voltage drop in gp relay - PeachParts Mercedes-Benz Forum

MotorCityMadman · #1 06-08-2009, 05:12 PM

It appears I may have not been diligent in my trouble shooting of my lack lustre starting 300SD. I took the gp relay apart and found 1 set of points(the smaller set) with a small arc of metal. I tried as I could to clean them up. After reinstalling I found 12.5v at the positive post on top of the relay. When I took off the large plug form the gp relay and tested for voltage in there was 11.5v. So I am losing 1v in the box. Is that normal or could it still be the points problem?????

MotorCityMadman · #2 06-08-2009, 08:00 PM

1v lost in the box when gp are not connected to the relay.

MotorCityMadman · #3 06-08-2009, 08:22 PM

Can someone pull their 5 pin plug and measure the voltage there relative to the positive post on the relay???

Diesel911 · #4 06-08-2009, 10:24 PM

I just pulled off the 5 hole connector going to the Glow Plugs and tested the 5 pins on my Relay during the Pre-glow cycle and I got 12.79 volts which started decreasing until the Relay shutoff.

This is a quote from the FSM: "The pencil element Glow Plugs for a current of 11 volts and are operated in parallel."

Also at this point I am wondering if you are using an Analog type Voltmeter (one with a pointer). If so it is possible that the meter is not accurate enough.

In any event from your other thread I think if your are getting 10.5 Volts at your Glow Plugs with your Glow Plugs in the Engine and the wiring hooked up properly that should be enough to heat the Glow Plugs. A 0.5 variation in voltage from the minimum 11 volts is not very realvent and I would say that the relay is working.

I know that some folks have cleaned up the Relay Contacts and continued to use the Relay. If I saw that there was some serious burning of the contacts I would most likly clean the contacts too but would start shopping for a New Relay and buy another when the price is right.

Another cheaper option is to pull a few Relays at the Junk Yard and hope that all or one works better then the one you have.

The main thing is the symptoms you are haveing.
If you still have bad Glow Plug symptoms and you think it is the plugs; pull the plugs and test each one on a fully charged Battery and see if the get that red-yellow hot color.
If you do your plugs are good.

Ether · #5 06-08-2009, 10:59 PM

With no current draw the voltage should be the same on both sides of the relay.
A 1 volt drop with no current draw indicates a significant discontinuity and probably means the relay should be replaced if cleaning doesn't help.

MotorCityMadman · #6 06-09-2009, 07:42 AM

Thanks guys. It started significantly better with 11.5v at the 5 pin plug. I imagine it will be even better if I can get the full voltage to the plug. A new relay may be in order.

911 thanks for going and checking.

MotorCityMadman · #7 06-16-2009, 06:49 PM

New Gp relay installed and perfect starts. The old one would give 10.5v sometime and 11.5 sometime and other times 12.5v at the gp pins in the box. This despite my cleaning of the points in the relay box. At 10.5v the cold starts were rough, at 12.5v the engine would snap to attention. Thanks all for the input.