Thermistor

While charging the battery in my 95 Riv (Series one engine w SC) I noticed that the thermistor was not attached to the battery cable, and was unplugged from the wiring altogether. It was just lying on the floor under the rear seat.

Should I be experiencing any issues with the thermistor not being active? The biggest electrical issue I have had since owning the car (4 years) has been the battery slowly discharging if the car stands for four or five days unused. Could this be from the unhooking of the thermistor?

I don't really know what the thing does anyway. What is it's function?

Thanks for any input.

designer1962 wrote:

While charging the battery in my 95 Riv (Series one engine w SC) I noticed that the thermistor was not attached to the battery cable, and was unplugged from the wiring altogether. It was just lying on the floor under the rear seat.

Should I be experiencing any issues with the thermistor not being active? The biggest electrical issue I have had since owning the car (4 years) has been the battery slowly discharging if the car stands for four or five days unused. Could this be from the unhooking of the thermistor?

I don't really know what the thing does anyway. What is it's function?

Thanks for any input.

The thermistor measures the temperature of the battery to modulate the charging rate. With it missing the battery will charge more slowly.

Dam, I didn't know that. How does it look?

It measures temperature at the battery. A cold battery likes a higher charge voltage. A hot battery shouldn't get much over 12V. Using a thermistor keeps the battery healthier, and is one of the reasons many of our original batteries lasted 10 years or more.

one fine summer day, when my 95riv engine was "awakened",
the instrument panel said “ALTERNATOR”.  This was a first.

These generation8 Riv alternators are v powerful and they are expensive.
I have a friend who was head mechanic at a CarMax and because he could get me the maximum
discounts at NAPA/autozone/Advance, we were most respectful.

Upon hearing of this problem my “certified master mechanic” pal announced that the
Riv's charging system is very simple, and he opined that I needed a new alternator.  
[ about $150 ]  He stated that the alternator itself was the only thing that could go wrong.

further investigation revealed that, even though the instrument panel was illuminating the
ALTERNATOR lamp, it turned out that,  at idle,  the battery voltage indicated 14.1 volts.

Idling output of 14 volts did not seem like a big crisis, so I got out the trusty Buick Service Manuals,
[ they were 2 volumes, printed by General Motors, and,  
the pair was bought on ebay for $100]
they are exactly what Buick mechanics used for reference when working on 8th-gen Rivs back in the day ]

Image – Buick factory service manuals
https://s279.photobucket.com/user/Lmckinley99/media/riv_service_manuals_1416.jpg.html?sort=3&o=5

page from GM manual here:
https://s279.photobucket.com/user/Lmckinley99/media/riv_diagnose_alternator_manual.jpeg.html?sort=3&o=0

turns out there is a diagnostic tree for alternator problems.  Also, it turns our that the info is
difficult to discover because the Buick service manual calls this device a “generator”, which is a charming anachronism, but not an accurate name for the Riviera's dynamo.  Grrrr....

so, I began to figure out this “thermistor”.
Results:
Rivs have them.  otherwise, only Caddy Eldorados use this part number.
It is a device that senses heat and translates that to varying resistance [ like a temp gauge sending unit ].
this thermistor is no longer available as a GM part.  There are none in the GM system now. [year 2014]

image:  thermistor collection
https://s279.photobucket.com/user/Lmckinley99/media/riv_thermister_collection.jpeg.html?sort=3&o=3

What the thermistor does for a gen8 Riv:
this device is clamped onto the battery positive terminal.
It senses how warm the battery is.
[ when the alternator pushes a lot of amps into the battery, the battery heats up ]
because the battery lives inside the passenger cabin, the system carefully monitors
the rate of charge. A runaway battery would be no fun for the backseat passengers.
The thermistor dials back the charger rate if necc.

The thermistor performs the monitoring mission and it sends temp data to the
alternator, and thus dials back the output of the alternator if the battery gets hot.

So:
if the thermistor fails, the charging system gets confused.
This is what caused this 1995 Riv's “Alternator” lamp to turn itself “on”,
even though the alternator was constantly producing it's maximum output.

The 1995 Riv's thermistor quit sending info to the alternator. This was caused by corrosion
on the pins inside the small white electrical connector leads which send info upline.
[ the corrosion was caused by H2O in the battery well, but that's another story ]

The fix:
cut off the excellent GM Delphi plastic 2-pin connector and simply re-connected
the wires using high quality squeeze crimp units.

Bottom line:
Thermistors were rarely used by GM.
New ones cannot be had.
Whenever I see a gen8 Riv at the junkyards, the back seat gets lifted and the thermistor
is removed.  At checkout, the cashier usually has no idea what she is attempting to
put a price on, so the cost is often $2 or, sometimes, free.

