Wiring diagram for GP relay, or how the purple wire mode fails for me. - PeachParts Mercedes-Benz Forum

winmutt · #1 12-12-2012, 06:13 PM

So I had the purple wire mod in place last winter and it really really helped. All the way until spring when the GP stopped turning off. I can only imagine the timing mechanism is burnt out. Thoughts? Diagrams? Gonna dive into this tonight after the kids are asleep.

TnBob · #2 12-12-2012, 06:35 PM

GP not turning off = a dead battery real quickly.

Very easy to confirm what is and isnt happening.

Needed tools are one cheepie DVM and 2 alligator clip jumper wires.

To prove / disprove that the GP is screwing up the process is very easy.

Meter is placed on the DC Volt 20 volt scale. If no 20 volt is there, any range that is over 12 volts DC.

Clip one alligator clip to the battery negative. The other end goes to the negative lead of your meter.

Second alligator clip, one end clips on any reachable glow plug. On a 617.952 #1 and #3 are the easiest. The other end goes to the positive meter lead.

With the motor off the reading should be zero.

I moved the meter up under the hood to face the windshield. Then reached inside and as the key is moved to the 2nd position voltage turns the GP on and a reading of 9+ volts DC should be read.

As the GP dash light goes out, the DC voltage reading should drop to 0 !! That means your GP relay is working properly by timing out rapidly as the GP light on the dash goes out.

Before I replaced my GP relay I was getting a reading of ~4 volts DC. Getting any voltage reading AFTER the GP dash light goes out means your GP relay is either failing or has totally failed.

Confirming each glow plug is very easy from there too. Disconnect the alligator clips from the battery and the glow plug.

I left the alligator clip connected to the common meter lead and clipped it to exposed metal on the block.

Pull the larger of the two plugs on the base of the GP relay. You will find 4 connections there for 240's and 5 for 300's.

BEFORE you make any measurements, change the meter to the lowest ohm range.
Connect the two leads together. Read and remember the reading. On my meter I got a reading of .3 ohms.

Once that is done, move the red lead (the no alligator clip lead) to any pin inside the larger plug. In my case I got a reading of between 1.0 and 1.2 across all my GP's indicating that they all were actually at a range of .7 to .9 ohm.

.7 ohms is the target number.

winmutt · #3 12-12-2012, 07:26 PM

Right, except I want after glow, which is what the purple wire mod does. With the purple wire removed, the relay stays on until the max timer is hit, except mine doesnt hit the max anymore. I was thinking maye its just a stuck relay.

compress ignite · #4 12-12-2012, 08:18 PM

You can dissemble the cover and Clean the contacts,Etc.Etc.

I know you're fussing with the '87,
Cause the '91 Has the "Afterglow" type "Intelligent" Pre-Glow relay.

winmutt · #5 12-12-2012, 08:31 PM

Pulled the purple wire again and its cutting off with the timer. Tested several times, will probably verify in am that it cuts off before I get on the road.