Really Weird Wireing Problem

I want to share this recent problem with anyone that has installed, or plans to install an electric cooling fan on their truck.

As most know, I installed and have made several "electric fan" posts on SS in the past. It works great in the winter. Without the constantly running water pump fan, the electric fan causes the engine to run in the 180 degree range.

With the new, hotter running LDS engine I decided the electric fan was too marginal for summer driving so I re-installed the water pump fan. I left the electric fan in place to act as an auxillary. When winter arrives I'll once again remove the water pump fan and fall back on the electric unit.

All went well until the other day when I noticed the tell-tale light that I installed on the dash to indicate when the fan is running was on. How could this be? The mechanical water temp gauge was only registering 170 and the electric fan isn't suppose to come on until 190. I checked the oil temp gauge. It indicated 180 so the water temp of 170 was logical. As soon as I would stop the check to see if the fan was running the light would go out.

This turned into being a real head scratcher. We did extensive checking and testing. I removed the fuse from the fan and the stupid light still came on. We then learned that the light came on when the engine was reved up. Soldier Number One, being a lot smarter than I am finally realized it was the whirling electric fan motor serving as a generator and feeding power into the tell-tale light. When the motor was reved up the water pump fan would pull air through the radiator and spin the electric fan causing a windmill effect.

Thinking it might be the alternator somehow feeding back current into the ground, we were about ready to pull the alternator. Simple things like this can give you fits.

I almost started typing before I finished reading what you found. I had a vehicle I wired up a few years ago where a pusher fan was wired directly to the A/C clutch. My thinking was that when the a/c was on, the fan would always be on as well. I wired it up before I charged the system in the winter time. 20 miles later, the dry turning compressor locked up and threw a belt. I now use relays for just about anything I add. That would work for you as well.

Glad you found the answer.

If you have this wired correctly, it shouldn't do this. How do you have the fan wired?

I don't have a schematic with me --- all is can say is that it is a simple system. The only thing we did that wasn't in the original schematic was to add the tell-tale dash light.

The tell-tale light would come on ever so dim as the engine gained speed until it was bright and then gradually go out when the engine went back to idle. Our first thought was that the alernator was somehow bleeding voltage back through the ground.

I need to find a take-out alternator. I've got quite a few spares but don't have a spare alternator.

bottleworks, if the fan is spinning and is a permanent magnet motor, it will act as a generator and light the bulb. perfectly normal and expected. what i do not understand is why ernie has to do all this to get the engine to run 180. all you should need is a good 180 thermostat. one thing i have noticed is that several trucks i have had all read 160 when they are really 180. ernie, i assume you have checked the engine temp with a known good thermometer??

Re: RE: Really Weird Wireing Problem

m-35tom said:

bottleworks, if the fan is spinning and is a permanent magnet motor, it will act as a generator and light the bulb.

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I do understand that. However, if it were wired correctly, the fan would be isolated when shut off. It is not wired correctly.

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ernie just has the light connected to the fan. that would seem to be the most correct way to wire it, if the light is lit there is power to the fan. the other ways of wiring it would only tell you that there should be power to the fan, but would not guarentee it.

Re: RE: Re: RE: Really Weird Wireing Problem

m-35tom said:

The other ways of wiring it would only tell you that there should be power to the fan, but would not guarentee it.

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Hehehe... No. There are other ways. I should point out that the fan is probably producing A/C voltage. Not good in a DC system.

I am not going to get into a pissing match. If Santa wishes to know of a correct way to wire it, then I will be happy to share.

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m-35tom wrote:

...what i do not understand is why ernie has to do all this to get the engine to run 180. all you should need is a good 180 thermostat. one thing i have noticed is that several trucks i have had all read 160 when they are really 180. ernie, i assume you have checked the engine temp with a known good thermometer??

Click to expand...

In the summer I don't have a problem pulling 180 degrees even WITH the water pump fan connected. In the winter, with the water pump fan connected, the LDS engine runs warmer than the old LDT but still won't built sufficient heat (180 degrees) when the weather is cold. I'm running a 180 degree thermostat BUT there is a bypass in the multi's cooling system. With the water pump fan running in the winter the real problem is in getting the engine to warm up enough to get the defrosters to work. If I could keep the frost off the windows long enough the get out on the road where I could put my foot in the pump I could get the temp up to 160 which was still barely enough to keep the defrosters working. As you know, it's also hard on an engine to run it cold. I have new S.W. mechanical water and oil temp gauges. I'm confident I'm getting accurate temperatures

When fall comes the water pump fan will come off and the electric fan will be turned back on.


Bottleworks wrote:

...I should point out that the fan is probably producing A/C voltage. Not good in a DC system.

I am not going to get into a pissing match. If Santa wishes to know of a correct way to wire it, then I will be happy to share.

Click to expand...

We removed the main fuse and the bulb from the tell-tale light. If you think the spinning fan could cause damage I can tie it off for the summer??

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SasquatchSanta said:

We removed the main fuse and the bulb from the tell-tale light. If you think the spinning fan could cause damage I can tie it off for the summer??

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Sounds like you got it handled.

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SasquatchSanta said:

We removed the main fuse and the bulb from the tell-tale light. If you think the spinning fan could cause damage I can tie it off for the summer??

Click to expand...

I had this happen on a car I owned. The electric fan that was installed was wired up directly to a switched power source, no relay. It kept killing the battery in the car, it'd last about 2-3 months and just go dead. There was never a problem before the electric fan install. It's since been switched to a relay system and solved the problem.
When the car was turned off, the fan was left spinning until it wound down. That was generating a current and feeding it into the system while everything was off. That turned out to be what was killing the battery. I'd suspect that you're damaging your batteries as well if it's wired up the same way, as suspected by the behavior of the light.

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ernie, i don't know what you are calling a bypass in the cooling system, but when cold all the coolant circulates through the engine only. there is absolutely no flow in the radiator. as the thermostat opens it allows coolant to start flowing into the top of the radiator and slowly shuts off the flow back to the water pump. your problem is most likely caused by the large amount of cold air the fan blows back on the engine. this could be solved by having a fan clutch, but no one has engineered one yet. someone was goint to send me an old water pump to work with but i never got it. perhaps someone has an old pump (a bad one) they would give me to work with.

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The simple solution would be to put a diode in the circuit to prevent a backfeed to the vehicle electrical system when the fan is freewheeling if you can get one rated for the current you are providing the fan. Otherwise, put the fan on a relay between the fan and the light that is controlled by your switch. The switch will be much happier not carrying the entire current of the running fan and the fan will perform better without the voltage drop through all of the wiring into the cab and back.

Later,
Joe

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guys, when the fan is 'freewheeling' that implies no power is applied to it. in that case it is an open circuit and nothing is going to 'backfeed' to anything. the point of having a light is to know the fan is really on, so the light has to be connected to the fan, not to the control circuit. of course you could get real fancy and sense the current draw of the fan and have that signal turn the light on...............