Fan Fail light blinking - only at highway speeds

[jawn101]

This is so bizarre. I haven't been able to figure it out.

I have Dave McKeen's fan fail box (the blue unit) installed, along with Hervey's low-current fans. Fans work great, come on and off as they should. The fan fail unit also works properly - if I run the fans with the car idling and unplug one or both fans, the dash light blinks with one or two flashes exactly as it should. And around town, the light is always off - just as you'd expect.

But, when I get on the highway - the light blinks double as if it detects both fans being failed, any time that they're on. It doesn't do it unless the fans are running, and the fans are definitely still running even when it's blinking.

I had thought it might be something with engine RPM (alternator outputting different voltage or something, perhaps?) so I did a test with the car parked and the revs held high - no flashes. Only at actual highway speeds.

The only (far-fetched) idea I can come up with is that the forward air pressure on the fans is causing them to draw much less current when spinning and the fan fail unit thinks there's no load? Like I said, it's far-fetched.

Anyone have ideas?

[DMC5180]

Have you contacted Dave directly? PM?

[Bitsyncmaster]

You could be right that maybe it's detecting low current because the forward air speed. Do you have my fused fan fail or the electronic fused unit? I will have to do a current check of the fans in my car at highway speeds. But I had Hervey fans with the fused fan fail and never saw blinking at highway speeds. I now have Toby's fans.

Is there a certain speed you see it happen? Are you using my circuit breaker bypass? What voltage do your normally have at high speeds? Low voltage to the fans would mean less current.

[jawn101]

Hey guys. Yeah, Dave you and I traded emails about this a few months ago and I didn’t want to keep bothering you directly with it! Your suggestion was that I increase the differential threshold on the unit, which I did - all the way up to 20%. My voltage at highway speeds is (if the dash gauge is to be trusted - I’ve verified at idle that it’s pretty darn close) right around 13-13.5

It’s the older fused unit, with 20a micro fuses installed. And I seem to see it at speeds around 60MPH and up.

[Bitsyncmaster]

I'm guessing you don't have a clamp on current meter to measure the fan currents. When I had the Hervey fans they would draw 7 to 8 amps each with the engine running. But since your getting the double flash, that tells me it's both fans so I'm guessing the current is to low. If you want to try my new electronic fused fan fail, let me know but I think I set the low current at 4 amps on both units.

[jawn101]

I'm guessing you don't have a clamp on current meter to measure the fan currents. When I had the Hervey fans they would draw 7 to 8 amps each with the engine running. But since your getting the double flash, that tells me it's both fans so I'm guessing the current is to low. If you want to try my new electronic fused fan fail, let me know but I think I set the low current at 4 amps on both units.

Hey Dave - no, I don’t have a clamp on meter. Maybe I can borrow one somewhere but I’m not sure. Is there any other difference between the electronic and fused units that could fix this? If the current detection threshold is the same I don’t know how it would change anything. But I’m not the expert here - that’s asking! :)

[Bitsyncmaster]

Hey Dave - no, I don’t have a clamp on meter. Maybe I can borrow one somewhere but I’m not sure. Is there any other difference between the electronic and fused units that could fix this? If the current detection threshold is the same I don’t know how it would change anything. But I’m not the expert here - that’s asking! :)

The two units use a different circuit to measure the current but your correct the design should have the same results if the problem is indeed current is to low. Now the question is are the fan currents really below 4 amps each when your driving and if so why. Getting the same result with my other fan fail would verify the fans probably are below 4 amps each.

[jawn101]

I’m open to try anything!

[Bitsyncmaster]

I just ran my car with fan fail jumpers and my otterstat jumpered. I had my clamp on current meter measuring the total fan currents. I have Toby fans and also have the Hervey rubber fan shield (to deflect hot air down to the ground). My alternator pretty much keeps 14.6 volts.

The cold start in the garage the fan current start at 20 amps and pretty quickly drop as the fan motors heat up to about 18 amps.

At 60 MPH the fan currents are 16.6 amps.
At 0 MPH the fan currents are 16.5 amps.
When I shut the engine off (my fans run an additional 20 seconds) the fan currents are 13.4 amps.

So I'm surprized the fan currents don't change much with the cars speed.

PM me your address and I will send you a loner fan fail unit just to make sure it's not something failed in the unit.

[vwdmc16]

I’m open to try anything!


Get some cardboard and block off half the radiator Inlet, unless it's a super hot day you should still have plenty of air to keep it cool on the freeway. Just for a quick test they should at least be able to make you confirm or deny air flow is the issue. Another ways you could tape off the back of one fan with masking tape and then go on your freeway test drive. If the warning only blinks once then you know you are confirming your theory.

