Temp Gauge Reads Higher with Headlights On

[Cardinal Grammeter]

I have a 195* thermostat and driving at night, the temp gauge needle is right at the max of the Operating Range interval on the gauge.  The difference between this and actual overheat is 1-2 needle's widths.  In the daytime, it's about at the 75% point in the operating range interval.

 

I know people talk a lot about the cluster mechanical voltage regulator and that would be a definitely Person of Interest.

 

Is this a common issue?  and if yes, how to fix?

 

Thanks

Tom

[xUltra510]

Following. 

[KoHeartsGPA]

Get an aftermarket temp gauge and save yourself from the guess work...Get and oil pressure and voltimeter too.

 

[Lockleaf]

^^^ that isn't a repair.

[DanielC]

Wacky electrical problems are most often caused by bad grounds.

[Cardinal Grammeter]

I don't deny careful study of the wiring diagram would probably reveal how headlights could effect temps.

 

Hell, I want to put LED's in my dash, that might even help things.

 

I'm not 100% sure, but it seems like headlights and NOT the high fan, affect the temp.

 

For now, I'll be happy if it doesn't overheat.  I sort of discovered you need to let the engine "burp" 2-3 times before you can put the cap on and run it.

[distributorguy]

I'd speculate that your engine ground, alternator ground, and headlight ground are not tied together.  That would be an easy repair to try first.  Grounding the gauge on its own could help too.  

[datzenmike]

The running voltage can vary from >13 to 14.5 volts depending on the ability of the alternator, how high it's revving (or idling) and the load on the system (headlamps/running lamps/brakes/heater all on) To prevent the battery voltage from varying the meters it has a regulated supply of about 8 volts. As the running voltage never goes this low the gauge readings for gas and heat remain even.

 

As the gauge does not have numbers you are assuming that the top of the run range is over heating. There is a reason they don't put numbers on the gauges. People interpret them as over heating and are more comfortable if it is within a 'running range'

 

First thing I recommend when there is a possible temperature 'problem' is change the thermostat. Summer's coming and 195F is too high for an L series any time of year, try a 180F. Spend at least $10 on a good one not a $2.99 one from WallMart.

 

Second, get or borrow an IR gun and 'shoot' the thermostat housing and see what the temperature really is. But change the thermostat to 180F first.

[KoHeartsGPA]

Get a failsafe thermostat.

[DanielC]

"I don't deny careful study of the wiring diagram would probably reveal how headlights could effect temps."

It should not.

 

About the only wires used in common with the headlights and the instrument cluster on almost any car or truck are the ground wires.

[datzenmike]

Thermostat is too high a heat range. Lower it to 180F and if it goes up two needle widths you won't worry as much as it will be in the middle of the run range. 

[distributorguy]

How to fix the voltage variation?  Read the variation at the gauge with a meter and report back.  It may or may not be electrical.  It sounds electrical, but may not be.  It may just be a randomly sticking thermostat, unless the gauge jumps religiously when you move the headlight switch.  

I don't know why people like 195 degree stats.  Late model cars need them in many cases, carbureted cars (and trucks) don't.  Ever.  

 

By the way, this is a common review for failsafe thermostats, cut and pasted:

1.0 out of 5 starsThese thermostats are a nightmare. Do NOT buy one. I chased an over temp issue for 18 months on my wife's car..

By Bobbi Dunne on July 20, 2013

replacing everything to try and get the temp to normal. I never went back to the thermostat because it was "new" and mentally ruled that out as the root cause. It wasn't until it completely failed and I replaced it, I was shocked to see my temps were rock solid.

A great idea but they don't work. Do a google search and you'll see I'm not the only one. Unfortunately, I didn't do that until AFTER it was fixed.

Bob

[Cardinal Grammeter]

Some updates:

 

195* THERMOSTAT CHOICE:

 

I was concerned about alum head on iron block.  So i check parts man and they did have a 190* stat listed.  I believe Beck Arnley makes one.  So I figured I could sqeak by with a 195. 

 

....which might have been a Fools Errand:  I was thinking in the winter, I want more cabin heat - but just thinking about it, if it is really cold and I turn the heater on, it's probably going to pull down the coolant temp below 195 anyway.

 

Would a 195 yield a warmer cabin in the winter?

 

 

TEMP GAUGE VARIATION SPECIFICS:

 

First some definition on what the Temp Gauge is indicating:

 

The Gauge has 4 Graduations as follows:  Call them Graduations 1, 2, 3, and 4.

 

LO  |  |______|  |  HI         I call the Range the 2 "marks" connected by the horizontal line.

 

With the Headlights on, the need lays right on top Grad #3. 

 

Yes, Headlights On increases Temp Reading by "2 needle's widths."

 

Further, its NOT the Headlights per se  since the Parking Lights cause it too.  Also, the fan on high does NOT affect Temp.  These are all completely repeatable.

 

I believe this means it's related to the cluster lights current draw.  I would bet when I replace lamps with LED's, this problem will go away. 

 

 

NEW WATER PUMP JOYS:

 

The old pump bear was bad - you could hear it grinding away 1/2 a block away.  When out, the radial clearance at the impeller was about 1/16" peak -to-peak.  (!)  wow.

 

Anyhow, with a new pump, I swear I picked up 5HP!  Seriously!  I can now make it up this local hill in 4th instead of 3rd.

 

I highly recommend ball bearings (!)  HaHa

[DanielC]

I have had Datsun 520, and 521 pickups since about 1973.

In my experience, Datsuns like to have the hotter thermostat.  This comes from somebody who used to tow a two horse trailer with a 521.  Trust me, you have to do something really extraordinary to get a Datsun with a good cooling system to overheat.

