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

pizzachef · #1 11-24-2008, 07:00 PM

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, 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

katja · #2 11-25-2008, 01:14 AM

I suppose you could wire a gauge in easily enough to monitor the current across the battery, but do you still notice the charge light dimming when you activate the glow plugs? And does your car start any better after installing the new plugs (like better idle, less smoke) or is it just a shorter wait time?

Does the heater coil in the salt shaker still glow? I would think it would, as it is in series with one of the plugs. I can see how that element would drop the voltage across the rest of the circuit though. I wondered if you could replace the heater coil with a small light bulb (keeping it in series with the glow plugs); maybe something rated over 12 volts so it would glow a dimmer orange & look more authentic.

A lot of people on this forum seem to recommend the change to the pencil-type glow plugs. What concerns me about in the conversion is that the pull start switch & wiring was never designed to handle the amperage (80, I think) of the pencil glow plugs vs the 50 or so the series plugs draw. Therefore I think there should really be a fused relay. I think the nicest way to do it (and I've seen a W115 240D where this was done) would be to put in a 300D speedometer that has the glow plug light built in, get the glow plug kit that comes with the conversion plugs and a relay/timer, and then wire it up so it turns on when you turn the key; you wouldn't even need to hold the knob out anymore.

BTW that VDO ammeter looks nice, though a future project I want to do is replace my analog clock with a VDO or similar-style diesel tachometer that gets its signal from the alternator. I think any car with a manual transmission needs a tach!

pizzachef · #3 11-25-2008, 07:35 AM

Quote:

Originally Posted by katja

I suppose you could wire a gauge in easily enough to monitor the current across the battery, but do you still notice the charge light dimming when you activate the glow plugs? And does your car start any better after installing the new plugs (like better idle, less smoke) or is it just a shorter wait time?

When I glowed for about the same amount of time (didn't clock myself) the start was easier sounding on the starter/engine, and quicker.

Quote:

Originally Posted by katja

Does the heater coil in the salt shaker still glow? I would think it would, as it is in series with one of the plugs. I can see how that element would drop the voltage across the rest of the circuit though. I wondered if you could replace the heater coil with a small light bulb (keeping it in series with the glow plugs); maybe something rated over 12 volts so it would glow a dimmer orange & look more authentic.

Yes, it still glows, but not as brightly. And after a few seconds (maybe 10) it dims a little. I had read that people swap out the indicator with 12ga solid wire (see this thread, post 11, hey you posted back there too

) I had thought about changing it with a light bulb or even putting it in parallel with the other plugs so it would still glow, but then it would just be for show and wouldn't really tell me anything (other than I'm getting electricity to the plugs). I still wouldn't know if I had one or two out...other than harder starting. How does the relay (on the newer cars) know when there a coupe plugs out? I think the dash light does something funny when a plug or two are out.

Quote:

Originally Posted by katja

A lot of people on this forum seem to recommend the change to the pencil-type glow plugs. What concerns me about in the conversion is that the pull start switch & wiring was never designed to handle the amperage (80, I think) of the pencil glow plugs vs the 50 or so the series plugs draw. Therefore I think there should really be a fused relay. I think the nicest way to do it (and I've seen a W115 240D where this was done) would be to put in a 300D speedometer that has the glow plug light built in, get the glow plug kit that comes with the conversion plugs and a relay/timer, and then wire it up so it turns on when you turn the key; you wouldn't even need to hold the knob out anymore.

I like the knob

And I don't think the glow plugs really pull 80 amps, that's just a peak. And whatever fuse is in there now (50 amps you say?) hasn't been blown, so its been less than that.

Quote:

Originally Posted by katja

BTW that VDO ammeter looks nice, though a future project I want to do is replace my analog clock with a VDO or similar-style diesel tachometer that gets its signal from the alternator. I think any car with a manual transmission needs a tach!

Indeed! A tach would be a good replacement for the clock. Mine doesn't work anyway, it probably just needs the clock repair with the two new transistors or whatever, but a tach would be much more fun.

Mike D · #4 11-25-2008, 08:41 AM

You went about solving this problem the hard way.

When you change to the "pencil" style plugs you should install the Mercedes timer-relay.

Go to a junkyard, ahem, recycle center and get the timer-relay from a 1978 or later Mercedes diesel. It mounts on the left side fenderwell or in the well behind the left side headlight.

Mount it, connect the glow plugs with the leads, feed the relay with an 8 ga. wire from the battery and wire an "ignition on" wire to it. The ignition comes on, the glow plugs glow and 10-30 seconds later (depending on the temperature) the relay "clicks" off. Start car.

Eliminate the "salt shaker" and the wiring.

If you get the relay from a 300D then obviously it will have one extra glowplug lead. Just dont connect it. I suppose if you wanted to you could run this extra lead to a light in the dash.

pizzachef · #5 11-25-2008, 09:49 AM

Does this relay have a wire to power the dash light, like in the 115 300D speedometer (i pilfered a photo from the internet, its attached below)? If so I could wire this to some little light somewhere that could be my glow plug light. And does it light up (or not light up) when there is a problem with one or more plugs?

Otherwise, all it really does is eliminate me having to hold the knob out, it doesn't provide any feedback on whether the plugs are working. That's what I was trying to do initially.

I didn't realize the relay was that simple though, thanks for bringing it up. Definitely an option I might explore.

vox_incognita · #6 11-25-2008, 01:58 PM

Quote:

Originally Posted by katja

A lot of people on this forum seem to recommend the change to the pencil-type glow plugs. What concerns me about in the conversion is that the pull start switch & wiring was never designed to handle the amperage (80, I think) of the pencil glow plugs vs the 50 or so the series plugs draw. Therefore I think there should really be a fused relay.

Actually,the amperage is identical...Instead of the ballast resistor,I mounted a 50A fusebox from a w123,just in case.The push-pull device is actually very simple and is not threatened.No burnt fuse yet. I don`t care about any glow-lights and by-passed the salt shaker.Will never install a relais,as I like things as simple as possible...

strelnik · #7 12-06-2008, 01:30 PM

Quote:

Originally Posted by vox_incognita

Actually,the amperage is identical...Instead of the ballast resistor,I mounted a 50A fusebox from a w123,just in case.The push-pull device is actually very simple and is not threatened.No burnt fuse yet. I don`t care about any glow-lights and by-passed the salt shaker.Will never install a relais,as I like things as simple as possible...

Speaking of push-pull switch for the 180, 190 and 200d cars, I'm looking for one for my 180D I am restoring and one for the 190 D I am driving. I would even take one of the older lever types if they were available.

Any takers?