Instrument panel circuit board problem

[Tim D]

Hi,
I've got an 89' 2.1 petrol van and have been having problems with the temperature gauge reading very high all the time and red light flashing etc. The van has a fairly new engine (less than 12,000 miles) and I've replaced the thermostat, gauge sensor and also the temperature gauge itself as a precaution but the problem continues; the rad is fine and fan is working (when the gauge is right over) and has a new sensor also. The one suspect area is the circuit board for the gauges as a small section behind the temperature gauge has been cut out where it would clamp onto the central thin post of the temperature gauge and there are three wires soldered onto what appear to be three circuits and these are currently connected the the central post via a spade connecter; is this likely to be causing the problem?

thanks
Tim.

[AngeloEvs]

Any mods to the existing membrane can cause problems but there is one further component that you need to check. The temperature gauge also connects to the Level Sender Control unit (LSCU) and this can fail causing the problem you are experiencing. The LSCU looks like a relay and fitted in the fuse box. Marked 43 (or 42 if a later LSCU has been fitted) and located in position 3. Remove it from the socket and switch the ignition on. If the fault clears then it needs to be replaced. See how you get on checking this first and post an update.............

[meggles]

Ojsmith had similar problem. I'll try to get him to reply on this forum. He had a split in some tank or other I think. Took ages to find it!!!

[Tim D]

Thanks for the replies, I can't wait to get this problem fixed its been bugging me for ages. The red light doesn't always flash and its possible to alter the position of the gauge, i.e. lower the reading slightly by putting the heater on which is what I did throughout the summer thinking the engine was cooking and as a consequence lost about a stone in weight.

AngeloEvs - I took the van for a run and removed relay 42 when the gauge was right over but unfortunately it didn't affect the gauge at all.

Meggles - that sounds a bit worrying, will be very grateful for any further information.

[meggles]

Not worrying at all. I seem to remember it was quite a cheap fix. Just finding it was a problem. Hopefully he'll be able to quicken your search up. I've pm'd him.

[AngeloEvs]

First, what does the temp gauge read when the engine is cold and ignition switched on (from your first post 'all the time' suggests deflecting when cold and ign on, or is this happening when the engine has been run?

If the fault remained after removing the LSCU then you have in effect disconnected the header tank sensor which eliminates any possible fault in that part of the electrical circuit. You are left with an overheating issue or a possible fault in the temp sender wiring/ dash wiring.

[Tim D]

Angelo Evs - sorry, by 'all the time' I meant when the engine is hot, the gauge rises gradually as the engine warms up but keeps on going until its way over to the right and sometimes the red light flashes also. This has got me concerned that the engine is overheating and so I've done some checks and it seems to be ok; the thermostat is new and is working, although it takes quite a while to open and the water seems to be circulating through the rad and the fan kicks in when hot and stationary but the gauge would indicate that the engine is about to meltdown.

[AngeloEvs]

Gut reaction is a coolant temperature problem given that you have replaced the sender and thermostat. Red light flashes when coolant temperature is excessively high and gauge three quarters across. Wiring faults at the temperature sender can cause the gauge to read high if there is corrosion and deposits across the sender terminals or the sender is faulty. It could be a problem with the membrane but I would be checking the coolant system first.

At normal coolant temp the sender has a resistance of around 100 ohms and drops to 47 ohms when critical over temp is reached. The gauge will be centre at 100 and three quartes across at 47 ohms. These resistance figures can be checked at the LSCU socket quite easily and you can also check the gauge accuracy/membrane panel using these values. If you want details to perform these checks let me know. A reading of less than 50 ohms indicates excessively high coolant temp.

[ojsmith]

hello

i had a very similar problem, which i traced to a split in the header tank. after changing for a new VW header tank and putting a new VW dalek cap on, then bleeding the coolant system, all problems ceased... cost about £50 for parts and was easy to do myself.

i remember it looked to all intents and purposes like it was an electrical issue, but turned out not to be...

do you have any indication of losing coolant? have you changed the dalek cap - if not it is likely to be 20+ years old, and probably isnt doing its job properly, so might be worth changing...

hope this helps, and keep us updated...

Oliver

[Tim D]

AngeloEvs - I've tested the sensor and it was about 107 ohms when the engine was well warmed up and the needle was more than three quarters of the way across; I would like to test the reading at the membrane/gauge next, I would be very grateful for advice on how to do this.

ojsmith - thanks for the advice, the header tank doesn't seem to be leaking anywhere and there is no loss of coolant, the dalek cap is probably the original so I will replace it now as a precaution.

[AngeloEvs]

Ok, 107 ohms should result in a temp indication below half way so the fault is now being narrowed down to the pcb. Best to remove the Dash assy next and test it on a bench. I use a 100 ohm resistor and a 47 ohm for checking gauge function (these are available from Maplin etc, 1W are ideal but any will do). With the dash on a bench you need to connect a 12V power supply across Pin 8 (12V) and pin 3 (0v Earth). Remove the plastic shroud from the edge connector and use small croc clips. You need to test the function of the membrane and, to do this correctly, means connecting the resistors in turn across pin 6 (temp gauge out) and pin 3 earth. With dash connected to an aux 12v supply I would do the following:-

a) Confirm Regulator output is 10V (and not 12V) by measuring the voltage to the right connector/nut on the temp gauge (viewed from rear).

b) Place 100 ohm resistor across the output nut (left when viwed from rear) and thye centre nut (earth). After a minute or so gauge shoul settle just under halfway. Repeat with 47 ohm . gauge should immediately flash and climb to 3/4 deflection.

c) repeat at the mutli-connector edge connections (can be tricky but possible with small croc clips)