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?
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?
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I am amazed it is working at all let alone just not heating... Have you thought about replacing the complete spa pack? 