The very fact that I have 4 thermistors under the workbench guarantees that i'll never have
a failure of the one which was installed in my RIV at Lake Orion, Michigan in 1995.

moral of this story:
the thermistors in our Rivs are rare and made of a “element” on the periodic table named
“un-obtanium”.
As a service to our community, whenever any one of has the opportunity, these devices should
be harvested.  [ if anybody requires one of these thermistors, contact me ]

Bottom line:
my original alternator continues to hum along after 130,000 miles.
Repairing the 2 wire connections extinguished the ALTERNATOR lamp.
The expensive GM service manuals paid for themselves then and there.

EDIT: the previous (this) post named the offending lamp as "alternator" but that was a mistake. the red lamp which illuminated had no written word description.
it is a little image of a battery located within the left-most circular "idiot light" gauge
and it is located directly above the headlamp switch.

To add a few details for clarity (I don't disagree with much of your analysis, but there is more for consideration):

As you mentioned, the thermistor protects against thermal runaway, and it also regulates (optimizes) charging voltage at colder temperatures. In very cold conditions, the alternator will deliver 15 volts or more. In hot summer, or when the battery is warm for whatever reason, charging can be just over 12 volts. What this does is keep the battery healthy for years of long life. Most lead acid batteries prefer higher charge voltages when cold.

Thermistors are not black magic nor rocket science. They're just simple components found in every modern car serving various functions, and normally they are very reliable. Elsewhere on the Riviera, thermistors report ambient temperature and intake air temperature - both are vital for correct engine and HVAC operation. The exact part for use with our OEM batteries may no longer be available, however thermistors are readily available in component form.

One source for raw thermistors is Omega Corp. When my intake air sensor failed (from exposure to elements), I did not bother finding an OEM replacement, I made my own from a mil-spec Omega thermistor ($15). The result was vastly decreased (faster) refresh rates for IAT. The write-up is here:

https://rivperformance.editboard.com/t8404-write-up-omega-intake-air-temperature-sensor-iat

For the battery thermistor application, I am not sure of the exact Omega component to use, but it would be easy to figure out with a few simple tools, resistors, and an ohm-meter. In bypassing the OEM thermistor and running a fixed 3k ohm resistor (a common value) in line to the alternator, the resulting charge voltage can be observed and recorded. Using a different value (e.g. 10k ohm) we see the adjusted charge voltage, and can begin to understand the charging curve and range. Choosing an appropriate thermistor to cover that range is now pretty easy.

Another option would be to just choose a fixed resistor that results in a proper charge voltage (14.4v is normal). A fixed resistor can be purchased for $0.25, or a multi-pack can be ordered for a couple dollars. There is a good chance that using a fixed resistor would be a forever fix, as they are extremely durable. Again, I don't know the correct value, but it would be easy to determine from trial & error, or using a trimmer (variable) resistor and ohm-meter.

Your idea to harvest used thermistors of course works well! I feel there will be a day when this option proves more difficult, so hopefully this thread can grow with new information to help Riv owners in the future, when parts are no longer available.

To AA and Lance:

The use of thermistor to modulate charge rate is clever but there's other ways to do it and I bet GM implemented a different strategy such as measuring voltage change over time via the PCM.

If when and as my thermistors quit I'd do what AA did, get a better spec thermistor as component and install. What I learned when rebuilding my HVAC control is that GM did use adequate components in the custom electronics but for $0.02 more could have used better and avoided some of the problems we see. However, that would cost a little more and might not be necessary to get through the warranty period.

Here is a good read on remote voltage sensing for batteries. Thermistor is mentioned in there.

http://www.madelectrical.com/electricaltech/remotevoltagesensing.shtml

The Aurora has one as well. The set point of the voltage regulator is 14.6V @ 77*F on our CS144 alternators. I built a high output alternator recently for my Aurora and audio system. There is a strict formula used for temperature compensation in the industry.

Quote :

Charging Temperature Compensation:

Charging voltages are based on a temperature of 77ºF (25ºC). For average operating temperatures below this range (colder than) the maximum voltage set point should be compensated with an increase at a rate of 0.018 volts / ºF (0.032 volts/º C.) For average operating temperatures above this range (warmer than) the maximum voltage set point should be compensated with a decrease at a rate of 0.018 volts / ºF (0.03 volts/º C.) Example: at 65ºF and 14.2 volt set point Corrected Voltage = 14.2 + ((77-65) x (0.018)) = 14.42 volts

My DIC voltage readout is ~0.2V higher than voltage at both of my batteries. I also run AGM batteries.

