fixed: temp gauge only sliiightly working
Posted: 23 Nov 2016, 20:33
I'm new to campers, mechanics and the forum - but today I fixed a thing after finding contradictory stuff online - here's the information I would have liked to have found, plus a couple of questions which arose.
Bluebell is a 1986 petrol 1.9, new to us. Temp gauge was moving but only a tiny bit - even at hottest the needle only just reached the furthest left end of the dial. The really useful quote from Smcknighty (below) helped with my hunch that this meant most relevant electrical things were connected and working, but that the sensor had deteriorated. The sensor was in general providing too much resistance, which sounds like a limping variable resistor to me.
NB what I would call a sensor is called a 'sender' in Haynes and on Just Kampers. Don't know what this means, but since JK were very right about also needing to buy a rubber washer thing (and new clip, less importantly) I'll go with it.
Things I found out:
1. There are two sensors on the thermostat housing. You want the one you can't see when you're standing looking in the engine bay. It is hidden on the side towards the front of the van. You can get to it by lying on your back under the van.
2. When you pull out the old sensor, coolant will pour out all over your face. Somehow nobody mentioned this would happen.
3. The new rubber washer goes on the shiny metal end of the new sender, not the plastic end. Having this on before you pull out the old one would mean you could do it all in one move: old one out, new one in. Because I had put the old one back in, in a hurry, I had to guess which end to put the washer on. Because I guessed wrong, I had to pour coolant on my face twice more in total.
4. This fixed the temperature gauge!
Things I don't yet know:
5. Whether it is a problem that loads of coolant fell out. I topped up the top-up tank and I'm hoping it draws in what it needs - or am I cruising for an airlock and will need to bleed the system?
6. Whether it is okay that the coolant whcih fell out (and that is in the main coolant tank) is an orange/pink colour even though the stuff that has always been put in is blue. To me this says: oxidation. Bad news?
I commend this to the wiki - thanks Smcknighty:
Bluebell is a 1986 petrol 1.9, new to us. Temp gauge was moving but only a tiny bit - even at hottest the needle only just reached the furthest left end of the dial. The really useful quote from Smcknighty (below) helped with my hunch that this meant most relevant electrical things were connected and working, but that the sensor had deteriorated. The sensor was in general providing too much resistance, which sounds like a limping variable resistor to me.
NB what I would call a sensor is called a 'sender' in Haynes and on Just Kampers. Don't know what this means, but since JK were very right about also needing to buy a rubber washer thing (and new clip, less importantly) I'll go with it.
Things I found out:
1. There are two sensors on the thermostat housing. You want the one you can't see when you're standing looking in the engine bay. It is hidden on the side towards the front of the van. You can get to it by lying on your back under the van.
2. When you pull out the old sensor, coolant will pour out all over your face. Somehow nobody mentioned this would happen.
3. The new rubber washer goes on the shiny metal end of the new sender, not the plastic end. Having this on before you pull out the old one would mean you could do it all in one move: old one out, new one in. Because I had put the old one back in, in a hurry, I had to guess which end to put the washer on. Because I guessed wrong, I had to pour coolant on my face twice more in total.
4. This fixed the temperature gauge!
Things I don't yet know:
5. Whether it is a problem that loads of coolant fell out. I topped up the top-up tank and I'm hoping it draws in what it needs - or am I cruising for an airlock and will need to bleed the system?
6. Whether it is okay that the coolant whcih fell out (and that is in the main coolant tank) is an orange/pink colour even though the stuff that has always been put in is blue. To me this says: oxidation. Bad news?
I commend this to the wiki - thanks Smcknighty:
Smcknighty wrote:Found this on Samba
http://www.thesamba.com/vw/forum/viewto ... 80&start=0" onclick="window.open(this.href);return false;
The electronics of the circuit board inside the temp gauge will react to an extremely short duration anomaly in the voltage that you couldn't see with a meter. I don't feel like going into a super detailed explaination of the circuit board right now but here is some. When you turn on the key 12 volt power is sent to the cluster, is stepped down to 10 volts by the voltage stabilizer, and then fed to the temp gauge module. The temp gauge internal circuit board has a capacitor on it. The capacitor takes time to fill up to capacity. Before it is full the led blinks. When it gets filled, the blinking stops. There is a current limiting resistor in the power feed to the capacitor. This resistor makes the capacitor fill up slower than it would without the limiting resistor. While the capacitor is filling the led blinks. The temp sender works by providing a leak for the power in the temp gauge to go to ground. The hotter the temp sender the faster the leak. (that is also what the gauge needle shows, the current flowing through the sender to ground, the temp gauge is a current meter) When the engine is too hot the temp sender drains the capacitor faster than the resistor will allow it to fill. That makes the led blink again when the engine is too hot. While the capacitor can drain in an instant it fills back up slowly because the limiting resistor restricts the fill rate. The led blinks until the capacitor it is full again. So just a tiny blip in the power feed can cause multiple blinks while the capacitor fills back up through the limiting resistor.
When the engine is cold the temp sender does not drain as much current from the gauge capacitor. So it takes more of a voltage anomaly to cause the false blinking when the engine is cold. Once the engine is warmed up there is always more current flowing from the gauge to the temp sender. That higher constant drain when engine is up to temp means a smaller interruption in the filling of the capacitor will cause the blinking with a warm engine than a cold engine.
(as a reminder, this blinking function has nothing directly to do with the coolant tank level sensor. That sensor goes to a relay and the relay connects to the gauge. The relay then provides an additional drain of the capacitor whenever the level sensor triggers the relay to do so. If you pull the coolant relay you remove the coolant level sensor system from any involvement in the gauge led behavior. This is a useful test to determine where the a false blinking is coming from. In this case this test has already been done and showed that the blinking was not from the coolant level warning system)