Overheating...Resistor?

[meiermat]

I have an '05 Aveo that started overheating last night. I haven't tested the thermostat or the Engine Coolant Temperature sensor yet, because I'm at work and the ECT is a pain to get to and I don't want to drain the coolant in the shop.

I have tested the fan with a DC power supply. It spins just fine. I tested the Cool Fan High relay by taking it off, applying voltage to the coil, and measuring the contact resistance (1 Ohm on a crappy multimeter), so I'm pretty sure that's OK. The Cool Fan Low relay measures the same. The fuse is fine.

I know the temperature feedback to the dashboard is working, because the gauge fluctuates as I drive, depending on how fast I'm moving and how hard the engine is working, so I'm pretty sure I can rule out the ECT. Also, the temperature DOES go down while I'm driving as long as I'm not working the car too hard, so I'm pretty sure I can rule out the thermostat. When I got to work this morning, I left the car running and popped the hood. The fan wasn't running, even though the temperature was at about the 5/8 mark. I imagine that's not high enough to trigger the Cool Fan High circuit.

Other than a bad wire, I think that leaves the resistor that's inline with the Cool Fan Low relay. It looks like it's cracked, but it's hard to tell. It's easy enough to access, but I have a few questions.

1. Where do I get one? The local stores don't turn anything up, nor does the local dealer. I tried Rock Auto with no luck, either. I might have a source on Ebay, but I'm wary because it lists as a DRL resistor.

2. It's hard to tell whether it's really replaceable. It appears to be hard-wired into the harness. At least, the wires don't have a clear connector on them from my vantage point. Is there a disconnect spot if I pull the loom apart far enough, or should I just snip the wires and crimp on new ones?

3. If it's not something I can buy, does anyone know what the resistance is? I can buy an appropriately-sized resistor and mount it down there, but there's nothing in the schematic that gives the value.

Thanks,

Matt

[xintersecty]

With any resister you can read the value using the OHM meter. You should be able to isolate the wire on both sides and read it.

[avguy]

Just making sure that your Aveo has no A/C, and just one fan, is that correct? Assuming so, you might want to jump test the low circuit before replacing the resistor. Fan operates at hi when the ECM gets 207F from the ECT.

[meiermat]

With any resister you can read the value using the OHM meter. You should be able to isolate the wire on both sides and read it.

It's broken, so I can't read it. It shows an open circuit with an ohmmeter.

Just making sure that your Aveo has no A/C, and just one fan, is that correct? Assuming so, you might want to jump test the low circuit before replacing the resistor. Fan operates at hi when the ECM gets 207F from the ECT.

It does have A/C (at least, the parts are still present), but it's just one fan for the cooling circuit. I clipped the wires off the resistor and shorted them through a barrier block for the drive home. It seemed fine, and I got the characteristic voltage sag when the fan kicked in at a traffic light after I got off the highway.

So, I'm 99% sure it's the resistor under the hood, but other than a pick-and-pull I'm not sure where to go get one. Anybody know what the resistance is supposed to be? Otherwise, I'll just match the fan coil resistance, and set low speed to half of full speed.

[avguy]

I measured 36KΩ from low fan relay pin #87 to fan connector pin #2. Interesting that you have the single fan with A/C installed. I was assuming the single fan was used only without A/C, as mine is. Guess they used multiple configurations, like with some of the other stuff on this vehicle.

[xintersecty]

I measured 36KΩ from low fan relay pin #87 to fan connector pin #2. Interesting that you have the single fan with A/C installed. I was assuming the single fan was used only without A/C, as mine is. Guess they used multiple configurations, like with some of the other stuff on this vehicle.

Thanks AVGUY for measuring and sorry to meiermat that I pointed out the OHM meter. Most people don't bother and I am glad that you did check it. I would go with a 1/2 watt or 1 watt resister and the 36 ohms. The wire resistance is low and would not change the target value.

[petrified.rabbit]

did you try jumping the relay to see if the fan comes on? I would verify each of the components still work before trying to trace wires.

[meiermat]

I checked the wiring. It shows continuity to the fan. I know that's not a perfect test, because the real system operates under load, but I figured it was good enough for now. I was able to find a local dealer that was helpful enough to find the part, and I ordered a replacement. I picked it up this morning. My multimeter shows 1Ohm for the resistor, but my multimeter doesn't do well for much below a couple Ohms, so I wouldn't use that as a solid reference.

I won't get a chance to put it in until next week, but I'll update with a final solution.

Matt

[avguy]

... the gauge fluctuates as I drive, depending on how fast I'm moving and how hard the engine is working ...

Given what you wrote above, I suspect that your Aveo has 2 separate issues. Based on your diagnostics, it definitely sounds like the low fan circuit does have a problem, and you will find out shortly if the new resistor resolves it. But IMO, what you described above shouldn't be caused by the fan not coming on.

You and I have similar climate, and I can say for certain that the heat gauge never goes over 1/2 or fluctuates, and the fan never comes on as long as my Aveo is moving. In the Winter it has a max running temp of around 186F, and it takes about 5 minutes of continuously stopped idling in order for it to hit the low fan setpoint of 199F. The fan would come on a bit faster on a really hot Summer day, but the coolant temp is still remains low enough to keep the fan off while moving, even on the hottest day. So unless you're driving in very slowly moving (long) stop-and-go, the fan in your Aveo shouldn't need to come on, if the cooling system is operating correctly.

So I'm thinking you have something going on with the coolant flow. The usual suspects would include a partially stuck thermostat, a blockage in the rad, or a broken water pump. Obviously the T-stat is the easiest (and most likely) so you might want to start there.

[meiermat]

Shorting the resistor definitely fixed the problem, and this morning I replaced the resistor. I've driven it twice so far, both of them long enough for the car to heat up, and everything is behaving normally.

So I'm thinking you have something going on with the coolant flow. The usual suspects would include a partially stuck thermostat, a blockage in the rad, or a broken water pump. Obviously the T-stat is the easiest (and most likely) so you might want to start there.

This is quite possible, and maybe I've never noticed it because the temperature never rises beyond the normal operating point. It's been probably 3-4 years since I last replaced the radiator, but I flushed the cooling system when I did the water pump/timing belt at 400k.

I can't measure my coolant temperature with your precision. I just have the idiot gauge in the dashboard, and I don't have a fancy scan tool; just a code reader/clearer. Likewise, I can't tell for sure when my fan comes on unless it kicks on at idle, and I can sense the load on the electrical system and engine. Almost all my driving is on the highway, but I do hit miserable standstill traffic in the summers when I travel to Detroit, Grand Rapids and Chicago. I've never had an overheating issue before except that one day when my plastic thermostat housing exploded, but it's possible I'm living right on the edge. It HAS been a while since I replaced the thermostat.