2005 Cherokee KJ CRD 2.8 Glow Plug Module Testing Sorted

Ludi Neethling · **Joined:** Sat Feb 02, 2013 11:04 am **Posts:** 87

I am suspecting that the glow plug module is not operating correctly due to the Jeep cranking too long in the morning to start. I've checked the glow plugs' resistance: 3 GP x .9 ohms and 1 GP x 1 ohms.

I've tried to find posts regarding checking the module. In one of the posts Joe indicated that this is a solid state electronic relay and not an electro mechanical one (which, for me, is easy to test with a volt meter and 12 power supply).

If I look at the relay pins I see the following:

With the tab lock sheath at the top there are four pins (left to right), G1, G2, G3 and G4 (glow plug connectors). Below them in the center of the relay are two pins marked 87 (left) and 31 (right) I assume this will be the circuit that opens/close to switch relay on and off?). At the bottom from left to right there are three pins, from left to right: small pin marked DI, large horizontal pin marked 30 in the middle (assuming the main power supply?) and a small pin to the right marked ST. I assume the pins marked DI and ST are for input from the ambient and engine temp sensors?

Can this relay be checked for functionality and if so please provide guidance.

PS: If the ambient temp and Engine temp sensors provide correct readings to the relevant gauges (I assume this is where the ECU obtains input for the glow plug module) can I assume that those sensors are not part of the problem?

Billybob · **Posted:** Sat Sep 15, 2018 12:05 pm

I am not sure that this is even a solid state relay as Pulse Width Modulation (PWM) ie. square wave pulses of varying duration are sent to the glow plugs through this module. Maybe this module does not itself generate this square wave but merely passes it though from the ECM. The fact that this glow plug module has to be programmed depending on if you have 7 volt or 5 volt plugs would suggest that the PWM is indeed generated by this module.
A lot of CRD owners find that running 5 volt plugs while the glow plug module is programmed for 7 volts works best in cold weather.

I will leave it to the experts on the newer CRDs...my 2002 Export 2.5 CRD has 11 volt plugs which do not have a glow plug module...two fat relays get energized and pass through the PWM wave from the ECM.
The ECM only checks these two fat relay coils are neither open circuit nor short circuit....no further tests are done!

diesel_guy86 · **Posted:** Sat Sep 15, 2018 1:38 pm

Its been a year since i was elbow deep in the glow plug system, but the best way to check if it is working is to unhook the glowplug harness connector. This is by the alternator, and is a square, 4 pin plug. Measure what voltage you have coming from the module. Last system i troubleshooted ended up being a bad ground on the frame by the fuse box and headlight.

Ludi Neethling · **Joined:** Sat Feb 02, 2013 11:04 am **Posts:** 87

Thank you diesel-guy. I unplugged the glow plug harness connector and used a 12v bulb as a tester. After switching on the ignition the bulb lights up and switches off after 10 seconds at each of the 4 gp connectors. As I am not an auto electrician (but learning everyday) could you kindly indicate what readings to expect (at cold start) if the relay is ok? When I tried to test the voltage I got readings between 99 ans 101 volts even after 10 seconds (when my "test light" switched off) ???

The local Jeep dealer quoted nearly $150 for a new relay, so if I can make sure of this one is ok I would be really happy. Sasquatch has the same module for +- $60 (excluding shipping,etc).

Ludi Neethling · **Joined:** Sat Feb 02, 2013 11:04 am **Posts:** 87

I've managed to check if the glow plug module is working. I used the old wiring harness (replaced due to damage to isolation) connected to the module plug and used my original ceramic glow plugs that are still serviceable. After I earthed them I switched on the ignition and they started glowing for 10 seconds and switched off. I am satisfied that the glow plug system works fine.

Billybob · **Posted:** Mon Sep 17, 2018 9:27 am

Did you actually physically install the original ceramic plugs and old harness or did you leave them out in the open and saw them glowing?

Only way to see what voltage the plugs themselves are receiving is with an oscilloscope as it is a rapidly pulsed square wave. The experts here who know how to program the ECM are able to see the changing duration of the programmed pulses over time.

Either way glad you have found the resolution to your problem Ludi.

Ludi Neethling · **Joined:** Sat Feb 02, 2013 11:04 am **Posts:** 87

I did it in the open with the old harness connected to the system and the glow plugs earthed to the battery negative and saw them glowing for 10 seconds. I wish I had the expertise/equipment to do it the right way, but this will have to do.

flash7210 · **Posted:** Mon Sep 17, 2018 10:24 am

The glow plugs actually do receive 12 volts when turned on, and you can see this with a voltmeter.
However, this voltage is modulated (rapidly turned on and off) so that it averages out to be about 7 volts. This is often referred to as pulse-width-modulation (PWM).
This is why you need an oscilloscope to really see whats happening with the glow plug voltage.

The modulation is done by the ECM.
The ECM rapidly turns on and off the glow plug module to maintain the appropriate voltage.
The glow plug module is a solid-state relay. Basically its just a pack of transistors. The transistors are capable of switching on and off much faster than any electro-mechanical relay can.

Its well known and documented fact that the Etecno 7 volt plugs are not as good as the original ceramic plugs. Many folks, including myself, have switched to Bosch 5 volt glow plugs and seen improvements in cold starting. This is done without any reprogramming to the ECM for 5 volt plugs.
So, the 5 volt plugs are actually getting 7 volts (PWM) and seem to handle it just fine. No one has reported any failures.

