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99 Suburban Daytime Running Lights (DRL's) not working

SAR 99 2wdSuburban
10-04-2006, 11:51 AM
I posted this in error on the Tahoe forum y'day. I'm chasing a DRL non-working situation on my 99 suburban & would appreciate help / suggestions. I started with a bad diode, but it seems in my fooling around, I've introduced another problem as I still can't get the DRL's to work even when I bypass the diode by shorting the leads (org & yel) which go to the DRL relay.

I've looked thru other posts & have a copy of the wiring diagram which I've used to get to where I am now, but am now stumped. On the wiring diagram I see what is called the DRL module, & suspect my trouble could be in what is called the DRL relay control on the wiring diagram, but have no idea where it is. Is it the convenience centre where the DRL relay plugs in or is there another relay I haven't found?

The summary of my situation is:
Headlights work OK. The only function not working is DRL.
My fuses are good (Fuse 15 for DRL / foglights & fuse 4 for gages). I can find voltage as expected at the appropriate contact of the convenience centre (where the DRL relay plugs in).
As described above, I've found the diode is bad, and expect it was my inital problem. However, with the contacts at the harness side of the Diode connector shorted (the Org & Yel at Diode 203), I can't get the DRL's to work with the relay in place. With the DRL relay removed, I can get the DRL's to work by shorting the contacts on the convenience centre (pin 87 - which goes to the DRL/fog light fuse & pin 30 which goes to the headlights).
The parking brake relay is OK.
The truck doesn't have fog lights
I've bought a new DRL relay but it doesn't change the condition - which is: Headlights work OK, but not the DRL's. Both the old and new relays give the same resistance (approx 80 ohms) across the contacts that should pull up the DRL relay (Pins 85 & 86) when the ignition is in the run position & voltage is applied. As expected, I see the voltage to the relay contacts on the convenience centre when the ignition is in the run position, so know that the wring is good, but can't figure why it's not pulling up the DRL relay. Other than measuring the resistance across the relay contacts I don't know how else to prove the relay.
Perhaps significantly, the DRL lamp on the dash isn't working. It was OK when I started the process, but in my fooling around to isolate the diode problem, it stopped functioning. I figured it had blown it while removing & replacing the DRL relay but, as noted above, now I'm wondering if I've done something to the DRL control relay or the DRL module itself..........(All other functions I see going thru the DRL module per the wiring diagram are working OK). As it seems like a lot of work to get at the bulb in the instrument cluster to see if its good or not, I thought I'd stop & seek advice before going any further.While losing DRL's isn't the end of the world, I'd still like to have all systems functioning properly. Any thoughts or suggestions appreciated.

10-04-2006, 07:58 PM
Scott: Welcome to the forum. The DRL system for the "old" body style is kind of complex. You've got a wiring diagram and from your description I'm going to assume you've got an "old" body style Suburban. That is the DRL's are the same as the regular low beam lights and not seperate DRL lights. So post back what you've got and what kind of wiring diagram you're looking at and we'll try to get it going..............Steve

SAR 99 2wdSuburban
10-05-2006, 12:11 AM
Sorry, I forgot to add that its the old body style in my earlier note. Until relatively recently, I'd been of the mistaken opinion that the new body style was introduced as a 2000 MY. I only found out when buying a part at the local dealer (as I bought it new & now have 91,000 miles on it, gives some indication of how little I've needed to buy parts)

The wiring diagram I have is from a Haynes manual & refers to 95 & later models. Not perfect, but it points me in the right direction & I've found it matches the witing on the vehicle. The contact (or pin) numbers I've referenced in my note are from the DRL relay body, not from the wiring diagram - which it doesn't provide that level of detail.

10-05-2006, 11:47 AM
OK. With the key in Run, Park brake off & light switch off the DRL module supplies a ground to close the DRL relay and turn on the DRL's and the DRL lamp in the inst cluster. When the lamp switch is turned on the DRL module removes the ground relaxing the relay and turnining off the DRL's and the DRL lamp in the inst cluster. Setting the parking brake does the same thing.

It sounds like you have checked for power at all the right places. Now you need to find out why there's no ground signal.

Pull the relay and check for ground at the lower right socket with the key in the run position. If you don't have it could be several things. 1. Park brake switch, 2. DRL module, 3. wiring to ground from the DRL module or the lamp switch.

Check the park brake switch by removing the one wire and trying the DRL's again. The DRL module is located under the left side of the inst panel taped to the inst panel harness and has about 10 wires going to it. Another ? When you set the park brake does the red brake light come on in the inst panel?

All the things I've mentioned go thru the DRL module.

If you have a fax, send me a PM(private message) with your number and I'll fax the diagrams from my '98 service manual.

SAR 99 2wdSuburban
10-05-2006, 02:42 PM
Steve. Many thanks. From my wiring diagram, I couldn't tell which of the relay contacts (top left & bottom right) should have had had battery & which ground & in what condition. What I'd found is that I have a short across the contacts. Ignition off - both have ground, Ignition in run, both have battery.

With your note in hand, I've just frogged the convenience centre sockets to the matching relay contacts & by taking the bottom right relay contact to chassis ground can pull up the DRL relay & get the DRL's to work - so that proves my DRL relay good.

That leads me back to the DRL module & from your description I think it must be a PCB (printed circuit board) located in a grey housing, with a flip cover, just above the convenience centre. With the PCB removed, the parking brake warning light stops working so I'm pretty sure its the DRL module. The board itself looks OK (ie no obvious burned out component), but I expect that I've blown one of the IC's on it as I've been fooling around which is why the short & no green light on the dash for DRL's.

I'll price a new PCB at the dealer this PM, but expect it will be $$$$, so will likely end up at a local salvage yard to see if I can locate one. I'm also after a new Intake manifold gasket as I have the 5.7 manifold coolant leak, so it will give me a chance to price the OE gasket before I tackle that job.

If you think I'm off on a tangent let me know. Otherwise, I'll (hopefully) report back that all is back in service shortly (BTW, I'm not planning to replace the bad diode as I really don't see any value in having the DRL's run at a lower output).


10-05-2006, 03:34 PM
Sounds like you're on the right track. I might note that fuse 15 (drl-fog) 20a is hot at all times and fuse 4 (gauges) 10a is hot in the run position.

SAR 99 2wdSuburban
10-06-2006, 08:30 PM
I priced the DRL module at the dealer & as expected it was dear ($185 Canadian!!). NAPA & the other local parts shop don't list the module, so weren't an option. Drove to a local wrecker & had my pick of full size trucks to source a module from, chose a 98 Suburban, & got the module for $10.

Plugged it in & all functions are 100%, including the green DRL lamp on the dash . I'm a happy camper.

Thx for your help Steve.

10-07-2006, 07:35 PM
Scott: It's always great to see a "post-back" success story. Great job on your end to find the problem. Hope you'll stick with this forum and share your knowledge and experiences..............Steve

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