Ammeter to monitor glow plugs? - PeachParts Mercedes-Benz Forum

pizzachef · #1 11-29-2008, 01:10 AM

I asked this over in the vintage section, but was hoping for some feedback from an electrical standpoint on whether this idea would or wouldn't work. Sorry for starting a new thread, but its more diesel related than vintage related anyway.

Let me know what you guys think of this...

I recently replaced the old loop-style glow plugs in the '74 240D with pencil style, replaced the toaster wires with jumper wires, and disconnected the ground. Shortly, I will also remove the dash indicator as well, to remove that resistance from the circuit. When I checked voltage at the plugs, I had a little over 10 volts, so I'm losing a little voltage somewhere and I think it may be dash indicator.

Anyway, now I have no indication that my glow plugs are good, or how many are good until I try to start the car. I was thinking of installing an ammeter (like this) to monitor the current draw, and I figured there would be a given amount of current for 4 good plugs, and diminishing levels of current for one or more plugs out.

I've never installed an ammeter before and don't really know how they connect or work, but the bigger question is, will the amperages that I read be of any use? Say I disconnect one plug and glow and read the amperage. Will that be THE amperage for when I have one bad plug down the road, or will the current draw fluctuate with the plugs' ages, the temperature, whatever?

Any electrical gurus, I'd appreciate your input. I guess I'm mostly doing this because I now have nothing to look at when I'm glowing, and the gauge would give me some satisfying feedback while I'm shivering and holding that knob

Thanks

barry123400 · #2 11-29-2008, 01:37 AM

First of all you can hook up your dash indicator if you wish as it is highly unlikely that is causing any voltage drop to the plugs. You were right to measure the voltage available to the plugs under load.

Really cheap maintenance things a to do. Remove important grounds. Battery to car body, body to engine block and clean them well. Then apply some agent even a little grease but prefferably a product designed to lessen corrosion in electrical connections in severe service enviroments. Then put your grounds back on.

Kind of a general maintenance item on an old car and never hurts to do even if there is no resultant benifit. Measure the plug voltage under load conditions again to establish if any improvement. Also clean your glow plug fuse holder connections. In fact any terminal in the direct high current circuit.

The normal way to locate an excessive voltage drop is really simple. Say you suspect a wire. Read any voltage being dropped over it. This is by simply putting one meter probe on one end and the other meter probe on the other end when the circuit is under load(glow plugs on in your case). Any voltage read is the actual voltage lost through resistance. Even ground connections can be read this way. You are looking for the largest voltage loss. Some loss is normal remember. I always like to see less than half a volt loss on any test of a connection or wire. This is not really practical always.

Do not forget the heavy relay contacts in your glow relay can get resistive to current flow with age and use as well. If practical to clean them up as well does not hurt . I would do everything else first though.

Of course if you did install an ammeter it would indicate clearly the loss of one glow plug but normally you kind of sense it is gone anyways if it fails. So this in my opinion is not a major issue. With the glow plugs on and since you do own a meter. Set it to a low voltage setting. Three volts would be about right full scale. Put one probe on the actual battery post and the other lead on the post clamp. There should not be much if any voltage lost unless your terminal and clamp are oxidized.

If there is loss clean them up and treat them as well. At the high current the glow plugs draw there will always be a voltage drop to some extent. I have no ideal what would be considered normal. The majority of losses will be caused by the size of the wire as is not large enough really if cleaning connections makes no difference. You may be able to run an additional wire alongside the original to lessen this voltage drop as well.

Many years ago I used to put pennies on the top of the terminals and battery as sacrificial metal so the corrosion was much less. They usually turn greenish and get eaten up with time. Our canadian penny is still made of copper basically. I am not sure of metal or metals used in the american penny now.

This is but one of many ways to increase the general reliability on an older electrical system and does fall under the general maintenance umbrella.

Jeremy5848 · #3 11-29-2008, 02:00 AM

Not much to add to Barry's words of wisdom. An ammeter would certainly work but would add resistance to the glow plug circuit where you really don't want any more. If you do install an ammeter, use heavy gauge wire – at least 8 gauge, maybe 4 or 6 – to minimize the voltage drop.

If you have installed all-new glow plugs then you will be good for a year or so at least and really don't need to check it every day. If the engine starts being reluctant to get going, then you can check each glow plug with an ohmmeter (about 1 ohm or less for a good glow plug).

Jeremy

yellit · #4 11-29-2008, 02:11 AM

Easiest to use a shunt for the total draw with a digital gauge....I made one for a forum member as an experiment...I will see if he has it running yet....and will post back...mounted in the ash tray alongside a voltmeter....
This way you do not have to run heavy current wiring into the cabin...

Jeremy5848 · #5 11-29-2008, 11:43 AM

Quote:

Originally Posted by yellit

Easiest to use a shunt . . .

Did you DIY the shunt or buy a commercial model?

yellit · #6 11-29-2008, 12:34 PM

Got whole setup on ebay...search digital ammeter and shunt
I will look for the seller and post back....if you have no luck...
The plugs will start drawing about 25-30 amps each for a second or two ...then rapidly fall to about 9-12 amps steady as they get hot....
So...the reading will start around 100 or so amps and then drop to about 35-40 for all the plugs total draw....
I thought about getting a read on each plug but that got too involved....a total read will let you see a history in case of 1 failing....you would see about a 25% lower total amp draw.....
I just shunted the glow plug circuit not the whole electrical system...
The little digital display fits well in the ashtray face ....takes some cutting and fiddling to get in but makes for a nice look.....also had a round needle DC meter beside the digital readout.....hooked to the ACC buss....