47 replies to this topic

[TerrA LX]

Thanx Yella SLuR
, can we stick this?

[rodomo]

Prepare to be "adjusted" in your diagnosis as you haven't stipulated the make, model and year of the vehicle you have repaired.

Edited by rodomo, 29 August 2006 - 01:16 AM.

[Dangerous]

Yella, please please PLEASE change your original post, and stop this dangerous practice! You should NEVER short out an LH/LX gauge. LH to UC and HJ (or is it HX?) to HZ (and early Commodore) gauges are a bimetallic strip gauge. They have two metal strips with dis-similar expansion coefficients, with a heating wire wrapped around the strips. The reading from the sender unit denotes how much current can flow through the heating wire, to heat the two strips. Because one strip expands at a faster rate than the other, the strip bends, causing a deflection on the gauge.

Now, LH/LX sender units have a minimum resistance of 10 ohms, with the voltage to all the gauges (except the voltage gauge) limited to around 10 volts by the voltage regulator. This means that the absolute flat out maximum current that can flow through the gauge is less than one amp, as the heater wire has some resistance.

If you short the sender wire to earth, the only resistance in the circuit is that of the heater wire, so a MUCH higher current than 1 amp can flow. This can do one of two things almost instantly, neither of them good. -

Firstly, it can cause a more rapid increase in temperature than the bimetallic strip is capable of handling, which can permanently warp the strip. This will cause the gauge to then permanently read high or low and/or slow. You've just stuffed the accuracy of the gauge.

Secondly, it can either burn out the heater wire, or break one of the bimetallic strips, either of which will cause the gauge to die permanently.

SL/R gauges are getting too rare to destroy them this easily. Would you flatten the throttle on your engine while it was running and in neutral (even if only for a short time) just to see if the engine was still working? I hope not, but that's the equivalent of what you're doing to the gauge.

Re the potentiometer, they can be used, but most don't have the current capacity to test the guage near its upper limit, and the potentiometer will burn out. Much better to use some power resistors, or get a 10 ohm 5 or 10 watt resistor, and solder in series with the potentiometer. Minimum settign one the pot will tehn correspond to minimum reading on the gauge, and effectively limit the current flow.

LH and LX sender units are the same in principle, although Holden did tend to change around the connector design in most Torana models.

Edited by Dangerous, 29 August 2006 - 03:09 PM.

[Tiny]

Hey Guys.

I've modified the above post as per dangerous's comments! Hope thats ok pat!!

Dangerous: Can you reccomend any way of diagnosis by a torry lover with minimal equipment? ( especially an in-car diagnosis?)

I know that guage repair people have a "dummy load" box wehre they hook up the guage and wind up the resistance checking the guage-read to a known resistance value. Are these available cheaply or is there another way?
Cheers for the good advise all round!

[Tiny]

Great stuff! I'll wait to see if dangerous has any more information, and then add that into the first post!

Cheers!

[Dr Terry]

Hi Guys.

I thought is was time to throw in my 2c worth.

The easiest way to check LH & LX gauges (& yes, they are the same, but UC is different) is to use substitute resistors (or dummy loads) as mention earlier. This works for all 3 gauges on LH/LX, by the way. Go to Jaycar, Dick smith or similar & buy three 1/4W resistors, a 75, 25 & 10 ohm. Just put these, one at a time between the disconnected sender wire & a good earth & switch the ignition on & wait for the gauges to warm-up & stabilise. The 75 ohm will make the gauge read to the empty mark (or cold or low for the temp & oil gauges), the 25 will move the gauge to 1/2 & the 10 ohm to full scale. That's it, if the gauge doesn't do this you have a gauge or voltage reg fault. You can also use these ohm readings with a mulitmeter to check a fuel sender out of the car. AKAIK the current thru the sender is less than 100 mA.

Dr Terry.

[Dangerous]

I think I need to clarify what I said vs what I meant. DA, you're right - what you have quoted me as saying about more than 1 amp being able to flow is not correct. I was using the '1 amp' as a hypothetical maximum current , and then trying to say that if you short the gauge to earth, a much higher current is allowed to flow than what the gauge was designed for. It certainly wouldn't need a current as big as 1 amp for the gauge to operate. Offhand, I do not know the 'hot' resistance of the heater wire element - the maximum current flow will depend on the cold resistance of this bit of wire, and any other internal resuistance in the circuit. It might be as little as 100 mA for normal operation, but the maximum design current will still be exceeded if the gauge is shorted to earth on an LH/LX.

The test method that Dr Terry has described is OK, except that you can't easily get a 75 or a 25 ohm resistor, but you can get a 68 and a 22 or 27 ohm resistor - the mid scale resistor should be roughly half way between the 10 ohm and 75 ohm range of the gauge, and the nearest preferred value would be a 33 ohm resistor. I'd also recommend at least 1/2 watt or 1 watt - not only because they can absorb more power, but they are also larger with larger leads, which make them easier to use. Resistors are only a few cents each.

