Measuring Electrical Loads

[ian010778 21]

Hi everyone,

 

Something that has been bugging me for a while is the affect that having electrical devices turned on has on the idle on my 205.

 

I know there will always be some impact but mine seems to suffer quite badly - for example, with the headlights on, heated rear window on and heater fan on half speed it will drop by as much as 4 or 500rpm on the rev counter depending on how warm the engine is and if the radiator fan has come on. I also find that if I press the brake pedal whilst the car is idling then this also drops the revs further and has led to stalling on the approach to junctions in the past. I wouldn't class these as extreme conditions (especially in winter) and I want to do some investigation to see if I have any faults or partial shorts that are causing a higher than necessary current draw.

 

I've done loads of work on the air and fuel sides and the car idles really well now when there's no electrical load but I can't help but think that I'll see my old cutting out on the approach to junctions problems come back when I get out on the road at night or in the rain. At the minute I have the idle speed set slightly high (about 1100rpm on the rev counter when warm) to compensate but even so, when warm, if I turn a few devices on and press the brake pedal repeatedly it very nearly kills the engine. I do find that the affect is less pronounced when the engine is cold and the SAD is open (fast idle) but once the SAD closes (squeezing the pipe has no affect on idle) then the electrical impact on the idle gets worse.

 

So, how do I go about testing all this?

 

My first thought is to place my multimeter inline with the battery -ve terminal and the -ve clamp and measure the increase in current draw as various devices are turned on, make a note and see if there are any trends that appear by way of the circuits that devices are on and combinations of devices but I'm not sure if I should do this with the engine running (would this be dangerous/would I be risking blowing my meter?) as I don't want to flatten the battery by off and oning loads of devices for a day. I then think it might be a case of testing individual circuits by removing fuses and completing circuits with my meter to see if any circuits are drawing any/high current when the engine is off as this might give me clue as to a partial short but this is only what my common sense is telling me - any other ideas would be welcome please.

 

Also, I don't really know what is an acceptable increase in draw for each device. The car doesn't blow fuses so I guess, if there is a/a combination of faults then I'm relying on subtle increases in the meter readings and I don't have any reference points - can anyone help me with this please?

 

The one that really freaks me out the most is the brakes - however I'm not sure how applying the brakes might impact the idle outside of the electrical circuits associated with the brakes (lights etc.) any ideas would be welcome please as I can't assume that the drop in idle speed due to pressing the brake pedal is 100% electrical? I don't really understand how the brake system integrates into other systems that could affect the idle and I need to eliminate any other causes too so again, any help would be appreciated please.

 

I should add that the alternator and battery are in good condition. The battery is a new 60ah battery and the alternator is from a 306 D Turbo - the voltage across the battery terminals is good regardless of the electrical load (14.8v when idling with no load, 14.3v with the headlights and fan heater on). Also, there is one unknown in that the idle/mixture screw hasn't been calibrated as yet - I have followed Damir's advice on here to achieve a ball park setting and this will be set up properly next Monday.

 

The final piece of this puzzle is that although I do get good voltage across the battery terminals, the battery light on the dash does glow faintly if I use a lot of electrical devices when the car's idling. I also find that operating the electric windows seems to dim the dashboard lights. When holding the car at about 2.5k rpm, all of this goes away - the dash lights are unaffected by anything and the rpm stays constant regardless of what i turn on.

 

After reading numerous other posts, I've made sure that the alternator exciter wire is good by watching the lights on the dash when starting and as far as I know all the earths are connected and in good order.

 

I'd be really grateful for any pointers that could save me a bit of time chasing around the car with my meter please. It'll be a shame if I can't get this sorted after all the work I've done stabilising the idle as it does idle really smoothly now (until I turn a few bits on )

 

Many thanks in advance.

 

 

Ian.

[GLPoomobile 958]

Ref cutting out under braking. Not sure if it's relevant, but your servo relies on vaccum from the inlet. Wonder if that's in any way related, such as if your mixture was only borderline correct, could the actuation of the servo be upsetting your mixture?

[Green Machine 3]

It sounds like your alternator is not putting out enough charge when running, but to rule out your "electrical problem when braking".

 

Remove the electrical side of the braking issue for temperary tests, go under the drivers dash to where the pedals are on the brake pedal if you folow it to the top you will see a micro switch, just unplug this and then try the brakes when engine is running, the micro switch is just a make when pressed switch so dont panic about reconnecting the correct pins.

 

if all else fails I can take a look as im in Leicester aswell

[ian010778 21]

Cheers both,

 

I've got next week off work to do 205 stuff so will have a look then.

 

Green - whereabouts are you? I'm in Thorpe Astley near the Meridian cinema - were you at either of the East Mid's meets in March and April?

 

 

 

Ian.

[Anthony 1,000]

My first thought is to place my multimeter inline with the battery -ve terminal and the -ve clamp and measure the increase in current draw as various devices are turned on, make a note and see if there are any trends that appear by way of the circuits that devices are on and combinations of devices but I'm not sure if I should do this with the engine running (would this be dangerous/would I be risking blowing my meter?)

Don't do this!

 

Most multimeters will only handle 5-10 amp, and you're liable to be pulling more than that with any kind of noteworthy electrical load applied - a quick estimate is about 9-10 amp @ 14.4v for the dipped headlights and tail lights alone for example, without any additional draw.

 

Heavy electrical load will drop the idle speed a little on a Jetronic 205 as there is nothing to compensate for it - no idle control valve and no way for the ECU to increase ignition advance. However, in my experience, if the setup isn't right with regards idle mixture, ignition advance etc, they can be particularly susceptible to excessively dropping and stalling, and yet they're normally fairly well behaved when there's only light electrical load - just as yours sounds.

 

Basically, Jetronic can be an absolute pig to get spot-on, and later Motronic management is usually much less sensitive.

 

The brakes having such an impact sounds strange though, and as mentioned above, I wonder if it's something relating to the vacuum/servo rather than electrical. Easy enough to verify, as you can remove the pipe and block it to prevent the engine sucking in unmetered air - just be warned that with no assistance the brakes will need considerably more pedal force for the same braking effort! The current for the brake lights should be fairly minimal (~3 amp) and I wouldn't expect any impact on idle speed/quality from that alone.

[ian010778 21]

Thanks Anthony - I wasn't sure about connecting the meter and I'm glad I asked about this first!

 

I'll have a look at isolating the brake servo and see what happens as a first port of call as I'm coming to the conclusion that I may have two problems here (one with the brakes and one with the general set up). I'm hoping I'll find a split pipe and it's just an air leak.

 

You mentioned something above that I've never looked at or considered until now - the timing. I've never (knowingly) had it checked in the 13 years I've had the 205 and given my questions/the advice I'd been given about fuel types a few weeks ago I'm starting to think I might need to have it looked at.

 

How sensitive is the timing adjustment on an 8v? I haven't got any equipment to check it properly but if I mark the current position to provide a reference and then carefully move it a mm at a time just to see if the idle becomes more stable with an electrical load would this be a big risk? If not can I do this with the engine running? I'm thinking, at least this way I can inform the garage what needs doing on Monday and save a bit of labour cost in them doing any investigative work if I at least have an idea if the timing's out and in which way (I already know it is over-fueling slightly at idle and if I can also tell them it needs to advanced or retarded based on me 'having a play' that would help).

 

 

Ian.