Temp gauge sky high but thermometer shows 69-degrees

[twodoorgaz]

Hi folks.

I've recently finished a full rebuild of a SWB SIII 2.25 petrol - she's had a turner engine and most of the cooling system is new, except the radiator which was pressure and flow tested.

During its first 500miles on running-in oil the temp gauge sat consistently at the top of the 'N' - higher than I've been used to in my previous SIII, but nothing to worry about.

This weekend I carried out its first service and sorted a few electrical gremlins from behind the instrument binnacle (the speedo bulb glowed bright and then blew, so I had to replace it).

However - when I took it out for a long run the temperature gauge went to the very top of the 'Normal' range, just shy of the red 'Hot' band. The needle sits there and never gets higher, however long the journey.

Panicing, I bought an IR laser thermometer from Maplins and measured the surface temp of various points in the engine bag (thermostat housing, radiator, etc) and I couldn't find anything hotter than 69-degrees C.

My understanding is that 74-degrees or even 82-degrees is perfectly acceptable as these are the standard thermostats.

So if its not overheating, and as the temp sender is new, the only thing I can think of is that the voltage stabilizer on the back of the speedo is at fault - perhaps I knocked it when fiddling with the speedo backlight bulb.

Is there any way I can test this theory (I don't have a multimeter I'm afraid) and would a replacement like this solid-state one off ebay be an appropriate solution or would I be better sticking with a pattern replacement for the standard one?

Many thanks.

[snailracer]

My first thought would be the voltage stabilizer. I'm pretty sure that when mine did similar the multimeter showed the voltage flicking about, not much help as you dont have a meter. It would possibly cause a test bulb to flicker if you made one up?

Also is the temp sender matched to the engine/gauge? I know there are fun and games to be had with 200tdi temp senders

[secondjeremy]

Sender must be paired with the gauge - so if you're fitting a 200tdi to a series - you use the series sender (assuming it fits) unless you're using a gauge unit from a 200Tdi installation.

[Serious Series]

I fitted electronic version from ebay awhile back works well not sure if it was from same seller as your post.

Check the one you have has not come loose as it needs to be earthed , my understanding of it is it acts like a relay switching voltage off and on to give approx 10V.

[twodoorgaz]

Thanks guys. Yep the sender, thermostat and gauges are all correct for a 2.25 petrol SIII.

Bit of an update: I was told to try it with the external and instrument lights turned off, dd so on a long commit this morning and hey presto the needle is back to sitting at the top of the "N" well clear of the red zone. So I think that this is confirmation that the stabilizer is at fault. Think I'll replace it to be on the safe side - the old one is looking very bedraggled and I'd like to minimise the amount of times I open the binnacle rats nest.

Thanks for your help.

[FridgeFreezer]

If it changes with the panel lights off, I'd suggest it's your dashboard binnacle earth connection that's at fault.

[twodoorgaz]

Thanks mate - did another test today. It goes up with the lights on regardless of whether dash lights are switched on or not. SO I think that yourself and serious series have it nailed - bad earth. Which could be a loose fitting to the speedo or a faulty stabilizer. Will report back once I've got it sorted.

[Pepé le Pew]

If the stabilizer is at fault the fuel gauge should have the same symptoms as the temp gauge.

Like said before the stabilizer is just a relais that switches on and of to give a crude 10V

You can't measure this 10V with a digital voltmeter because the stabilizer switches relatively slow and the digital meter will try to follow that and give a very erratic reading. It works better with a analog meter.

Seeing that the temp gauge goes up when you put the lights on i also put my money on a bad earth.

Cheers,

Eric.

[Snagger]

I had exactly your fault. It's the earth connection on the engine side of the driver's foot well top slope. It's a bolt sticking up through the sheet steel with a bunch of earth wires with ring terminals - you need to make sure there is clean metal on the foot well panel for the terminals and bolt to touch.

[twodoorgaz]

Right, replaced the stabilizer with a spare I had AND ran a short earth from the stabilizer to the negative power socket behind the binnacle. None of this made a difference - stil the same high reading.

I fitted the loom myself and the single earth behind the bulkhead is bolted onto the steering bracket. I'll take it out as per dnagger's advice and rub back to bare steel, see if it makes a difference.

[twodoorgaz]

Update: I removed the earth from the steering bracket (on most SIIIs this is on the footwell in the engine bay) and took it to bare steel before refitting. No effect.

[Snagger]

The effect of the lights proves it's an earthing fault, not a sender or gauge problem. You may have broken or corroded earth wires inside the dash, in which case you need to run a new wire with ring terminals from speedo to combined cluster, and a second new wire from either of those to a good earth point on the body. Just because the earthing post is OK, it doesn't mean the wiring between it and the gauges is doing its job...