K75 Temp Light Always On

[Steveboot]

Took my 92 K75 out for the first real ride yesterday and at about 25 miles the temperature light came on. This may be pure coincidence but just prior to it coming on I had turned on the grip heaters (they didn't heat up) and I had heard an odd moaning sound, just briefly, that I couldn't trace (did not sound like a bearing and it went away). I pulled over about a mile after the light came on and shut her down. There was no obvious problem and the engine case was pretty warm but not hot enough that I couldn't rest my hand on it. I rode the ten miles home with no issue but the temp light stayed on. Parked her and after a couple hours in very cold temps I turned her back on and the temp light is solidly lit. Same thing this morning after sitting overnight.

My Haines manual says likely culprit is a bad frame ground so I removed and cleaned the one on the bellhousing, but the light stayed on. The manual says there is another frame ground behind the steering head but I can't find one... Getting ready to pull the relay but having trouble getting the hoses off the tank to remove so thought I'd ask here if anyone has similar experience before I go out and force it.

[Laitch]

The manual says there is another frame ground behind the steering head but I can't find one...

When you find it, clean corrosion from the plate, the bolt, and all the terminals.

[Steveboot]

Thanks Laitch. There was no corrosion but I cleaned all the surfaces down to bright metal and reconnected. Unfortunately the light is still on. The manual says the only way to test the relay or the sending unit is to swap them out with known good ones. That sounds about right since applying jumpers turns the light or fan on not off. Any other suggestions?

[Laitch]

How much coolant is in the radiator?

[Laitch]

Did the fan start?

[Steveboot]

I jumped 12v to the A2 terminal of the fan relay socket and the fan started. It did moan/squeal for a few seconds and that matches the coincidental sound I heard on my ride. There is coolant in the overflow bottle but can't be sure the system is full unless I drain it. That will be a task for tomorrow. I'll probably order the sending unit as well so I can swap it out when I replace the coolant. Unfortunately the relay is not available from my local shop. 

[Steveboot]

Nix that. Was too easy so just drained it. There was a fair amount of orange coolant but it doesn't look like 3 liters to me. My wife frowns on me using kitchen tools in the garage so I'll pick up a measuring container tomorrow.

[Laitch]

I'll probably order the sending unit as well so I can swap it out when I replace the coolant.

If you do the sensor swap and the coolant replacement simultaneously—which you seem to be implying—and the temperature bulb isn't lit nor the fan running after you start then ride the Brick for another 25 miles, how will you know which procedure stopped the problem?

[Steveboot]

Hmmm. The light is on solid when the bike has been sitting for more than 24 hours so I don't think the coolant would be the cause, but I know the coolant is old and should be replaced. It is simple enough to put in new coolant and run it to see if the light goes out so that makes sense. I can always drain it into a clean container and then replace the sensor later. I'm picking up the sensor along with the coolant today so I'll at least be prepared. The relay, however, is unavailable so I'll have to order a used one on line. One step at a time.

[Laitch]

I think I'm getting the picture but when you state, The light is on solid when the bike has been sitting for more than 24 hours, are you intending to convey that the light is "on solid" when after 24 hours the ignition key is turned to ON?

[Steveboot]

The light is on and remains on when the bike is started. Key on, light on. Bike running, light on.

Among all sorts of things that are confusing to me is the manual says the light being on all the time is likely caused by a bad ground. I would think that means the absence of a ground allows a sneak circuit to energize the lamp, but the lamp is bright. That would seem to be a short to ground that bypassed the fan relay.

[Laitch]

You could have a multiple problems. If the fan starts when the engine starts, there's something wrong in the relay circuit. If it starts miles later, there could be problems with the coolant system—low coolant, degraded coolant, stuck radiator thermostat.

The overheat warning bulb lights when there is no resistance in the coolant temperature sensor circuit. A cold engine creates high resistance in the temp sensor circuit; therefore the warning bulb won't light. As engine temperature increases, resistance in the sensor circuit decreases. First, if the engine gets hot enough, the fan will be activated. After that, if the engine continues to heat up at an increasing rate, resistance in the temp sensor circuit will eventually reach its low point, the temp sensor circuit will be grounded, and the warning bulb will light. It is conceivable that your Brick's coolant temperature sensor or circuit has become defective and doesn't detect when resistance resumes increasing during cooling so the bulb stays lit. It's relatively uncomplicated to check temperature sensor function.

[Steveboot]

Going to fall on my sword here; What is the Motronic unit? Nothing like that on the wiring diagram and no devices with both  21 and 32 terminals. And the term Motronic doesn't appear in the index of the Haines manual.  K75 diagram has the Ignition Control Unit and the Fuel Injection Control unit, both also don't have both a 21 and 32 terminals. Bound to be something I'm overlooking. Really appreciate the time your spending helping me.

