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Old 01-02-2011, 08:58 PM   #1 (permalink)
PrivatePilot
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Glow plug relay override, construct & install.

Ok, further to my recent glow plug replacement in this thread I wanted to do a manual glow plug override.

The factory glow plug controller, as we all know, only cycles the glow plugs for a set amount of time, varying somewhat based on sensed engine coolant temperature. These default calibrations were sufficient for the original AC Delco 9G or 11G glowplugs, but for many alternative glowplugs such as the AC Delco 60G's we installed, the factory setup provides insufficient glow time for extreme cold conditions.

My initial tests today indicate that the factory glow system can fire the 60G's for a sufficient period of time to achieve an easy engine startup in temperatures of around -5C (23F), but I expect (and have heard) that as temperatures drop to more frigid levels the factory glow cycle becomes insufficient - 60G's need a minimum of 10 seconds to preheat fully but the factory glow setup may only power them for 6 or 7 seconds in this scenario. The factory ECM doesn't know you've replaced the glowplugs after all, and reprograming it isn't simple or straightforward - overriding the glow system, however, is.

So, to remedy this issue I fabricated and installed a simple glow plug override today.

There are several ways to go about this. Initially I had considered simply running a wire from a momentary button inside the cab to the stock glowplug relay and connecting it to the trigger circuit. When energized, this switch would cause the stock relay to close and stay closed as long as you held your finger on the button.

On further thought we had concerns about the fact that there would be voltage backfed to the ECM while you hold this button - the ECM would continue to attempt to control the glowplug relay for normal starts, but when the override was used the trigger circuit would continue to be fed voltage even after the ECM commanded the relay to disengage. This voltage "backfeeding" to the ECM seemed potentially problematic. Yes, a diode or something could be wired in, but this added complexity.

I decided instead to install a complete secondary relay with all separate wiring. This has several benefits - one, when energized manually the factory relay is no wiser that it has been overode, and there is no backfeed voltage being fed to the ECM, therefore (perhaps unfounded, but IMHO) less risk for ECM damage. The second benefit is that in the case of failure of the factory glowplug relay there is a completely secondary manual system in place that will still allow me to get the truck started.

I haven't personally had any issues with the factory glowplug relay, but it does pop up as an issue here occasionally, so it's clear they do occasionally fail. If it occurs on my truck, it won't stop me!

So, here's what I started with:

1/ Auto supply store generic relay. The one I bought was $10 and rated for 80 amps - sufficient (just) for 8 glow plugs.

2/ Two lengths of 4 gauge wire. 4 Gauge is way bigger than technically required, but I had it handy already in my garage and I often opt for overkill as opposed to marginal - I want to make sure that I can always get plenty of amps to the glow system in extreme cold.

3/ Crimp on connectors for said cables.

4/ Momentary button (not shown in pics) and indicator light (optional).



Second, I fabricated the cables. Strip back the insulator, insert into cable ends, crimp.



One of two completed cables:



I covered the wire that will go from the secondary relay to the factory relay in a protective covering since it will be in the vicinity of parts that may vibrate - this should protect against chaffing and damage to the wire.



I test fit the other wire leading from the battery to the location of the secondary relay and fabricated it in a similar fashion. It's grey in the picture below, just a different style of cable, again, what I had kicking around.



So, I mounted the new secondary relay on the bracket that the ABS module uses - there was 2 empty threaded bolt holes just begging to be used, so I dug through my junk drawer and found bolts that fixed. This worked well since it provided a good ground to the relay without need for extra wiring.



Another picture of the relay mounted:



So, I started wiring. One lead from the relay to the battery through a breaker (not visible) and the other end up and around the master cylinder towards the factory glowplug relay.



My drivers side battery had a nice terminal installed that made it easy to slide in the new lead to the relay underneath the existing cable:



So, the wire is then tucked up and around the engine bay to the factory glow plug relay and connected to the LEFT terminal (looking from the front of the truck). The left terminal is the one that actually feeds the power FROM the relay to the plugs once the relay is closed by the ECM. Yes, when I close the secondary relay voltage is backfed into the relay, but this is of no concern - the feed side and ECM side of the relay remain isolated from any stray backfed voltage, so the ECM is safe.



I did take one closeup of the actual factory relay with the lead from the secondary relay hooked to it, but the picture didn't turn out. I'll post it here anyways as a placeholder and replace it tomorrow with a better pic:



After that was done I ran a single wire into the cab and wired it to a momentary (Push and hold) button which is in turn wired to a +12V switched power source. The other end of the wire is hooked to the trigger (close) circuit of the relay. I will add pics of these two things again later as well.

So...now I have two choices - Allow the ECM to control the glowplugs normally - my secondary system in no way effects or disables the factory system. If I opt for this the system will fire the plugs as normal, although in extreme cold, perhaps not for quite long enough, but (after testing) more than long enough for near zero, and certainly long enough for summer temperatures.

Alternately, I can fire the glowplugs manually by simply pushing and holding the new switch inside the cab for as long as wanted. Since 60G glow plugs (as well as the other popular choice, Bosch Duratherms) are self-regulating there is no fear of burning out the plugs by leaving them turned on too long (within reason), but take note that other brands of glowplugs may not be self regulating and may need to be operated under very time-limited cycles, so make sure you cycle your glowplugs properly under manual control.

