Fluctuating Temperature Guage

[David Mirylees]

My 1988 chassis Caterham has the problem of gaining +10 degrees on the water temp guage whenever the headlights or the cooling fan (etc) are operated.

I have tried fitting a solid state voltage stabiliser (from my Europa) to see if it improved anything but it made no difference.

Any suggestions as to the reason for the initial problem and a subsequent cure would be very much appreciated.

Cheers

david

[Normans_Ghost]

david, that sort of fault is usualy related to faulty earths. But not in this case I suspect.

 

have you tried a new sender? I can't think why it should be affected by voltage fluctuations but if it's faulty anything can happen.

 

^{Norman Verona, 1989 BDR 220bhp, Reg: B16BDR, Mem No 2166, the full story here}

You and your seven toThe French Blatting Company

[alextangent]

This has happened to me on a tintop; I first noticed it when the lights ran extra bright *idea* . For all of about 10 minutes before they popped.

 

It could be a problem with the alternator on the way out. When the system is under load, the larger current draw causes the voltage to dip, and the alternator pumps more juice at a constant higher voltage than the battery to compensate. If the rectifier or the feedback circuit is malfunctioning, the voltage may go far too high. As the temp gauge is just a voltmeter across a resistor in the sender, the needle moves up (and your headlamps burn brighter too).

 

The thin sensor wire provides the feedback for this behaviour; it measure the voltage across the battery terminals back to the alternator. Check the voltage across the battery, engine running, with lights/fan off and on. It should be failrly stable. If there's a big increase in voltage, my money is on the alternator.

 

Alex McDonald

Loud, louder, loudest...

[Eugene]

Hi Dave *wavey* long time...

 

I've had both the sender unit do this, and the alternator after the regulator was fried... but I didn't notice 'til too late.

 

Worth checking both.

 

Small Boy - with Loud Toy!

Co-founder of the BOG Club

See Eugene here

and see what I do here

 

[David Mirylees]

Thanks for the responses - I'll check the Alternator but that route is unlikely as the problem has existed for at least ten years and 70k+ miles!! I have not bothered too much about it but a few years ago on the way to the 25th Swiss meeting, whilst crawling through Basel, the temperature was showing maximum 'Red'. It was actually OK but very disconcerting at the time. I have also managed to run out of fuel (in the UK) when the guage was reading 'just' OK :-( So I thought that now was perhaps a time that I ought to fix it!

david

 

[Normans_Ghost]

David, Assuming the tank unit connections are clean and (fairly) tight the best way to adjust the unit is to drain it dry, refill with 1/2 gallon and remove the tank unit and bend the arm so that it's on empty about 1/16" off the "E" . This way you'll have a small reserve.

 

But I guess you know that

 

^{Norman Verona, 1989 BDR 220bhp, Reg: B16BDR, Mem No 2166, the full story here}

You and your seven toThe French Blatting Company

[David Mirylees]

Done the bending bit - as you so rightly surmised :-)

Mind you the first time was a failure as I had a non vented fuel cap and a mechanical pump on the (old) engine which collapsed the tank enough to cause the float to stick at mid-tank level - and yes, I did run out of fuel.......javascript:insertAtCursor(document.postpage.message,%20' *redface*')

...It was a long time ago and I hope that I now know better!

[David Mirylees]

Successful outcome!

For the record:

Checked the voltage as suggested - negligible drop.

Took a separate earth lead and touched it to one of the dash rivets whilst engine running and headlights on - 5 degree drop (no difference when the lead was touched to the dash itself).

Decided to run a short individual earth from the battery to the horizontal scuttle and realised that the main battery earth was not 100% tight. Cleaned the terminal and bollocked up the bolts - problem cured.

Would normally check this sort of thing but fitted a battery cut-out in 1988 so never do. I changed the battery in 2002 so it may well have been loose since then :-(

Thanks guys for all the help!

david