Mysterious Black Box on Drivers Fender

Howdy, all!
I believe the glow plug system in my new M1009 is not working, and while tracking it down by the TM, I discovered this interesting little box on the driver's side fender. I believe the police dept. converted the truck to 12v when they replaced the driver's alt with an A/C compressor, and this appears to have something to do with that. Anyone have any idea what this box is?

Image attached.

(Side note: I believe it's the plug system because it starts in warmth but not "cold" [Texas cold being under 50*F], I see white smoke when it's trying to start, and the plugs were all good except for one. The relay is good according to the TM [the -20, 2-4.19], and it's getting power, but I don't know how to test the rest of the system. The manual glow plug button kicks the relay over but I don't think it actually warms the plugs. If anyone could offer insight on that, I'd be very grateful, but I understand it's a "read the TM" kind of question.)

That looks like an electric fan controller unit, as the silver-colored probe that you shove between the radiator fins is visible (at right) in your photo... It opens and closes an electrical connection based on temperature input from that probe, and the knob on the box allows you to adjust at what temperature that electrical connection occurs.

Where do the wires inside that plastic split wire loom go? Also, if you can post more photos of your batteries and how they are connected to your glow plug relay, that'll better allow us to help you troubleshoot.

Ah, ok - thank you! I noticed the electric fans up front - is this a common modification for the CUCVs? I'd heard the 6.2s ran pretty cool. Should I put the temp probe in the radiator & adjust the controller, or just leave it tied up where it is?

I'll get some photos of the batteries and glow plug relay connection when I head to my shop this afternoon. I've started to track down the 12v modifications... kind of wished I'd known this before I bought the truck, but then again, those 115* days make me thankful for the A/C!

Thanks again for the help!

If you intend to keep the electric fans, I would definitely install the probe between the fins per the manufacturer's instructions, and verify that the controller is turning on/off the fans at appropriate temperatures to prevent overheating. The instructions for a Flex-a-Lite 30332 (which looks very similar to yours) can be found at Summit's website here.

As for the glow plug relay, first make sure that the PD bypassed the resistor bank (on the firewall behind the air cleaner)... In a pure 12v setup, the cable feeding your glow plug relay needs to be run directly from the (+) terminal on one of your batteries to the input lug on the relay. Since you said the relay does indeed "clunk", you'll need a voltmeter (or a test light) to ensure that the relay is passing 12v+ to the output lug while the relay is closed.

In the first photo, the loom comes in at the top right of image, where one wire (B.) is connected to the top of the glow plug relay and the second (A.) is connected to a ground on the firewall. This must be where the power comes from, as the other set of wires goes forward to the dual fans mounted in front of the radiator.

The second photo is a more macro view, where the batteries connected in parallel instead of series are visible - which is the second piece of evidence suggesting that the truck has been at least mostly converted to 12v. The third photo is a close-up of the rails behind the batteries. I believe the relay is getting voltage, because the top is energized and the bottom outputs 12v. Would I be able to check each glow plug lead for voltage by putting the voltmeter between the plug and its terminal, to see if the plugs are in fact getting any voltage?

EDIT: Posted at the same time! How funny. Thank you for the help on the fan controller, I'll be finishing their installation tomorrow. I can't remember if the lead at the top of the relay is running directly to the battery, but I am certain it's getting 12v there. When the relay is closed, it does pass 12v to the bottom lug. Am I right in thinking that this is a glow plug problem? Thank you again for your help!

kyleJ said:

In the first photo, the loom comes in at the top right of image, where one wire (B.) is connected to the top of the glow plug relay and the second (A.) is connected to a ground on the firewall. This must be where the power comes from, as the other set of wires goes forward to the dual fans mounted in front of the radiator.

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kyleJ said:

EDIT: Posted at the same time! How funny. Thank you for the help on the fan controller, I'll be finishing their installation tomorrow.

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I believe you are correct on all counts. The power feed for the fan controller is piggybacking off the hot/input lug of the glow plug relay, and the ground wire for the fan controller is terminating at the firewall in the same area. The fan controller is acting as a relay to switch power to the fans, in lieu of a traditional relay.

Beware and Caution: the full current draw of your glow plugs AND your cooling fans (via the controller) are passing through the red wire present at the annotated (B) lug of the glow plug relay... This wire appears to transit the upper firewall towards the bus bars, and appears to terminate at the lug furthest towards the passenger side of the upper (+) bus bar.