[Bitsyncmaster]

Get some cardboard and block off half the radiator Inlet, unless it's a super hot day you should still have plenty of air to keep it cool on the freeway. Just for a quick test they should at least be able to make you confirm or deny air flow is the issue. Another ways you could tape off the back of one fan with masking tape and then go on your freeway test drive. If the warning only blinks once then you know you are confirming your theory.

I did run a fan current test a long time ago on an OEM fan. Blocking air to the fan increased the current which is why I thought air blowing on the fan would decrease it. But after thinking about it more, the fan speed may just be increasing and still drawing the same power. Another test I can do with and OEM fan is to power it up and use my compressor blow gun to see if the currents change. I have two OEM fans, one is still in the stock shroud bolted to the stock radiator. I used those fans when bench testing my fan fail design.

[jawn101]

These are cool ideas guys, thanks for the suggestions. I could see if Clint’s test helps prove the theory at all when I get som time this week

[hippieman9]

Did you ever figure out what was causing this, because a car I’m working on is experiencing the same thing. I have been in contact with Dave in reference this as well. Thinking the Hervey fans are causing this issue, gonna convert to Toby’s fans.

[jawn101]

Did you ever figure out what was causing this, because a car I’m working on is experiencing the same thing. I have been in contact with Dave in reference this as well. Thinking the Hervey fans are causing this issue, gonna convert to Toby’s fans.

I never did, no. Does the car you’re working on have the Hervey fans too?

[hippieman9]

I never did, no. Does the car you’re working on have the Hervey fans too?

Yes it does.

My personal car has Toby’s fans, Dave’s fan fail relay and Dave’s fan relay, no issues at all.

Going to jump the otter stat and drive the car on the highway and see if the light comes on. If it doesn’t I’ll bet the “windmill” effect is causing the the light to flash.

[Bitsyncmaster]

I've been thinking about this problem. My new fused fan fail signals a failure when one or both fans have less than 10 volts (a blown fuse would result in low voltage). My micro runs down to 2 volts. But with a very low voltage (fans generating voltage by windmilling) the fan fail light would be so dim you would not see it. My unit drives the fan fail light with input voltage.

Since one customer has said the fan fail light is bright, all I can think of is the fan fail socket is not getting a good ground. But why would that only be a problem at high speed?

I did some testing of windmilling with OEM fans a long time ago and don't remember it generating much voltage.

[jawn101]

I've been thinking about this problem. My new fused fan fail signals a failure when one or both fans have less than 10 volts (a blown fuse would result in low voltage). My micro runs down to 2 volts. But with a very low voltage (fans generating voltage by windmilling) the fan fail light would be so dim you would not see it. My unit drives the fan fail light with input voltage.

Since one customer has said the fan fail light is bright, all I can think of is the fan fail socket is not getting a good ground. But why would that only be a problem at high speed?

I did some testing of windmilling with OEM fans a long time ago and don't remember it generating much voltage.

And I installed your relay strip ground bus, which didn’t fix the issue - and no other evidence of bad grounds (all of which have been cleaned and super upgraded at this point) anywhere else in the car.


Yes it does.

My personal car has Toby’s fans, Dave’s fan fail relay and Dave’s fan relay, no issues at all.

Going to jump the otter stat and drive the car on the highway and see if the light comes on. If it doesn’t I’ll bet the “windmill” effect is causing the the light to flash.

I never managed to try Clint’s suggestion to block off the radiator temporarily with some cardboard and see if it still happened - was always too hot where I live and then I just forgot! Maybe give that a try too. It’s a smart test.

[Bitsyncmaster]

What I will do when I get time is to solder a wire to a fuse (only need to do one since both fuses are connected to the input pin. Then I will attach my meter to that wire and go drive at high speed and look at the voltage. I have Toby's fans so that may not help knowing what John's fans produce.

If you still have the fan breaker, you could measure the voltage there. Of course it is only valid if the fans are now powered to read the windmill voltage.

[Bitsyncmaster]

Thanks Ryan for testing the voltage with the Hervey fans. He got 5.75 volts at 80 MPH.

If you have my AC panel LED board this would not be a problem since it has no light at 5.75 volts on the fan fail light circuit. My board will start to glow (very dim) at 6.75 volts.

Probably would not see any light also if you have an incandescent bulb in the fan fail location.

You also would not have a problem if you are using a standard fan relay with that center pin still grounded in the fan relay socket since that shorts the fans when the fan relay is off.

I will look into a change in the fan fail unit to shut down the warning light until the voltage is above 7 or 8 volts. If the car voltage is that low your engine would probably not run.

[Bitsyncmaster]

If you have this problem of fan fail light flashing at high speeds due to fans windmilling, I have a fix I can install in your fused fan fail unit to correct it. You would need to return your fan fail unit to me.

All new sales will have this fix installed.