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Discussion Starter · #1 ·
Hi, I'm putting right the final few things that don't function after my 1980 CX custom rebuild.
The temperature guage within the tachometer doesn't register at all so I've done the following. My electrical knowledge is sketchy!
Connected the guage to a battery and it rises all the way so have assumed this is functioning.
Putting a meter to measure current from the sender to the live connection (from the 7 volt regulator) shows 4 volts with the ignition on and engine cold.
Am I right in assuming as the water temperature rises the thermistor lets more current through so more voltage gets to the guage and the needle rises? As the circuit seems to be complete with 4 volts flowing am I right in thinking the sender is dead or am I missing something?
 

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yes, the sensor resistance in Ohms is less when it is warm and lets more current through the gauge.The correct values are in the shop manual.

I hope you didn't apply 12volts to the gauge. Many Analogue swing-needle gauges are delicate beautiful things to look at when you see how they are made and really it hurts to think of ways to burn their tiny windings out .

This is just me remembering how to roughly test these little analogue swing-needle gauges, but I recall you can often just start at the highest scale of a multimeter's ohms and STOP when it moves at all. Don't jerk it to full deflection. Ouch. :(
 

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Connected the gauge to a battery and it rises all the way so have assumed this is functioning.
Now it's damaged.
Putting a meter to measure current from the sender to the live connection (from the 7 volt regulator) shows 4 volts with the ignition on and engine cold.
I'm very sure You will not get Volt values.

The sender gives Ohm readings/values.
For example:
20°C = 477 Ohm
60°C = 131 Ohm
100°C = 34 Ohm
 

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CX500 Z 1978 , Rickman Polaris fairing, currently GL500 front, Ignitech etc. subject to change
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Don't worry, the meter is not a coil type but uses a little bimetal strip heated by the current which increases as the resistance of the sender goes down when the coolant temperature rises. It will not break even on the full 12v.

You should see 7 volts from the regulator at all times (ignition on), voltage is constant, current is temp dependnnt. If you measure 4Volts most likely your 7v regulator has died, just replace it.
 

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^^ what he said. You can even build a simple replacement, check in the WIKI.

Joel in the Couve
 

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Don't worry, the meter is not a coil type but uses a little bimetal strip heated by the current which increases as the resistance of the sender goes down when the coolant temperature rises. It will not break even on the full 12v.
Really?Thanks for the correction,brinkcx. I haven't had mine apart....still, I hope people will be careful. For instance, if you can't find a voltage source low enough on some kind of test, in a pinch you can often put a small dash lightbulb in series to act as a current limiter/voltage dropper.
 

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Putting a meter to measure current from the sender to the live connection (from the 7 volt regulator) shows 4 volts with the ignition on and engine cold . . . As the circuit seems to be complete with 4 volts flowing am I right in thinking the sender is dead or am I missing something?
If what you mean is you connected your voltmeter (+) to the +7V regulator output and the voltmeter (-) lead to the temperature sending unit . . . it's really an inconclusive test. There are two bits of data you would need to know to make sense of that reading:

(1) What is the resistance of the gauge

(2) What is the specified resistance of the sending unit at "cold engine".

With the engine "cold" there's really no spec on the sending unit. The resistance could be in the 300 - 500 ohm range, but they really pay more attention to getting the resistance vs temperature right in the normal to hot range. Your test indicates some current flowing through the thermistor, which is a good sign, but we don't know how much current is flowing or whether it's the correct level.

If you want to check out the gauge, you can shunt the sending unit with a 50 ohm resistor - the gauge should indicate normal temperature. Then shunt the sender with a 20 ohm resistor - the gauge should indicate hot.

Since you're there with the voltmeter in your hand, I presume you verified that the output of your 7v regulator is correct?
 

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Discussion Starter · #9 ·
The 7v regulator is only showing 2 volts so looks to be the culprit. Looking at replacing with a BA178M07T ROHM SemiconductorSTANDARD REGULATOR POS 7V 0.5- cost £1.50!
Resistance from the sender reads over 200ohms with cold engine which fits the scale in the workshop manual (60c:104ohms, 85c:44ohms) so hopefully, as you say this is a good sign.
Can't test it hot as am rebuilding the starter and waiting for a casing bolt replacement from Holland.
Thanks for all the advice.
 

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The 7v regulator is only showing 2 volts so looks to be the culprit. Looking at replacing with a BA178M07T ROHM SemiconductorSTANDARD REGULATOR POS 7V 0.5- cost £1.50!
Resistance from the sender reads over 200ohms with cold engine which fits the scale in the workshop manual (60c:104ohms, 85c:44ohms) so hopefully, as you say this is a good sign.
Can't test it hot as am rebuilding the starter and waiting for a casing bolt replacement from Holland.
Thanks for all the advice.
Yep, two volt out would be a problem. But how does this square with your previous measurement showing 4V across the gauge (plus whatever the voltage across the thermistor was)? Something is inconsistent in your data. Just to be sure you're not chasing your tail, be sure the regulator input and ground are correct and its output isn't just pulled down by excessive load.

