Power input to glow relay - 85 300d - PeachParts Mercedes-Benz Forum

Zacharias · #1 12-25-2012, 02:33 PM

Should the power-in to the glow relay (the big black wire to the bolted terminal) and 80A strip fuse have juice with the ignition in the OFF position?

I got 11.36v with the ignition off, but almost nothing once the car was started.

Tks and Merry Christmas.

Diesel911 · #2 12-25-2012, 04:13 PM

Quote:

Originally Posted by Zacharias

Should the power-in to the glow relay (the big black wire to the bolted terminal) and 80A strip fuse have juice with the ignition in the OFF position?

I got 11.36v with the ignition off, but almost nothing once the car was started.

Tks and Merry Christmas.

As in the Pic you should have Battery Voltage at the parts the Arrow Points with the Engine off. Of coures if your Fues is good you will have voltage at the other Screw that holds the Fuse down.
That is for the Main Power that goes to the Relays inside.
The Power to activate and close the Relay and to send power to the Glow Plugs comes from the Red/Black Wire during the normal Preglow cycle by way of the Ignition Switch and the Violet Wire while the Starter is canking.

Glow Plug Wire colors on the square Connector and where they go.
Here is a pin out description:

1)rd/bk (Black) ignition switch

2)vi (Violet; Purple) On turbo models goes to a Wire Terminal/Junction block and mates with a White Wire coming up from the Starter Solenoid. When you Crank the Starter + Voltages goes through the White Wire to the Violet one and turns the Glow Plug Relay On.

3)blu/wt (Blue with white strip)glow indicator Light

4)br (Brown)ground

vstech · #3 12-25-2012, 04:17 PM

if you've only got 11.36v with the car off... the battery is dead, and energizing the 100 or so amp load of the glow plugs will be too much for it.
a weak battery is only putting out 12.1 v a partially charged battery will be around 12.5v... full should be 12.76v

Zacharias · #4 12-25-2012, 04:28 PM

I will check the battery again but I believe I got about 12.6v at the battery with the ignition off and 13.86 with the car running when I checked it earlier.

Edit: I had had the battery charger on it for a couple hours (since I started the thread). I now have 13.0 volts key off, 14.29 running at the battery. I have 12.39 at the relay with the key off.

Battery terminals are clean, I clean them prior to winter every year.

So is it looking that I need to claim on this AC Delco battery? Its only about two years old.

Zacharias · #5 12-26-2012, 03:03 AM

To follow up on my earlier post....

Why would I be getting 12.6v across the battery terminals, with the key off, but only 11.36v at the relay?

Bad ground?

Or should I be getting better than 12.6 at the battery with the key off?

pmckechnie · #6 12-26-2012, 09:44 AM

One of the following will explain it.

1. You have a bad connection between the battery and the relay.
2. The relay is staying on and pulling the voltage down.
3. You are moving the ground lead of the meter to a different ground.

You are getting a 1.24 volt difference which can only be explained by one of the above. Connect one meter lead to the battery positive and the other to the fuse on the relay. Tell us what you see. Do this with the Ign off.

Paul

vstech · #7 12-26-2012, 10:25 AM

I agree... you have a bad connection somewhere. the terminal on the relay and the battery should be the same voltage.

barry12345 · #8 12-27-2012, 02:03 PM

Where where you grounding the meter? Under no load the power wire to the glow plug relay should show full battery voltage. You may have issues in your ground circuits.

This may be reflected if you used the body or engine block for a ground point when measuring that voltage for example. I suspect you were not using the negative terminal of the battery for your ground.

From the negative terminal of the battery to the power input of the relay you will have full battery voltage in my opinion. This all is based on you must have at least some current flow to drop voltage across resistance. A meter for all practical purposes presents almost no current draw. If you have less than full battery voltage then that feed has issues.

The voltage missing if it really is after the car has started and the relay dropped out is strange. It sounds like you may have measured it with the glow circuit still active but not up to scratch perhaps so it sagged down to zero volts. This line is only fused by the 80 amp fuse. Other than that it is a direct connection to the primary power sources. So it is hot or has voltage all the time normally. I would run this down rather than it errupting in some fashion while away from home with the car.

Zacharias · #9 12-27-2012, 03:40 PM

Thanks for the replies.

Those who suspected I was using the engine block as a ground were correct. Here are today's readings, taken using the negative battery terminal for a ground:

Key OFF:
Battery 12.36v
GP input bolt connection: 12.36v
At 80A fuse: 12.36v
At connector going to pin 15 on the relay (small plug): 0v

Key ON (engine not running, accessories off):
Battery 11.78v
GP input bolt and fuse: 11.58v
At connector going to pin 15 on the relay (small plug): 10.93v
At glow plugs, with relay activated:
- 3 @10.50 (approx)
- 1 @ 9.78
- 1 too hard to reach with probe

I'm having a hard time working this out. I did wonder about the meter but I did the entire routine twice and it gave me consistent readings.

pmckechnie · #10 12-27-2012, 05:01 PM

All of those numbers look ok except the battery is a little low. It should be at least 12.5, but that is ok for now. We need to know more about the problem on the car.

Paul

barry12345 · #11 12-27-2012, 06:37 PM

Go back to using the engine block for a ground. If the voltages are lower it is time to remove,clean, treat and install the grounds and the surfaces they contact. I use a little grease or a dielectric . No shame in a car this old and good preventative maintenance as well.

Zacharias · #12 12-28-2012, 12:39 PM

Quote:

Originally Posted by pmckechnie

All of those numbers look ok except the battery is a little low. It should be at least 12.5, but that is ok for now. We need to know more about the problem on the car.

Well the problem was unpredictable behaviour of the glow system. It started with it working intermittently, but when I would see the dashboard glow light, I would have glow.

In my past experience, all roads have led to the relay... so I acquired a used one and installed it. The problem then became less predictable -- sometimes I would not see any glow light, but it would glow, and other times, vice versa.

HOWEVER, before you waste effort in replying, late yesterday I think I found the issue, and if that is the case, I will be asking for the official peachparts forum corner stool and white cap. More later as I am not sure I have actually fixed it.

That still leaves the issue with the 12.36v key-off battery output, however. (I have 14.29v with the car running.)

Quote:

Originally Posted by barry12345

Go back to using the engine block for a ground. If the voltages are lower it is time to remove,clean, treat and install the grounds and the surfaces they contact.

I removed and cleaned up the main ground strap (at the battery I mean) this fall when I cleaned the battery posts and cables.

Where are the engine grounds?

qwerty · #13 12-28-2012, 01:12 PM

Quote:

Originally Posted by Zacharias

That still leaves the issue with the 12.36v key-off battery output, however. (I have 14.29v with the car running.)

That sounds like a typical tired battery. If you hurry, you can take advantage of the Advance Auto battery discount and rebate.

Zacharias · #14 12-28-2012, 01:50 PM

Quote:

Originally Posted by qwerty

That sounds like a typical tired battery. If you hurry, you can take advantage of the Advance Auto battery discount and rebate.

Thanks for that, but I'm in Canada.

The AC Delco that is in there is just over two years old... which means just over its two-year free replacement window

Diesel911 · #15 12-28-2012, 03:19 PM

Quote:

Originally Posted by Zacharias

Thanks for that, but I'm in Canada.

The AC Delco that is in there is just over two years old... which means just two years over its free replacement

If you look on the internet for a chart you will find that the Batter Voltage drops when the Battery is cold.

Of you have a Battery that has removable Caps check the Electrolyte level.
One time I also found on a Realatives Car that the Battery had a layer of scum covering the whole top of the batter and it was conducting Electricity between the Battery poles.