Battery warning light at low revs

This site contains affiliate links for which LandyZone may be compensated if you make a purchase.
guineafowl21 said:
Hi Joe

I have a diode tester on my multimeter which proves handy for fault-finding battery charger output boards and the like, but I've just bought an old Hameg 20 MHz scope and would like to get some use out of it on the car. Would a duff output rectifier on an alternator show up as a ripply output?

Also, what else could it be used for? - I was thinking ABS sensors, MAF, MAP, etc as well as injector and solenoid PWM signals.

When I get some spare time I'd like to start scoping stuff all over the car so I know what normal looks like. Any advice you might have would be much appreciated.

Cheers, GF
Click to expand...
Hi GF, nice one, I like the old Hamegs - what did you get ? a 203 ? .. nice units, aint seen one for a while. I got rid of all my old and quality stuff when we went live aboard sailing - started using the DSO units - they get better all the time. Anyway, yes, you can see a definite ripple but it can be damped by the battery depending on charge state - still noticeable though.
Hall sensors on reluctor pickups etc (ABS) - - great with a scope ! :) - perfect use - you can even check compression on the cylinders lol :) - seriously - you can do a 'relative' compression test on an engine - diesel or gasoline - by looking at battery voltage drop on every compression stroke - you simply hook the scope input to pos and neg on the battery, obviously a recording or DS scope is better but you dont need one - you can see if a pot is down as the load on the starter at each compression stroke will cause a slightly less voltage drop. It is better with a dc current clamp adaptor linked to the scope but they are expensive ! - and the voltage method is fine :) :) . on a petrol disable ignition and use WOT, on a diesel disconnect the shutoff solenoid.
Maf and Map, certainly better than a multimeter as the digital ones are too slow to react and the analogue ones (if you can find a good one) are too low impedance.
Injector pulses yes no worries - just be aware of some of the dangerously high voltage boost on some modern units at high current levels - not an issue for the scope but just don't hold on to the connector :). You will need a bnc to dual crocodile clips and some back probes really to make up a kit.
Any PWM it is perfect for. I used mine recently to test the PWM to the 'commencement of injection solenoid' on the VP37 pump on the L series.
For baseline checks the DSO is a real boost. I think the 208 Hameg had DS ?.. for a lot of work on a car the small scope in a box usb adaptors are pretty good and cheap now. Linked to a laptop you can save waveforms etc - and also record and replay periods back.
Great tool. :) enjoy.
Joe

edit - just checked mine - I couldn't remember the make lol - the bench one is a Hantek 5102P, the knock about one is a UNI-T 2025C. both DSO. -
 
Last edited:
Hameg HM 203-5. It's got a fancy (to me, anyway) little component tester function that graphs diode responses and all sorts of other stuff. Great fun for £30 on Gumtree.

I vaguely remember a ScannerDanner vid (youtube) where he showed a waveform from an alternator with a bad diode, and he said to make sure you probe right at the B+ terminal, not at the battery. I'll try that at some point.

Yes, I might look into a simple DSO - there are some kits on eBay that might suit my simple purposes - the Hameg is a heavy bulky thing but I'm sure would have been used 20-30 years ago to do car stuff as you suggest. Many thanks for your advice.
 
Hi GF, nice unit !.. Yes do probe on B+ as otherwise the battery will damp it over the cable run.

Also, for other purposes it is worth noting that you 'could' also use the battery cable as a crude ammeter shunt and connect the scope from B+ to Batt+ or from starter + to Batt. Most times in automotive use the B+ and Starter + are connected together . (we used to do that in marine work with long HD cable runs and mostly in marine the B+ and starter are not wired directly together .. :) ... worked well. ) - you would be then measuring proportional current flow. - that can also be used for relative comp testing :) :) (as can the Negative Batt to Body if the run is long enough - usually not in automotive - again good in marine but I digress)

I like Pauls channel (scannerdanner) - wouldnt mind his book either. Also have you ever watched Eric O at South Main Autos ? - he is pretty damn clued up and his diagnostic work is absolutely first class. (not the eric the car guy one - he is occasionally fun to watch with his ford fairmont build but he aint got a clue on diags unfortunately - or welding lol)
Eric O at SMA (South Main Autos) is well worth subbing on youtube. Of course he uses the Autel and the Snap on units but the scope stuff can all be done on a standard scope no worries.,
He is also a great advocate of one of my favourite tools - a good quality test light with a couple of 100 mA load via the bulb - the 'scope on a rope' :)
You will have great fun with Eric O at SMA. .