That BA178M07T looks like it will do, but (speaking as a professional circuit designer) not with much margin. Keep in mind that you're dissipating 2 watt in that regulator when the gauge gets into the hot region. And it's under those same conditions that the environment is likely to be hottest. Given it's rated thermal resistance, it will be running at 125C junction temperature unless you fix it to a nice heat sink with air cooling.

If you can find a way to bolt it to the frame, you should be in good shape. Also be aware that these regulators often need a large electrolytic capacitor on the output to prevent oscillations. Rohm is notorious for leaving important details like that off their data sheets.

By the way, you can test the thermistor hot. Just take it off and put it in a pot of boiling water. A temp of 212F is very close to normal operation temperature of an engine, and is an excellent way to get a single point calibration.
 

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Discussion Starter · #11 · (Edited)
Not too experienced with electrics but it measured 4 volts yesterday. I recharged the battery overnight as it had dropped down to 9 volts having been on and off whilst testing earths etc..over the last few days. When I measured the voltage from the regulator today it only gave 2 volts. Assuming I'm using the meter correctly - red lead to the regulator output lead and black lead to frame earth? I've read that the regulators often give variable output before failing?
Have a heat sink recycled from a microwave (about 2" square) that will fit nicely to the regulator.
I saw this regulator used to replace a similar one on a GL1000 on YouTube without any other components. Should I be thinking of adding a capacitor in line with the output?
 

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Not too experienced with electrics but it measured 4 volts yesterday. I recharged the battery overnight as it had dropped down to 9 volts having been on and off whilst testing earths etc..over the last few days. When I measured the voltage from the regulator today it only gave 2 volts. Assuming I'm using the meter correctly - red lead to the regulator output lead and black lead to frame earth? I've read that the regulators often give variable output before failing?
Have a heat sink recycled from a microwave (about 2" square) that will fit nicely to the regulator.
I saw this regulator used to replace a similar one on a GL1000 on YouTube without any other components. Should I be thinking of adding a capacitor in line with the output?
A two inch square heat sink would be more than enough. Even 1 square inch. As long as it gets some exposure to air for convection cooling. Two important things to understand are:

(1) The gauge draws more current as it gets hotter

(2) This sort of regulator throttles back its output voltage to protect itself when current becomes excessive.

The upshot is that you can end up with a gauge system that works fine with a cold or normal engine, but refuses to go beyond the normal range when the engine gets hot. Or it may indicate hot for a few minutes before drooping back into the normal range. The $1.50 regulator's clever design to save itself from overheating can cost you an engine.

I do think your design will work, but I would absolutely test it with an 18 ohm 2W resistor for ten minutes to simulate a hot thermistor.

In cases where they oscillate, the usual symptom is that the DC output voltage is off. If you measure 7 Vdc and zero Vac at the output, then you're good.
 

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Wekadog, I am not familiar with THAT regulator and the datasheet kinda sucks (normal for Rohm in my experience) but on MOST of the linear regulators in a TO-220 -3 package the metal mounting terminal is at the same voltage potential as the output. He needs to test that mounting terminal before using any heat sink and taking extra care using the frame as a heat sink.
Easy to test but important consideration.

If your familiar with that regulator I am happy to defer to your experience but a simple test will not hurt him.
Assuming the temp does not cause limitations he should be OK. I would worry a lot more about putting a bypass cap on the input so
the spark transient does not cause a problem.

Just my 2 cents and everybody is encouraged to do their own homework on whatever I say.
:)
 

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. . . on MOST of the linear regulators in a TO-220 -3 package the metal mounting terminal is at the same voltage potential as the output.
The part number he posted had the TO220-AB package with fully insulated tab. It has higher thermal resistance than a metal one, but it's good to mount directly to ground.

Hood Automotive exterior Auto part Tool accessory


Actually, most of the common positive, fixed voltage, 3-terminal regulators have pin 2 (and the mounting tab) grounded. The 7808 below is typical.

Technology Circuit component Transistor Electronic device


The 7808 is what I would use if I were doing the job. I like the lower thermal resistance and higher shut-down current. Yes, it's 8 volt rather than 7, but a diode or two in series with the output lead would drop the voltage into range.

An large electrolytic capacitor directly on the switched 12v would draw a large inrush current. You're absolutely right about arcing causing wear to switch contacts. But by putting it at the output of a current-limited regulator, the surge is limited too.

http://www.fairchildsemi.com/ds/LM/LM7805.pdf
 

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Discussion Starter · #15 ·
Soldered up the the new 7v chip with a capacitor on the output as in the wiki and a little heat sink and works a treat.
Thanks for all the advice.
 
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