I'd suggest running a separate wire from the (+) bus bar to the power input terminal on the fan controller, and be sure to include an inline circuit protection (fuse, etc) to prevent the fan motors from pulling too much current. Further, I'd suggest upping your feed wire size from the bus bar to the (B) lug of the glow plug relay, as this wire is destined to carry approximately 80 amps of current, based on some other posts I've seen here on the forum... I believe most of the 12v conversions are using a 4-gauge cable, but don't quote me on that.

kyleJ said:

The second photo is a more macro view, where the batteries connected in parallel instead of series are visible - which is the second piece of evidence suggesting that the truck has been at least mostly converted to 12v. The third photo is a close-up of the rails behind the batteries.

Click to expand...

Based on these photos: your front battery is feeding the starter, your rear battery is feeding the upper (+) bus bar, there's a non-original wire feeding your glow plug relay from the (+) bus bar, your rear battery is grounded to the engine block, and it looks like your glow plug resister bank is still connected to the (+) bus bar (via a blue fusible link), but is NOT connected to the glow plug relay... All appears 12v, as you've noted.

kyleJ said:

I believe the relay is getting voltage, because the top is energized and the bottom outputs 12v. Would I be able to check each glow plug lead for voltage by putting the voltmeter between the plug and its terminal, to see if the plugs are in fact getting any voltage?

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kyleJ said:

I can't remember if the lead at the top of the relay is running directly to the battery, but I am certain it's getting 12v there. When the relay is closed, it does pass 12v to the bottom lug. Am I right in thinking that this is a glow plug problem?

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Based on your comments here, I'd say your glow plug relay is functioning normally and can be excluded.

To verify that you're getting 12v down at each glow plug, simply disconnect any of the connectors from the glow plugs and insert your (+) probe into the connector, and ground the (-) probe (or alligator clip) to a suitable engine ground... No voltage at the GP connector indicates a problem with the harness somewhere between the relay and the plug connector(s).

While you have the connector removed, you could recheck each glow plug's resistance value to ensure that none are shorted or open... You stated that they tested good, so I'm assuming you may have already performed this test on each plug.


Additional diagnostic questions:
1) What type of glow plugs are you using, if known?
2) Is your glow plug controller card installed?3) How long are your holding down the manual glow plug button before attempting to crank the engine?

Additional diagnostic photos:
1) Macro shot of the entire firewall area, from the bus bars to the glow plug relay

kyleJ said:

Thank you again for your help!

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No problem!

Awesome, I think that tells me what it is! I tested the voltages going to the glow plugs from the harness and none of them ever crested 0.17v... no wonder nothing was happening! So is the next step to trace it back from the glow plugs to find the problem? I'm worried that, since a little voltage is leaking through, where there rest is going and what the break looks like.

1) What type of glow plugs are you using, if known?

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It looks like the PD put AC Delco 60Gs in there except for one, in cyl 1, which appears to be the original Wellman. I don't know for certain but I believe, since it was the only one still unchanged, that last Wellman is swollen or stuck in the block or something. That'll be the next problem to look at!

2) Is your glow plug controller card installed?
2a) Does your glow plug relay automatically cycle, at all?

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It is indeed still installed and functioning - just tested that today.

3) How long are your holding down the manual glow plug button before attempting to crank the engine?

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I hold the button down for a total of 1 minute 30 seconds, in 30 second increments with 15 second rest periods, but as it turns out I didn't need to because the plugs weren't heating up or getting voltage anyways.

So glad to finally be making progress and narrowing it down, can't wait to drive my Blazer again - I've been away on business for the last 4 warm weeks here in Texas, so it's been too long!

If your glows are working right, you should only need 15 seconds of holding the manual button and it should start. I wouldn’t hold it more than that and then start it. If it doesn’t start wait a minute and repeat.

johnjr said:

If your glows are working right, you should only need 15 seconds of holding the manual button and it should start. I wouldn’t hold it more than that and then start it. If it doesn’t start wait a minute and repeat.

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I'll definitely keep that in mind once I track down this wiring issue - I started out low but tried to ramp it up in hopes of it actually starting in the cold.

kyleJ said:

Awesome, I think that tells me what it is! I tested the voltages going to the glow plugs from the harness and none of them ever crested 0.17v... no wonder nothing was happening! So is the next step to trace it back from the glow plugs to find the problem? I'm worried that, since a little voltage is leaking through, where there rest is going and what the break looks like.