[Laitch]

Stick a pressure bandage in your sword wound and have one of your minions press on it with two hands. You'll recover.

I confused you with another correspondent who has a 4-valve. Not that you aren't unique and worthy and full of potential and memorable. It isn't you, it's me. I'll delete some of that post and revise. It's the LE Jetronic we're testing.

Anyway, test the coolant temperature sensor. Same resistance values. Bridge pin 10 to pin 5 or frame ground at ambient temperature. Should be 2500Ω.

[daveson]

Here's one low tech way of looking at the problem.

Remove the wire from the temperature sensor and turn the key on. If the light is off, the sensor is probably bad.

If the light is on, there's a short, then remove the temperature relay and repeat the test. Is the light now on:

Yes; then there's a short between the relay and the warning light.

No; then there's a short between the sensor and the relay, or the relay is bad. With a test light between battery positive and this earth wire, if the test light glows, then there's a short on this wire (because the wire is disconnected at both ends)

[Steveboot]

Finally got a chance to go out to the garage... resistance between terminals 5 and 10 is 3160 ohms. So higher than expected. I also measured from terminal 10 directly to the frame ground point with the same result. The new, non OEM sensor I picked up reads 2600 ohms. I did not reinstall the tank just to make sure the light hadn't magically extinguished. I'll play around with it more tomorrow. 

[Steveboot]

Made progress. If I can count learning more about my bike as progress... Temp light is still on. However, I did drain the coolant, remove the radiator, replaced and tested the temperature sensor (old one and new one have very similar resistance values), refilled with fresh coolant, reinstalled everything with no leaks and fired her up with no issue, other than the lamp is still lit.

I didn't check the condition of the light with sensor connector open (forehead slap) but will pull the fan relay again and the lamp should go out. If it doesn't I can start chasing a ground on the neg side of the lamp.

Proactively I've ordered a fan relay,  and an oil dip stick thermometer so I can at least have an idea what the engine temperature is doing.

I'm still wondering if me switching on the heated grips caused a problem, since that preceded closely the temperature light coming on. The EWD shows a connector for the "optional" heated grips so I'll see if I can trace that down

[Steveboot]

More progress; there is a partial ground on the violet/black wire on terminal A3 of the fan relay connector (relay unplugged). That wire is a straight shot from the fan relay to the #10 terminal of the gauge/cluster housing. With the connector at the housing unplugged the ground on the wire goes away and there is continuity from the fan relay connector to the housing connector.

With the connector unplugged there is about 6k ohms from pin #10 of the housing to frame ground, which was higher than I expected for the lamp/resistor, but when I turned the key on it was a dead short.

If I am reading the diagram correctly the wires that have a main green color are positive through the key. I only had the first connector (pins 1-12) pulled so power must be coming through the second connector (pins 13-24). I'd guess with the key off I was reading through the bulbs and with the key on and connector open there might be a sneak circuit.

[daveson]

First up, one quick question which I don't think has been asked yet. After turning on the heated grips, the fan came on and the temperature warning light came on. If you turn the heated grips off, does the temperature light also turn off?

[Steveboot]

No. Once the temp light came on it stayed on. I disconnected the wiring to the grip heater switch (wires were kid of fouled but not shorted) but haven't found how the grips themselves are wired.

[Laitch]

I'd guess with the key off I was reading through the bulbs and with the key on and connector open there might be a sneak circuit.

It's more likely there is either a break in the violet/black wire from the instrument bulb to the temperature relay, a break in the violet/green wire from the coolant temperature sensor plug to the temperature relay, or the relay itself is the problem. Have you checked for continuity in those two wires?

[Steveboot]

The violet/black wire from the relay to the connector at the gauge housing is continuous but I did not measure the violet/green wire from the sensor to the relay. I can't measure the violet/green wire directly without dropping the radiator again (that will be the last straw), but I can measure from terminal E of the relay to ground through the temperature sensor so I'll do that. My new (used) fan relay is supposed to be delivered today so I'll pop that in as well. 

[Steveboot]

Violet/green wire from terminal E of the fan relay to frame ground reads 2350 ohms. That should be the resistance of temperature sensor so wire is good.

[Steveboot]

And, drumroll please, replaced the fan relay and the temperature light is no longer lit! Dip stick temperature gauge installed and ready for a test ride tomorrow. I know I saw somewhere in my searches a reference for what the temperature should be when reading it from the crankcase but I'll have to find it again.

Thanks folks for all of your help. I'm sure I'll have more questions as I get to riding her a bit, after all, she sat for 6 years without being started so I will be surprised if nothing else comes up.

[Laitch]