Can somebody pls help me understand how this circuit works and figure out a workaround?

DangerBoy

Well-known member
Oct 4, 2018
141
Calgary/Alberta
This is a challenge for the electrical engineers and techs who can look at a schematic and understand how a circuit works. I'm an engineer myself but not an EE and my understanding of circuit diagrams and how circuits work is only basic and I need the help of someone who has more expertise in that area.

Below is the schematic for my '95 Watkins Hot Spring Grandee. The problem I'm having is that the heater is failing to turn on when it's supposed to and from what I've been able to determine, the reason for that is because the 12V control/trigger signal to the heater relay K4 is too low in voltage. I'm measuring less than 1.2 V which is the unlatching threshold voltage for the relay.

Here are the things I know:

1. The flow/pressure switch is working fine. There's continuity through it but the voltage on the circuit it is very low.
2. The T-Stat thermistor is good. I checked it's resistance and it was bang on what it's supposed to be at that water temperature so the system should know to be calling for heat when the water temp cools off. Continuity to and from it is good too.
3. The trigger voltage going into the Heater Interlock relay K5 seems low too but the heater interlock relay is latched and sending power over to the heater relay K4 like it's supposed to. The problem is K4 is not latching but the relay is okay and working. As far as I can tell, it's the trigger voltage that's too low. That makes me wonder if there's something wrong with the T-Stat control board (P1) or the PWA interlock timer board at the top of the diagram. If either of those boards have failed, I'm in a bit of a pickle because both boards are NLA and 100% pure unobtanium.
4. When I was trying to troubleshoot this problem on Saturday, the heating system suddenly started to work again but I don't know why and the trigger voltage on the heater relay K4 still seemed quite low. The heater worked for a couple days but now it's back to where it was again with K4 not being triggered despite the water cooling down well past where the heater should be kicking on. This leads me to wonder if something in the system is starting to fail but came back to life for a while on Saturday and may or may not come back to life again.
5. I looked carefully at the thermostat control board P1 and could not see any visible evidence of a component failure like a burned resistor or an exploded capacitor. There's no discoloration on the board either. It is 25 years old though so probably well past its life expectancy. The same is true of the PWA interlock timer board at the top of the diagram.
6. I checked the diode between the trigger terminals on heater relay K4 and it is working properly too, only letting current travel one way.
7. Relay K2 is working fine and the circ pump is running constantly. I didn't think to check to see if the power terminals unlatch like they're supposed to when the power is off the relay. If that relay is fused, that could explain why that relay is latching and K4 is not.

Can someone please tell me how that heater relay trigger circuit works? The way I interpret it is that my 12 VDC signal voltage comes down out of terminal 6 on the lower right hand side of the top board and into the + trigger terminal of relay K2. That 12V then powers relays K2 and K4 and also runs the signal current through the flow switch. The circuit grounds through the light blue wire running out of the - side of relay K4 and at or through the T-stat board. Is that correct? 🤔

If that's correct then I'm confused because relays K2 and K4 are triggered by the same 12V line and K2 is latching when it's supposed to but K4 is not - at least not all the time. That might make you suspect something's not quite right with K4 but I tried putting in a brand new relay into K4's position and that relay wouldn't latch either. Again, the trigger voltage seemed like it was too low.

So all this makes me suspect that there's something wrong in the main board at the top and the voltage coming down the blue wire from terminal 6 into K2 is lower than it should be. For some reason relay K2 is closing but K4 is not. One possible reason that could be is that K2 is an old relay, (probably original to the tub) so it could be stuck closed/latched. I've never checked for that so it is a possibility. I don't think that's the case but that's just a guess.

If I'm missing something here please let me know but if I can assume for the moment I'm correct in guessing the problem is that the voltage being supplied by the main (upper) board through the blue wire into K2 is too low, could I not solve the problem by supplying the 12V it's supposed to have from someplace else I know has got 12 Volts? For example, I know the red wire coming out of the T-stat board has 12 Volts on it as I measured it on Saturday. As a workaround, could I not jumper a wire over from there to the positive signal terminal on relay K2 and power the trigger circuit that way? That way I'd just bypass whatever's wrong on the main (upper) board and get the needed voltage into the trigger circuit where it's supposed to come into relay K2. Is that right?

1995 Grandee electric.jpg
 
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cowboycasey

Gold Supporter
LifeTime Supporter
TFP Guide
Jul 3, 2013
4,429
Fletcher, OK
25 year old hot tub parts, stick a fork in it cause its done :) I am amazed it is working at all let alone just not heating... Have you thought about replacing the complete spa pack?


Lets see if we can get some assistance :) @RDspaguy what do you think?
 
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CuckooChris

Active member
Sep 14, 2016
28
Coplay, PA
Not an engineer, but can you check what voltage you have coming out of the transformer? Also, how are you checking the signal wires? I believe you should unplug both from the relay and insert meter leads and make sure your meter is set to DC.
 

