Project: GPR Bypass
There have been many threads on
the glow plug relay bypass idea discussing how and when to turn off the relay,
and even whether or not it is a good idea at all. I have come up with a
combination of ideas that I feel is the best scenario.
How the glow plugs work depends
a lot on the ambient temperature that the computer reads from certain sensors.
The computer determines how cold it is outside and sends a signal to the glow
plug relay to stay closed for a certain amount of time. The colder it is, the
longer the relay stays closed, up to approximately 2 minutes. You can tell when
the relay opens by watching the voltmeter after the truck has been started. The
needle on the voltmeter will swing to the right indicating less drain on the
charging system. The computer also sends a signal to the ‘Wait To Start’ light
on the dash. The light is on longer when it is colder outside, about 8 seconds
or so, but it is nothing more than an idiot light that should be used as an
approximation of the time needed for the glow plugs to warm up.
When the temperature is below,
say, 30 degrees, the ‘Wait To Start’ light would stay on for about 8 seconds,
but the glow plugs are on for about 2 minutes. It is a good idea to wait even a
minute or so after the light comes on to make starting the engine easier. Also
remember that every time you start the truck, the glow plugs are active.
Essentially you are combining the drain of the glow plugs with the drain of the
starter, which ultimately takes it’s toll on the batteries. Some of the later
PCM tuning, however, does cut off the GPR cycle when the motor is up to
When the temperature is warmer,
the PSD will start willingly with no waiting for the glow plugs to warm the
combustion chamber, so the plugs are on during warmer months when they really
aren’t needed. The intention, then, is to decrease the amount of time they are
on to extend the life of the glow plugs and relay.
Overview of Project
Some feel the glow plugs should
be left to cycle on and off as designed to minimize the chance of carbon
build-up, which would diminish their effectiveness as well as hinder replacement
of the glow plugs. Others say to turn them off completely in the summer when
they are really not needed at all to extent their life. What I wanted to do was
to combine ideas from posts that I read on the ’94-’97 forum, and I need to point
out that the ideas are not mine and I have tried to identify the authors of the ideas. Nonetheless, my GPR bypass system consists of
three parts. 1) The actual disengaging of the GPR after the truck is warm to
prolong glow plug life; 2) The actual disengaging of the GPR during starting to
prolong the life of the batteries and to ease starting; 3) An indicator light on the dash that shows
exactly when the glow plugs are on.
1) NAPA # TS6709 Coolant
2) NAPA # AR201 Relay
(normally closed contacts)
3) Spade type weather proof
fuse holder and 3 amp fuse, for indicator light.
4) Dash indicator light,
your choice on color/style/size.
5) Approximately 40
feet of 16 gage wire, various colors help.
6) Spade, and ring
terminals, and heat shrink tubing.
HERE to see an electrical diagram
of the GPR Bypass System.
Building the GPR Bypass
the GPR after the truck is warm. This is accomplished
by installing a temperature switch in the pipe plug located on the water neck
below the thermostat on the front of the engine. This switch is off of a mid
1980’s S-10, and is basically what turns the electric fan on when the engine
warms up, on those vehicles. The thread type is exactly that of the plug on the
water neck. When wired as shown in the diagram, once the engine is up to about
175 degrees, it will cut the signal from the PCM, opening the circuit in the GPR.
This way, the glow plugs are guaranteed to cycle at least once a day, every time
the truck is first started, and will not come on when the engine is warm, like
when you have several stops to make and end up starting your truck several times
through the day. This is what killed my first Glow Plug Relay about 2 years
after I bought my truck. I was working as a construction supervisor, so I was
starting the truck 15 times a day, and eventually the relay gave out.
Installation of the switch
isn’t too difficult. Getting to the plug is tight, and make sure you have the
new switch nearby, with Teflon tape applied, because when the plug comes off a
stream of coolant with impressive force will want to wash down the front of your
engine. Unscrew the plug then quickly screw on the new switch. When
tightening, pay attention as to the location of the connector prongs. Having
them turned as shown in the picture will assist in attaching the spade
Disengaging the GPR while cranking to start the engine.
This is done by using a relay with normally closed contacts. The one I
used is a standard automotive relay. Take a wire from the starter relay on the
right fender, from the same terminal as the small wire that goes to the starter
telling it to activate, and run it to the new relay as the power source.
Connect 2 wires intercepting the PCM signal to the GPR on the 2 other terminals
of the relay as shown in the drawing. Now, when you crank the
engine, the same wire that tells the starter to turn is opening the contacts in
the new relay, thus cutting the signal from the PCM to the GPR. When the truck
has started and you stop cranking, then power is terminated and the normally
closed contacts connect, continuing the PCM signal to the GPR. This should
help extend battery life and assist in starting on those really cold mornings.
I wired this to work all the
time. If wired according to
the diagram, it will cut the signal regardless if
the dash switch is left in ‘Automatic’ or ‘Stock’ position.
light. I wanted a more accurate way to determine when the glow plugs were
actually on than by using the factory Idiot light or by relying on looking at
the voltmeter. I installed a light in the blank spot on the
dash to the right of the ‘Wait To Start’ light, grounded one side, then ran a
wire to the Glow Plug Relay on the terminal leading to the glow plugs. It is important to have a fuse located as close to the relay as
possible, because if the wire should short, there would be a lot of current
flowing, and the results would sure to be an electrical fire. I installed a ring terminal as
close as I could to one side of the spade type fuse holder. Then, use a 3 or 5
amp fuse in the wire to the light, and whenever the glow plugs are active, the
light will illuminate.
The indicator light is an ideal
test light for a couple of things. It can be used to confirm that
the GPR is working properly. It can also be used to know if
the engine has cooled down enough that you need to wait for the glow plugs to
come on. For example. If you drive over to a buddy’s house to help work on
his PSD, and when you go to leave the light comes on, then you know the
engine has cooled down to the point to where the temperature switch has closed,
sending the PCM signal to the GPR, so you should wait and let the plugs do their
Also if you’re working on the
glow plug system, you might as well take the time to switch the large wires on
the relay. By switching the large terminals you can
extend the life of the relay because most of the wear comes on the hot side of
the relay. By switching the large wires, the hot side will now wear on the
other terminal. If your GPR is old, this may not help much. But if
it’s a couple years old, you may get a couple more years out of it.
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