts stone cold (closer to freezing). It also had plenty of life yesterday when starting up after standing for 8 days.

I risked switching the ignition on and everything seems OK with a slight -ve reading on the ammeter. With lights on it had a slightly more negative reading.

I had quick look under the bonnet and noticed the fan belt is loose. If I'm really lucky perhaps this is the cause.

I still don't understand ammeters.

I think you made a good point about a volt meter being better with an alternator. I read something similar in Practical Classics many years ago.

Chris

[ATE399J]

The reason volt meters are preferred to ammeters these days is that an ammeter requires all of the current running the car to pass through the unit i.e. Up behind the dash. This is considered a bit of a fire hazard with modern 60amp alternators. A voltmeter is simply connected across the battery and measures its voltage so very low current.
Agree with previous post regarding odd reading, it could also be the regulator built into the alternator (assume you aren't still running a dynamo) that's throwing a hissy fit.
If all is well now then l'd just keep an eye on it for odd behaviour.

[Dollyman1850]

ATE399J said:

The reason volt meters are preferred to ammeters these days is that an ammeter requires all of the current running the car to pass through the unit i.e. Up behind the dash. This is considered a bit of a fire hazard with modern 60amp alternators. A voltmeter is simply connected across the battery and measures its voltage so very low current.
Agree with previous post regarding odd reading, it could also be the regulator built into the alternator (assume you aren't still running a dynamo) that's throwing a hissy fit.
If all is well now then l'd just keep an eye on it for odd behaviour.

Key info. Is your car running a dynamo or Alternator? If an original dynamo then the voltage regulator controlling the charging circuit to the battery could be at fault.
If you have an Alternator then I don't really see the point of an ammeter. A voltmeter will give more information.

Ammeters used historically with dynamos so that you could see realtime the current draw from the battery as you drive. This is still sometimes useful on cars with lots of ancillary electrical equipment but less so with modern high output alternators..especially on a vixen.

if your fan belt is slipping then I would expect to see more toward the negative on an ammeter.

If you have an alternator then the high positive is perhaps just a reflection of the greater output charge?

N.

Edited by Dollyman1850 on Monday 24th November 20:52

[prideaux]

Chris like myself I know you like to retain as much as possible original features however when it came to deciding on my Tuscan I decided that firstly running an alternator rather than a Dynamo I would not fit even though I had it restored and it will go on another Friends Tuscan when he restores the car I really did not fancy that amount of current going behind the Dash maybe with a Dynamo that would be different.
Will be interested to hear if you find the reason
Andrew

[Mr Tiger]

Thanks for the replies, info and advice.

Just to clarify, the car has an alternator. I've checked the handbook and an alternator is correct for the model. It states it's a Lucas 17 ACR. A quick Google reveals this is a 36 amp unit. The chances of it being original are probably zilch so it may well have a more powerful replacement.

I think an ammeter is correct for the Vixen chassied 2500 and maybe a volt meter for the later S4? The handbook just lists the gauge as a battery condition indicator. I've just found a few pages further on a small paragraph about the ammeter.

It's looking like TVR built a fire hazard into these cars, along with just two fuses as standard. Luckily mine has a modern fuse box and a battery isolator switch. It should be safe for now, at least until I try running the engine again!

Chris

[Mr Tiger]

I tried tightening the fan belt but that made no difference. I ran it briefly with the alternator unplugged. It ran rough but not surprisingly stopped the dangerously high reading on the ammeter. I put a volt meter across the fuses one by one and could find no electrical flow. I then got a new alternator from my local motor factor. It cost all of £36 and even came with a fan & pulley. The spec sheet said its 36 to 43 amps so I hope it'll be safe. I fitted it on the weekend and touch wood it seems to have sorted the problem.

I still think a volt meter would be better. Does anyone know if AC made them?

On the other hand would it be possible to fit a hidden switch and probably some extra wiring so the car could run with the ammeter disconnected most of the time and just switch it in occasionally to check things are OK?

Chris

[Dollyman1850]

Mr Tiger said:

I tried tightening the fan belt but that made no difference. I ran it briefly with the alternator unplugged. It ran rough but not surprisingly stopped the dangerously high reading on the ammeter. I put a volt meter across the fuses one by one and could find no electrical flow. I then got a new alternator from my local motor factor. It cost all of £36 and even came with a fan & pulley. The spec sheet said its 36 to 43 amps so I hope it'll be safe. I fitted it on the weekend and touch wood it seems to have sorted the problem.

I still think a volt meter would be better. Does anyone know if AC made them?

On the other hand would it be possible to fit a hidden switch and probably some extra wiring so the car could run with the ammeter disconnected most of the time and just switch it in occasionally to check things are OK?

Chris

An ammeter runs via a shunt in series. all of the current passing between alternator and Battery pass through it. It measures flow into and out of the battery so will fluctuate depending upon what is happening. If you are periodically showing a large flow into the battery then it could be pointing to the regulator on the alternator being at fault or a flat battery or damaged ammeter.

A volt meter is connected in parallel and as such very little current passes through the gauge behind your dashboard.

I am not sure where you are feeling you are getting a dangerously high reading? If you are passing charge to your battery but no flow from it via various items switched on then you will show a positive input when the car is running and the alternator charging. If you have a load of accessories switched on when the car engine is switched off then you should be showing a negative discharge.. if it is fluctuating from one extreme to the other then you may have a knackered gauge, knackered battery not holding charge adequately or a blown diode / regulator in the alternator

N.

[phillpot]

Mr Tiger said:

The spec sheet said its 36 to 43 amps so I hope it'll be safe.

A Lucas 17 or 18 ACR? It'll be fine, will only put out as much as is being "asked for" to a max of 43amps

Mr Tiger said:

I still think a volt meter would be better. Does anyone know if AC made them?

Nice cheap 'n simple option ... clicky

Mr Tiger said:

switch it in occasionally to check things are OK?

Trying to add a switch (or relay) to high amperage cables is not a good idea, imho, just adding possible issues. If the ignition light is out everything is fine!

[Mr Tiger]

I think the new alternator has solved the problem - although I haven't driven the car since fitting it. The ammeter had suddenly started reading +40 when driving with no electrics on. I thought this was probably dangerous. It now reads around zero with the engine running at about 2000 rpm. It shows a negative reading when cranking and about +15 with the radiator fan on with the engine running. I hope this all sound about right.

Thanks for the input and ideas. I hadn't heard of a plug in volt meter before.

Chris

[Stevo302]

I got one, they are pretty cool. Its the only LED thing in my dash. Nice to know all readings wile you are at standstill and driving.

[whitewolf]

Pictures would help alot!

One of the gauge maybe?