I just moved a 1998 Sundance Altimar spa to a new location and it is not coming up correctly. The control board is a Sentry 850 LCD and it is wired to 240V/50A.

When it first came up, the temp read 45 degrees, but climbed to 95 within 15 minutes or so. Eventually, the display would go to OH and watchdog. Aside from the temp reading, everything (pumps, blower, heater) seemed to be working properly before the watchdog condition.

After a few days, the behavior has changed. Now, after power is restored, the display reads OH for a second, then flashes "---" (watchdog), and then the tub shuts down and the display blanks. This happens even after shutting the breaker off for a few hours.

I initially suspected a temp sensor, but both read about 65K ohms at the PCB plug. My next guess is that the circuit board needs to be replaced. Before I commit to a $400 board, is there anything else to check?

Thanks in advance,
Tom

Check to be sure you have enough water circulation through the circ pump and heater. You can disconnect the heater power leads to prevent it from being energized - and hopefully your oh condition won't come back.

My guess is that there is not enough water flowing through the heater, which will make it overheat quickly. And your board logic is responding appropriately.

The temp sensor is in not in the heating line, so I'm not sure why poor circulation would cause a mis-reading on it. It should be able to accurately measure the water temp in the spa itself, but instead it reads way too high.

I hooked up the hi-limit sensor in place of the temp sensor and the temperature read correctly and the tub operated normally. A replacement temp sensor has now fixed the problem.

Interestingly enough, the temp sensor read a reasonable 66K ohms on a meter but the spa incorrectly reported the temperature at 108 degrees. When I temporarily replaced the sensor with a 66K ohm resitor, the spa reported the temperature as 46 degrees, which seems about right for that resistance. I'm not sure why the sensor behaves differently in the circuit, but the ohm meter was giving me false confidence that the sensor was OK. Swapping the two sensors turned out to be a much better way to troubleshoot the problem.

Anyway, problem solved.

Thanks for your help.

The symptoms you originally indicated are exactly what happens when a low flow condition occurs in a system where the temp sensor (and in all cases, the high limit sensor) is at the heater.

Spas are a weird bunch. One diagnosing them from afar, cannot assume that the sensors are the right ones, that the sensors - actually exist where they are supposed to, and that any standards originally set by the manufacturer are still present in fact, at any particular spa installation.

This particular scenario doesn't apply to you, but imagine this:

You could've had a genuine Jacuzzi brand spa, with a balboa controller as originally installed by the manufacturer, that you got from a friend that happened to be an electrical engineer that preferred to do things his way.

So he modified the controller or sensors to his liking. Perhaps when the spa was at his house, before you got it to yours, he decided to add a gas heater and keep the original plumbing, original, but substituted his own for the time being while he conducted his experiment. The only differences anyone would have noticed after the spa was moved to your place would have been a couple of 2" diameter holes in the side.

He installed his own concoction of temp sensors, and cut off the original's which might've been installed through the tub wall.

In haste to get rid of the tub he replaced those sensors because the old one mounted thru the tub wasn't reading correctly. So our Mister Engineer in his infinite wisdom has put the new sensor directly under the heater sensor flange where it belongs on many other spas - just not his brand. However it works well.

You get this spa.... start to ask questions because it doesn't work right wrt temp sensors... You ask the right questions, and get appropriate answers. But you respond that the temp sensor isn't in the heating line. I ask you to double check. You do and find that the tub sensor wire is buried under some foam insulation and it doesn't go anywhere.

I ask you to follow the wire that's plugged into the circuit board at the position labeled temp sensor, and you find it's actually on your heater.

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Like I said, this situation doesn't apply to you but I have to give the most direct and 'soft' diagnosis possible without creating an idiot/problem situation.

It's nice that your installation was still up to factory spec. We see and hear of them all the time where someone before modified this, or changed that and nobody knows what they're looking at. Which makes for quite screwed up troubleshooting when the knowns, aren't anymore!

Since you didn't specify a particular configuration - that your sensor is hanging out in the filter cavity which is where it `should be` - I gave you a rather quick but soft diagnosis that applies to 90% of all other installs where these symptoms exist.

Since you are more adept at electronics troubleshooting than most, you were already ahead of the power curve, and had all of the right logic. If you were looking for validation or verification of your own diagnosis, then I apologize for not addressing it up front with more detail. I prefer to err on the side of economy and safety, and if there are unknowns left to chase down, not to do it in a forum but directly on the phone, or visually through emailed digital photos.

Having firm 'known quantities' in this biz is nice, but without them, it's a gut-shot straight draw at best. Even your own resistance measurements led you to a false conclusion - as it would me as well.

So go figure.

What you ended up with was a thermistor that was faulty - but only when under power, and the only way to troubleshoot it in the end was to swap out a known good one, in which you had to pay to play the odds.

All that said, haha... can you imagine what it feels like if I told a customer his temperature sensor was probably bad, he bought it, then it didn't fix the problem. Yeah right.

He wants a return on the sensor, and buys a circuit board - then come to find out both were defective at the same time. But then he checked the new and the old sensor with an ohm meter and both indicated the same resistance.... Perils of this business.

Well best of luck and thanks for the feedback. I wish that all of them were as simple and cheap as yours was to check out. At least you were in a position to be intelligent and knowledgable enough to diagnose, and take a chance at fixing it without having to waste a bunch of money on a circuit board.

This one gets a sticky.

Thanks again.

That era of Sundance spas is notorious for bad temp sensors. If it is where is supposed to be it is in the filter area. They did use some PVC pipe as conduit so there are simple to change.

Not sure if that is the issue, but it is a known problem.