Ludi Neethling · **Joined:** Sat Feb 02, 2013 11:04 am **Posts:** 87

Thank you Flash. So that is why the voltmeter fluctuated when I tried to check the voltage. Interestingly when I used the globe I could actually see it dimming/brighting within the 10 second period. I assume that the 5v Bosch glow plugs will be available in South Africa. For now, seeing as it does not get that cold here and the amount of work it took to install the 7v Etechno's I will keep this on my to-do list.

GordnadoCRD · **Posted:** Mon Sep 17, 2018 9:18 pm

flash7210 wrote:

The glow plugs actually do receive 12 volts when turned on, and you can see this with a voltmeter.
However, this voltage is modulated (rapidly turned on and off) so that it averages out to be about 7 volts. This is often referred to as pulse-width-modulation (PWM).
This is why you need an oscilloscope to really see whats happening with the glow plug voltage.

The modulation is done by the ECM.
The ECM rapidly turns on and off the glow plug module to maintain the appropriate voltage.
The glow plug module is a solid-state relay. Basically its just a pack of transistors. The transistors are capable of switching on and off much faster than any electro-mechanical relay can.

Its well known and documented fact that the Etecno 7 volt plugs are not as good as the original ceramic plugs. Many folks, including myself, have switched to Bosch 5 volt glow plugs and seen improvements in cold starting. This is done without any reprogramming to the ECM for 5 volt plugs.
So, the 5 volt plugs are actually getting 7 volts (PWM) and seem to handle it just fine. No one has reported any failures.

There are more options than the Bosch 5V ones.
I like the Etecno1 design, as the heating element is larger with more surface area, but yes, the 7v ones were problematic.
I've been using the Etecno1 5V GPs, part no GX2123, for, going on 1.5 years, flawlessly. Love em.
Got em from IDParts.com

Ludi Neethling · **Joined:** Sat Feb 02, 2013 11:04 am **Posts:** 87

Thank you for your input GordnadoCRD.
I did an "experiment" this morning. The temp in the garage was 64 degree Fahrenheit (irrespective of the temp below or above 0 the morning start is always an issue). I turned on the ignition and waited for 10 seconds, turned it off and on again for 10 seconds and the engine started immediately without a lot of smoke/vapor at the exhaust. This is even though I did not bleed air out of the filter head (which still needs to be replaced/repaired).

I am not sure about the Etechno 7v's but when I tested the original ceramic glow plugs yesterday only the tip glowed.

The fact that after 20 seconds the engine started easier tells me the glow plugs do not heat up fast/effectively enough within the first 10 seconds. It then makes sense to me that 5v GP's connected tot he 7v system would heat up faster/warmer in the same time, thus enabling more effective initial combustion.

I am assuming the the GP module is operating normally, ie. providing enough power to the GP's to heat up effectively.

The irony is that the glow plug indicator light on the dash actually goes off in less than 2 seconds, so the GP's only just start heating up at that point. Quote from Operating Manual: "Watch the "Glow Plug Indicator Light". It will glow for 0 to 2.5 seconds, depending on engine temperature. When the "Glow Plug Indicator Light" goes out, the engine is ready to start."

Thanks to LOST I was aware that the aforementioned info is incorrect.

GordnadoCRD · **Posted:** Tue Sep 18, 2018 3:38 pm

Sometimes the GP indicator light it seems doesn't hardly blip on at all, even when the temp is cold enough that one would think it should.
That said, that's only the indicator light. The ECM control to the GPs works properly, and independent of the indicator light.
I believe it provides power to the GPs up to 30 seconds, so if you turn it on and wait 20-25 seconds as you did before, it should start similarly.

Quote:

The fact that after 20 seconds the engine started easier tells me the glow plugs do not heat up fast/effectively enough within the first 10 seconds. It then makes sense to me that 5v GP's connected tot he 7v system would heat up faster/warmer in the same time, thus enabling more effective initial combustion.

That is precisely the case. I just pointed that both Bosch and Etecno1 make 5V GPs, and both brands seem to perform and endure equally well with the 7V programming.

mass-hole · **Posted:** Tue Sep 18, 2018 4:43 pm

GordnadoCRD wrote:

Sometimes the GP indicator light it seems doesn't hardly blip on at all, even when the temp is cold enough that one would think it should.
That said, that's only the indicator light. The ECM control to the GPs works properly, and independent of the indicator light.
I believe it provides power to the GPs up to 30 seconds, so if you turn it on and wait 20-25 seconds as you did before, it should start similarly.

Quote:

The fact that after 20 seconds the engine started easier tells me the glow plugs do not heat up fast/effectively enough within the first 10 seconds. It then makes sense to me that 5v GP's connected tot he 7v system would heat up faster/warmer in the same time, thus enabling more effective initial combustion.

That is precisely the case. I just pointed that both Bosch and Etecno1 make 5V GPs, and both brands seem to perform and endure equally well with the 7V programming.

Yeti insists the GP’s are internally self regulating so the applied voltage shouldn’t matter.

My Jeep had the Bosch 5V plugs installed when I bought it. I ordered a GDE tune before I knew about this 7V vs 5V thing. I went back to them and asked and they told me the tune was 7V but in general my GP’s have been ok. I think I’ve replaced two in 5.5 years or so and, like I said, they were already on the Jeep so they were older than that.

2005 Cherokee KJ CRD 2.8 Glow Plug Module Testing Sorted

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