What's striking me as remarkable here is that GM went to so much trouble to adjust the charge voltage based on the temperature swing of a battery that is inside the car. The temperature swing Riviera batteries see can't be anything like what a battery under the hood of the car sees, and I wonder if that isn't the real reason why they last longer. I can hardly believe the battery even needs to have the voltage adjusted because the Riv's interior will not be -20 or 120 degrees for very long after start. You're going to get it to 70-80 degrees within minutes. How much charging is the alternator going to do in a few minutes?

Jack the R wrote:

What's striking me as remarkable here is that GM went to so much trouble to adjust the charge voltage based on the temperature swing of a battery that is inside the car.  The temperature swing Riviera batteries see can't be anything like what a battery under the hood of the car sees, and I wonder if that isn't the real reason why they last longer.  I can hardly believe the battery even needs to have the voltage adjusted because the Riv's interior will not be -20 or 120 degrees for very long after start.  You're going to get it to 70-80 degrees within minutes.  How much charging is the alternator going to do in a few minutes?

Battery temp not necessarily same as interior car temp due to heating while charging; have to have some way to detect an prevent thermal runaway if you're going to boost the charge rate.

Think of it this way: of all the Rivs owned by people on this forum what % have had the thermistor crap out at all?

Right.

Not that many.

Enough to give us something to talk about and not enough to make an epidemic.

@AA: It would not surprise me a bit to find that the IAT and battery thermistors are electrically the same.

albertj wrote:

Battery temp not necessarily same as interior car temp due to heating while charging; have to have some way to detect an prevent thermal runaway if you're going to boost the charge rate.  

That's a good point. I've never used a temperature sensor when using a battery charger though. Is the alternator charging at a higher rate than a battery charger?

Jack the R wrote:

albertj wrote:

Battery temp not necessarily same as interior car temp due to heating while charging; have to have some way to detect an prevent thermal runaway if you're going to boost the charge rate.  

That's a good point.  I've never used a temperature sensor when using a battery charger though.  Is the alternator charging at a higher rate than a battery charger?

I don't know, and I don't think it matters because there are other ways to figure out how to goose the charge rate.  Here are some comments that may be of interest:

1) BTW I just bought a new battery charger, a Stanley.  You can see it as Walmart #: 552936580 on the walmart web site.  Surprise, surprise... it works well; it has a reconditioning cycle that uses frequency pulses to break up sulfation and extend battery life; it automatically modulates charge rate - apparently as a function of the state of the charge on the battery as it is charging (it is possible to do that).  My guess is that as the battery charges and the voltage at the terminals goes up, this charger trims the charging amperage downward.  This could be done by switching off the charge and switching in a circuit to measure battery state, using that information to adjust charge rate.  I imagine it done with a microprocessor (think BASIC STAMP not PC) and some switching circuitry.  No idea how to adequately protect that circuit from reverse polarity at that battery charger's price point, though, so I'd be diligent about polarity. Very diligent. Making some level of current available to the battery, for charging, isn't that hard.  

2) An alternator can economically charge a battery at a  MUCH higher rate than a household or small commercial battery charger.  At idle even, you can get 50+ amps out of an alternator depending on how it is set up.  At speed, you can get the rated amperage (80, 100, 140, or what have you).  Suppose you are getting 80 amps at 14 v +/- from the alternator.  You could charge a car battery well within a couple hours if deeply discharged; in minutes (I am thinking 15-20) if it was well-charged but used to start the vehicle (starting is a fairly big hit on a battery).  Using the thermistor arrangement you could easily maximize the charge rate with essentially no guesswork and no switching circuitry to add electrical noise to the car's systems -- important due to the PCM, BCM and other electronics.  

3) Why the thermistor?  Because in practice car charging systems are usually designed to top-off good batteries not deep charge them, and the Riv has a high parasitic drain.  Meaning that in a normal drive cycle it probably needs a pretty good recharge for the battery not just a top-off -- in order to make up not only for the start but also for all the power that got used when the car was just sitting "off." (hint - the Riv is NEVER really all the way Off...)    Also by the way charging the battery so hard that it overheats causes big problems,  they will emit lots of hydrogen gas while they overheat, and if they survive the fast cycling it won't be for long.  Basically the "average" onboard auto electrics are sized to run the accessories on the car and as well reasonably quietly charge the battery. The Riv is above average in that respect and will more quickly charge its battery by boosting the charge rate somewhat, modulated by that thermistor.  

Albertj