DangerBoy

Well-known member
Oct 4, 2018
141
Calgary/Alberta
I thought about it some more and realized that my workaround won't really work. While it will allow the tub to heat up, it won't allow for any regulation of the temperature unless that 12V red line on the T-Stat board is regulated by the thermostat. If the voltage is constant then the tub will just keep heating up and never stop until the high limit is tripped and all systems are shut down.

I do have a used complete IQ2000 system (control box, control panel, etc.) here I could swap in for the system that's in there right now. I would prefer to avoid having that if I could but I realize I may have no choice. It's winter here and I have to keep the water in the tub heated or it will be a catastrophe. :(
 

jseyfert3

Silver Supporter
Bronze Supporter
Oct 20, 2017
1,036
South-Central WI
3. The trigger voltage going into the Heater Interlock relay K5 seems low too but the heater interlock relay is latched and sending power over to the heater relay K4 like it's supposed to. The problem is K4 is not latching but the relay is okay and working. As far as I can tell, it's the trigger voltage that's too low. That makes me wonder if there's something wrong with the T-Stat control board (P1) or the PWA interlock timer board at the top of the diagram. If either of those boards have failed, I'm in a bit of a pickle because both boards are NLA and 100% pure unobtanium.
4. When I was trying to troubleshoot this problem on Saturday, the heating system suddenly started to work again but I don't know why and the trigger voltage on the heater relay K4 still seemed quite low. The heater worked for a couple days but now it's back to where it was again with K4 not being triggered despite the water cooling down well past where the heater should be kicking on. This leads me to wonder if something in the system is starting to fail but came back to life for a while on Saturday and may or may not come back to life again.
The mention of trigger voltage seeming low, yet relays work (or sometimes work in the case of K4), makes me wonder if you are not measuring the voltage correctly.
  • How exactly are you measuring the voltage for the relay signal?
  • What voltages are you measuring exactly (i.e. not just "low")
  • Do other relays, such as the jet pump relays, have a control voltage that is 12 volts, or do they also read "low"? If the latter, that also confirms that you're not measuring something correctly.
P.S. Mechanical engineer here (by degree), though I do a lot of hands-on stuff at work, including a decent amount of electrical building and troubleshooting.
 

DangerBoy

Well-known member
Oct 4, 2018
141
Calgary/Alberta
@jseyfert3 I'm quite sure I'm measuring voltages and resistances, etc., correctly. In another life I had very technical jobs in the oil service industry running different types of trucks that gathered various types of geophysical data. We had to be able to troubleshoot trucks full of specialized electrical panels down to the card level and we also had to be able to troubleshoot problems with the cabling and connectors. For that, I received a lot of technical training and I also worked a lot side by side with the lab technicians learning how to troubleshoot and diagnose problems with the various types of downhole geophysical data gathering tools we used.

The trigger voltage I'm getting on the K2-K5 trigger circuit is only 0.12 - 0.15 VDC. Way too low to latch one of those relays.

I figured out why the circ pump was working despite the fact that there isn't enough trigger voltage to latch the K2 relay. It's because they jumpered that relay in the factory and you can see it in the wiring diagram. They only jumpered it on one side because the circ pump only runs on 120 VAC.

The trigger voltage on K6 is 4 VDC. Even that voltage is low as it indicates that circuit is on the 12 V rail in the wiring diagram.

When I turn on a jet pump, the trigger voltage at the jet pump relay is 12.1 VDC.

The voltage at the red wire going from the T-Stat board into the top of the top circuit board is 15 VDC.

Right now I'm heating up the water by connecting the heater directly to the heater interlock relay, K6. In that mode I can limp along keeping the water hot and the tub alive as long as I want in the cold weather we're having just by periodically flipping the 30A breaker for the heater circuit on and pumping a bit of heat into the tub.

I'm still measuring ~0 ohms through the pressure switch so that component is working perfectly. I checked the resistance on the thermostat thermistor on Saturday and it read right in the range of where it's supposed to according to the resistance vs temp graphs available on Backyardplus.com so there's no issue there either. The only other safety systems that'd prevent the heater from coming on is an overheat or underheat condition and I don't have either of those.

It bums me out to say it but it's looking more and more like that 25 year old T-stat board has finally given out. Or it could be something on the main board at the top of the diagram. Whatever the issue is, that blue wire running down to the + side of relay K2 from the main board is not getting the voltage it's supposed to and there doesn't seem to be anything I can do to change that.

So now I've got a choice: I can retrofit with a used IQ 2000 control system from a similarly configured Tiger River spa that I have on hand or I can be adventurous and modify the existing system by transferring the heater control duties over to an off-the-shelf Digital PID Temperature controller and replace the general purpose heater relays with solid state relays to avoid the premature dying problems I've been having with the heater relays. (See my other thread ). Not sure what I want to do right now. Better decide soon. ;)
 

RDspaguy

In The Industry
Mar 21, 2020
912
Cabool, Mo
I would not recommend rerouting any circuits on the board as you may bypass safety features.
Control voltage enters the relay board at the connection on the upper right of the relay board. Check that connection for corrosion or debris.