Click to expand...

If your glow plug relay is indeed pushing 12v+ through the output lug, and all of your glow plugs are indeed fully functioning, then the problem could be the fusible links (the smaller-diameter green wires) that drop out of the harness and directly feed the glow plugs... The two orange wires coming off the output lug of the GP relay will separate towards each side of the engine and divide themselves into fourths to power all eight glow plugs.

Burned fusible links will typically exhibit very soft, rubbery, and sometimes broken outer insulation, and should also exhibit some deterioration of the copper strands as well. The burned links I've found and dissected on my trucks have had discolored, cooked-looking innards.

I'm at a loss for how the PD managed to fry all eight of the fusible links, though... Maybe they attempted a 12v conversion while still powering the glow plug relay via the resister bank?? I would expect the output voltage at the relay to drop to ~6v, which should then double the current draw of each glow plug as they attempt to glow...?

kyleJ said:

It looks like the PD put AC Delco 60Gs in there except for one, in cyl 1, which appears to be the original Wellman. I don't know for certain but I believe, since it was the only one still unchanged, that last Wellman is swollen or stuck in the block or something. That'll be the next problem to look at!

Click to expand...

kyleJ said:

I hold the button down for a total of 1 minute 30 seconds, in 30 second increments with 15 second rest periods, but as it turns out I didn't need to because the plugs weren't heating up or getting voltage anyways.

Click to expand...

Probably swollen, and the PD was reluctant to expend the required effort or purchase the specialty tool to remove it. Easier to run 7 good plugs and let the remaining cylinder catch up as the engine warms.

The 60G's should be self-limiting, so I don't see any particular issue with your manual glow routine...but I agree with johnjr, you shouldn't need to burn them for so long in such relatively warm weather; something is definitely amiss between the relay and the glow plugs.

So before I began to trace the wiring from the relay down to the plug, I figured I'd test the relay once again - at which point it read the same low voltage as I had been seeing at the end of the links. I then found out that something is broken either with the lug or inside the relay, as when I put pressure on it while testing (as I had originally, leaning from the side over), it produces 12v when energized. Otherwise, the lug seems to spring out barely 1/16th of an inch and outputs a fraction of a volt. This means I ought to replace the relay, right?

I figured that since the relay is just a switch, I'd just jump the 12v input to the output directly, and see if I could get the engine to start. With 2 or 3 minutes of glow plug activation (in 30s increments), the truck fired up and ran like a charm. I drove it around, let it warm up, and shut it off to get some lunch, after which it fired right up because the engine was still warm.

The truck simply would not fire with only 30s/60s of glow plug time, and would just crank over dry as it had. So this leads me to my final questions: do you know what kind of relay that is? It doesn't look like what I remember a CUCV relay as. Also, what's proper glow plug etiquette - it needed at least 60s with only 45 deg ambient temp, do I hold the manual button for that long? Also, do I heat the plugs up while cranking, or just prior to cranking as I have been?

Thanks again for all your help! I never would've figured this out on my own.

Glad you got it tracked down, and luckily you won't have to dig into the glow plug harness at all! I would definitely replace that relay (which appears to be a Ford-style starter relay) if the output lug is loose or otherwise unsecured inside the relay body. The FAQ at the top of the CUCV section should have some part numbers for you to reference, with the NAPA ST85 being the original go-to relay. You specifically need a relay whose mounting bracket is electrically isolated from its ground circuit.

The only thing I can figure with your extended glow times are that you're not getting enough volts to the plugs themselves, and they're not reaching the correct temperature. Proper etiquette is to glow prior to cranking the engine (preglow), and sometimes after the engine has started (afterglow), depending on the engine's ambient temp. The glow plug controller card consults a temperature sensor on the rear of the driver's side cylinder head to determine the engine's ambient temp.

If you haven't seen this video, produced by antennaclimber here on the forum, I assure you that it's worth the full 8 minutes to watch:

CUCV Glow Plug System Video

Thank you so much for your help, Gary! I've been out of town on business and running around but finally had a chance to put on the new relay last Thursday! My new ST85 relay is in, replacing the old Ford style relay, and the truck is working perfectly now! The old manual glow plug switch was wired directly from the 12v to one of the control lugs. I've done the new one the same way and the relay is fastened down at both ends of the relay, much better than the one screw holding the Ford relay on. I really appreciate your help!