And that's about it! Intact automatic control with optional manual control to extend the glow cycle...and act as an emergency backup as well.

On a funny side note... I started by tarping over the hood of my truck and firing up a propane heater under there to keep the wind and cold out. It was a bit miserable this afternoon...but nice and cozy so long as I stayed hunkered up in the underhood area.

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Old 01-02-2011, 09:11 PM   #2 (permalink)
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Nice and simple. I'd think a remote mount starter solenoid off a ford would do the trick for that too.
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Old 01-02-2011, 09:17 PM   #3 (permalink)
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only thing i can recommend is some form of fuse in the line from battery to solenoid, for safety's sake.

When i installed my override all i did was jump the 2 contacts on the factory relay, since one side the the GP harness, and the other is an always hot lead from the batteries with a fusible link built in.
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Old 01-02-2011, 09:33 PM   #4 (permalink)
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Quote:
Originally Posted by IamDave0887 View Post
only thing i can recommend is some form of fuse in the line from battery to solenoid, for safety's sake.
I have a 105 amp breaker installed. Didn't make it into the pics.

Quote:
Originally Posted by IamDave0887 View Post
When i installed my override all i did was jump the 2 contacts on the factory relay, since one side the the GP harness, and the other is an always hot lead from the batteries with a fusible link built in.
As mentioned I thought about going this route but was concerned about the backfeed issue since the ECM would be seeing voltage it wasn't expecting. I'm sure this method has been done anyways (you have, so clearly it works) but I didn't want to risk it. The fully independent system also acts as a backup, just in case - if the stock relay ever takes a crap I'm still good..and given my Murphy's Law thing, this will happen to me way up in Northern ontario when it's -30 out and my hood is frozen shut.

I had also contemplated just putting the relay on the firewall right near the factory relay and using the hot line from there, but I had more then enough cable to go with the direct-to-battery option, so again in the interest of redudancy, I went that route.

For a $15 total investment it seemed worthwhile to go the secondary relay route as opposed to manual control of the stock relay.

I should have mentioned that this is NOT the simplest method of going about this task..just the way I opted to go.
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'97 C3500 Crew Cab Long Box Dually 6.5 Turbo Diesel.
Rebuilt with Navistar 506 block spring 2012
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Find my list of reference information HERE
New members, please introduce yourself HERE

Last edited by PrivatePilot; 01-02-2011 at 09:36 PM.
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Old 01-02-2011, 09:53 PM   #5 (permalink)
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I am a little concerned about your crimp or lack of I guess, Smashing it in a vice does not shrink the diam. as a crimp does I think in your case without a proper crimping tool you should have soldered it IMHO You just wont have a very tight connection on it
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Old 01-02-2011, 09:59 PM   #6 (permalink)
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Quote:
Originally Posted by racedaymechanic View Post
I am a little concerned about your crimp or lack of I guess, Smashing it in a vice does not shrink the diam. as a crimp does I think in your case without a proper crimping tool you should have soldered it IMHO You just wont have a very tight connection on it
Good point - I did actually have one come loose when I tugged on them after the initial crimp in the vice - I had the same concern.

Subsequently I took the ball peen hammer to the crimped portion and really snugged them down. After that, even standing on the terminal and pulling on the cable with both hands yielded a tight fit.

If I was to do this again I would buy pre-fab battery cables off the shelf - the one end of one cable I used (the one hooked to the battery) had a prefab cable end already on it which is much more slick/secure.
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Old 01-02-2011, 10:03 PM   #7 (permalink)
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Just remember that th GPs pull 105-110 amps. You may end up tripping that breaker.

When i made my 1/0 gauge battery cables i did the vice method. Crunched the connectors down real good, and took a metal working hammer to the connectors after to crimp it down. Then i used heavy duty heat shrink with glue inside to weatherproof the connections. Before i used the heatshrink tubing i tried to separate the connector from the wire, and couldn't.

Cables turned out very professional looking, and the truck certainly cranks like a champ.
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Old 01-02-2011, 10:11 PM   #8 (permalink)
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Nice install, but you type to much haha!

I'll make sure to add this to the new FAQ thread i've been working on.

BTW (for current readers), the indicator lamp is only needed on '96+ trucks since the computer on OBDII controls the WTS light while on pre-96 the light was triggered by power to the glow side of the relay.
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Old 01-02-2011, 10:14 PM   #9 (permalink)
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Quote:
Originally Posted by IamDave0887 View Post
Just remember that th GPs pull 105-110 amps. You may end up tripping that breaker.


I was sure I had read that 60G's were rated 10 amps each. Sure enough, now I see 13.5 amps each for a total of 108 amps.

That's exceeding the technical (although probably not actual) limit of that relay, as well.

Hmm...
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Old 01-02-2011, 10:16 PM   #10 (permalink)
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Quote:
Originally Posted by Green Machine View Post
Nice install, but you type to much haha!

I'll make sure to add this to the new FAQ thread i've been working on.

BTW (for current readers), the indicator lamp is only needed on '96+ trucks since the computer on OBDII controls the WTS light while on pre-96 the light was triggered by power to the glow side of the relay.
98+ trucks. my 96 the light will come on when i jump the relay.

98+ the glowplug light is ECM controlled. 97 and down(including the 6.2) the light would come on when the relay is jumped.
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