If it were me (and it was, cos I've bought them ) you could buy a 72 ohm, 10 ohm and 33 ohm resistor. 72 ohm bridged from the gauge wire to earth will give empty/cold/low, the 10 ohm will give full/hot/high, and the 33 ohm will give a mid range reading.

You can just hold the resistors in place with your hand and you won't get electrocuted as the voltage is way too small, or you could buy some miniature jumper lead sets when you buy the resistors.

For LH/LX, it's the only Torana where the three gauges (oil, temp and fuel level) are identical in their operation with all three sender units using the same resistance range for operation, so any sender unit will cause an identical reading if attached to any one of the three gauges.

As far as in car diagnosis of LH/LX gauges goes;

(1) If all three gauges (not the voltage gauge, as it's hooked straight into the 12 volt rail, bypassing the voltage stabiliser) suddenly head towards full scale for no apparent reason, turn off the ignition immediately, as this is a sign that the instrument panel voltage stabiliser has failed and is short circuiting, and is allowing too much voltage to be fed to the gauges. The car can still be driven, but I'd recommend disconnection the oil, temp and fuel gauge sender wires until you're sure that the voltage stabiliser is replaced or working OK. Removing the wires will protect the gauges.

(2) If all three gauges fall to zero for no apparent reason, the voltage stabiliser has probably failed in an open circuit - no immediate danger to the instruments. Unfortunately the voltage stabiliser rarely fails like this - more often like described above.

(3) if all three gauges move down a bit when the park or headlights are turned on, it's a sign that the instrument cluster earth is not good enough (it's barely good enough from factory). Run an extra earth lead from the screw holding on the voltage stabiliser on the back of the cluster to one of the four 7/16" UNF bolts holding the steel steering column bracket onto the column. DO NOT use any of the larger three silver bolts, as they form a part of the collapsible steering column assembly.

(4) If the fuel gauge dies, or reads low, it's usually that the tank isn't properly earthed (once again, no real earthing from factory). Run a piece of wire from a small hose clamp attached to the metal fuel pickup line on the fuel pickup/sender unit to a good earth on the body.

(5) If one gauge dies, eg the oil pressure gauge, and you think you've still got oil pressure, swap over the oil pressure sender wire with the temperature sender wire (you might need a bit of wire to extend it to reach). The Oil pressure gauge should then read engine temperature, and the temperature gauge should then read oil pressure. If the temperature gauge then shows accurate oil pressure but the oil pressure gauge does not show accurate temperature, then your oil pressure gauge is probably faulty. If the temperature gauge does not show accurate oil pressure, then you probably have a faulty oil pressure sender unit. The same goes to test the fuel tank sender unit.

(6) As I mentioned, having a known working spare fuel tank sender unit is handy for checking any of the fuel, oil or temp gauges. Connect the sender unit connector on the spare fuel sender unit up to the sender wire for the gauge to be tested, earth the body of the spare fuel sender unit, and as you raise and lower the float, the gauge under test should raise and lower with it (but slowly).

(7) Dr Terry's method, with resistors as outlined.

For other early holdens, HK to HQ and LC and LJ gauges are a different design. The test method is as above, except in this case, for the fuel gauge the resistance ranges are 0 to 30 ohms (ie you can short the fuel gauge sender cable for the low reading), temperature is 315 to 22 ohms, and oil pressure is 10 to 190 ohms.

For UCs, as DA has mentioned, fuel and temp resistance ranges are 282-40 ohms, but oil is 10 to 180 ohms. Not sure if these changed through the life of the UC or not.

Sorry if my previous posts caused confusion.

[Dr Terry]

Hi Guys.

Dangerous said:- "the mid scale resistor should be roughly half way between the 10 ohm and 75 ohm range of the gauge, and the nearest preferred value would be a 33 ohm resistor. "

This is not correct, the thermister (sender) is non-linear. A 33 ohm resistor will result in a gauge reading closer to a 1/3 scale.

You are correct about the commonly available resistors, but you can make a 75 ohm by using two 150 ohm ones in parallel & you can make a 25 ohm by using a 47 ohm & a 56 ohm in parallel.

Dr Terry.

[Dangerous]

^ ahh, OK. I was going off the specified VDO sender units listed for the UC. Maybe they've made another mistake?

[Dr Terry]

Hi Guys.

Devils, I was using the temp sender (thermister) as an example. The 3 gauges senders are all non-linear, look at the way the fuel float arc swings, it too is non-linear.

In answer to Yella's query, it is easier to substitute for the sender, it saves removing it from the tank. But having said that, if the sender is out check it using the ohms scale on a multi-meter. The advantage of using dummy loads (substitute resistors) is that it not only checks the operation of the gauge, regulator & wiring etc. it also checks the gauge's accuracy, which is difficult if you're just checking the sender.

Dr Terry