I just picked up the Lynx diag kit a few weeks back - that is pretty awesome too !.. again hooks to a laptop.. you can even do full one man brake bleeds via the ABS unit processor hook up. It also purges the ABS as normal bleeding with abs can be a real pain. covers most anything you could want on the freeby.
Joe
 
here is another way of testing your alternator wait until dark the start your engine then put everything on then while everything is on put you foot on the brake peddle this should make the warning light glow more, if it does glow more then there is a problem with alternator and like said it will be more than likely diodes
 
Good idea to use the battery cable as a shunt - I suppose I could use an injector signal on the second channel to mark cyliner 1, or even use the external trigger.

Never seen Eric O - will give it a shot.

I was thinking of one day getting a proper OBDII thing for our 04 Td4 - Pscan and iCarsoft have been suggested too. I wonder which is best on price and functionality?
 
guineafowl21 said:
What do Halfrauds do? Surely they just measure output voltage and if it's below 13.5 or so try to sell you a new one?!
Click to expand...
Yup.... they even have a little fancy printer (handheld) to give you a printed bit of paper to say the same thing. :eek:
Anyone can test the alternator with a multimeter. low voltage but rising above battery on higher rpm and warning light on at lower revs will be rectifier. No output, lught on permanently or no signs apart from battery boiling usually means regulator. nice and simple. ;) . some of the modern ECU's will put the money light on if Batt + is over volt.
 
Steady on MHM - so, (I'm interested), I can see a battery analyser has real value, but how does the 131 tell you more about an alternator than a DMM?
 
guineafowl21 said:
Steady on MHM - so, (I'm interested), I can see a battery analyser has real value, but how does the 131 tell you more about an alternator than a DMM?
Click to expand...
It is extremely simple - it follows the same algorithm that anyone with any basic knowledge would. It looks at voltage drop on starting (current with the optional probe) - the recovery time from original state voltage and charging voltage (even is high tech enough to have an ambient temperature sensor - OOooooooooooooo !!!!) .. that's it in a nutshell.
I am sure even a numpty like MHM could use one with a hell lot of practice ....
:rolleyes::D
 
... So they wouldn't differentiate bad reg/rectifier or something else, just charging voltage under load?

Sounds like what I used to do (before I got my super scope!) - stick DMM on battery, check voltage drop on cranking, also cranking drop from +ve batt to +ve starter, and -ve to -ve drop along earth strap. Then switch on headlights and other stuff and check output doesn't drop below 13.5 V or so.
 
guineafowl21 said:
... So they wouldn't differentiate bad reg/rectifier or something else, just charging voltage under load?

Sounds like what I used to do (before I got my super scope!) - stick DMM on battery, check voltage drop on cranking, also cranking drop from +ve batt to +ve starter, and -ve to -ve drop along earth strap. Then switch on headlights and other stuff and check output doesn't drop below 13.5 V or so.
Click to expand...
No, they are more interested in a go - no go situation. The alternator side is a simple measurement at idle, and then at a preset rev range - most of the time the 'technician' does it by ear. All the BS about CCA (Cold Cranking Amps) and AGM Wet etc is simply smoke and mirrors. there are recommended float levels for each type however with advancements in glass mat tech the difference is now negligible. Even then it cannot recognise any of these factors, the user has to enter them - but never will !.
Note, one of the main selling points is that you can have your business details added to the little money ticket print out to keep the little darling customers happy and ensure repeat business...
About sums it up.. Great tool for a job seeker with half an O level and a budgerigar on loan to somewhere on a zero hours contract.:D
 
Sorry, MHM, I suppose 70,000-odd posts don't necessarily make for a deep knowledge or insightful comments. I didn't consider 'they load the system and attach a BOSCH DIAGNOSTIC DEVICE' to be sufficiently informative - more designed to impress than inform.

Telling a man who's designed battery analysers that he knows nothing about battery analysers - well, OUCH. No wonder you fooked off.

And indeed, as I suggested and Joe_H confirmed, it means nothing more than: they switch your headlights on and check the output voltage. But, hey, what do you expect for free? A printout?!
 
not at all... just had better things to do.

I do not know everything, never have said I do. I am surprised that you know more than Bosch, but so be it. good luck in yo fault finding.
 
You must log in or register to reply here.

Similar threads

R
P38 Alternator Fault
Replies
2
Views
469
Datatek
Datatek
L
Disco 4 (LR4) battery light on
Replies
9
Views
1K
Graculus
Graculus
Kerlaan
Freelander 1 SRS oddity
Replies
6
Views
598
Kerlaan
Kerlaan
HeywoodFloyd
Freelander 2 wont start, short crank
Replies
16
Views
1K
Nodge68
Nodge68
s8ndst0rm
Transfer box clunk, low revs, whilst turning
Replies
11
Views
958
lynall
L
Back
Top