 

Light, and ignition power get separated from each other pretty close to the positive battery terminal.  Ignition power is used to operate the gauges.  Additionally, the positive feed to the gauges has a second gauge voltage regulator, after the ignition power gets tapped off.  Almost all cars have a gauge voltage regulator.  They do this so minor variations in the positive voltage do not affect the gauges.

 

Because of the gauge voltage regulator, the switching on and off of headlights, heater, or wipers should not affect the gauge reading at all.

 

That is why I think you have a grounding problem.

[Cardinal Grammeter]

Since heater and wipers do not affect the Temp reading, it is not a "alternator load" issue.

 

I thought the headlights were a load issue, but since parking lights also do it, then it's not a load issue.

 

So what do head and parking lights have in common?  Dash cluster lights.

 

My guess is the more current draw from the cluster including gauges, the lower the regulated voltage.

 

Since the FSM does not state if the temp gauge resistance has a positive or negative coefficient, it's not possible to speculate.

[datzenmike]

First thing I recommend when there is a possible temperature 'problem' is change the thermostat. Summer's coming and 195F is too high for an L series any time of year, try a 180F. Spend at least $10 on a good one not a $2.99 one from WallMart.

 

 

Now it will run lower and if it moves up slightly with or without the lights on it won't look so bad. This is easy fix, stop over thinking it.

 

My vehicles run just a hair (needle width) below half way on the 'run range' all the time, and the highest is a hair over half at highway speeds in the hot of summer. My 620 was a needle lower and hit half way in the summer. Probably the larger 3 core rad cooling system.

[DanielC]

Back to electrical grounding issues.  How it the instrument cluster grounded?   On a 521, the instrument cluster has a dedicated ground wire.  The instrument cluster can be held in your hands, not touching anything on the truck, and work normally. 

The instrument cluster has to have a good ground reference voltage to the engine.  If the ground connection to the instrument cluster is a little dodgy, the increased current flow from the instrument lights going through a poor instrument cluster ground will change the ground reference voltage compared to the engine.  If the instrument voltage regulator uses this instrument ground reference voltage, it will not know the difference from the engine ground voltage.

 

Do you know how to do a voltage drop test? 

 

The alternator, the battery, and the (alternator) voltage regulator have to have good grounds to each other.  To insure this, on every 521 I have seen, that has not had the wiring hacked a lot, the battery negative cable connects to the cylinder head, by the fuel pump.  This cable has a pigtail crimped onto it, that goes down to, and grounds the alternator frame.  On the alternator, there is a second wire, also connected to the alternator frame, this wire goes to the alternator voltage regulator, and at the alternator voltage regulator, there is another ground wire that goes into the wiring harness, that supplies a ground for the headlights, and I suspect the other ground connections made with the wiring harness.

 

A good engine ground comes into play, because the temperature sender grounds to the engine.  It is a variable resistor, that has less resistance when hot.   The temperature gauge is just a bimetal strip, heated by a resistance wire, connected in series with the temperature sender.  The instrument voltage regulator supplies a reference voltage to both the gas, and temperature gauges. 

When the engine is cold, the temperature sender has high resistance.  Little current flows in the temperature bimetal heater, it stays cold, the needle does not move.  As the engine heats up, the temperature sender allows more current to flow through the temperature gauge, it get hot, and moves the needle higher.  A quick way to test the temperature (and gas) gauge is to ground the hot sender wire to each gauge, at the sender.  Doing this should cause each gauge to go full scale, depending on what sender was grounded.

[Cardinal Grammeter]

UPDATE: 

 

FUEL GAUGE:  It also goes up about 2 needle's widths when the Head or Parking Lights are on. 

 

GROUNDS:

• Neg Bat terminal:  bat cable goes to fuel pump
• Neg Bat terminal:  braided ground strap to radiator mounting bolt
• Alternator ground wire from loom
• Regulator has no apparent ground
• Cluster:  have no idea where it is.

 

CLUSTER LIGHTS Primary Suspect:

• Pin #2 of the Cluster Connector goes to ground.
• All Cluster Lights AND the VR ground through Pin #2.
• The VR is actually a current regulator.
• If the lamps are on and there is resistance from Pin #2 to ground, there will be a dV at that point when the lamps are on.
• Hence the Regulated 8V has been increased.
• Since the gauges are directly grounded, they do not see that dV at their ground.
• THEREFORE, there is greater voltage applied to the gauges and their values increase.

CONCLUSION:  There is a connector or ground issue at PIN #2.

 

OVERHEATING was probably due to failure to BURP system when filled. 

 

I discovered this time around since I let the engine idle w/cap off to be sure everything was OK.  What appears to happen is trapped air causes a hot spot that boils just enough water to push a big slug of steam into the radiator.  When this happens the coolant level goes up quickly and probably a quart's worth.  When the bubble clears the cap, the level is below the top tank.  Add water and let it happen again.  It will do this 2 or 3 times, then it is stable. 

 

I think what happened when I took it on the highway was that it burped which blew a lot of water out the overflow.  The second time I did this, I was able to get the cap off to see the radiator full of water AND boiling noises coming from the block.

 

When it was doing this, the Temp would reach the last graduation with the 195 stat, head/parking lights on.

 

So far, it has behaved but I have not given it a long run on the highway yet.

 

195* STAT Thoughts: 

 

What does suck is that genuine overheating is about 3-4 NM (needle widths) beyond the running range.  So with the 2 NW issue, there isn't easy to know where you are on the Temp Gauge.  I even think the TG may be non-linear with more sensitivity in the running range (2nd to 3rd grad.)

 

Parts manual calls for a optional 190 and I know for certain that that would lower the reading 1-2 NW.

 

I may put the 180 back in.  But I need someone to tell me, Will the 195 will make the cabin warmer in the winter?  I'm guess engine won't make enough heat to get the whole system up to 195 during the winter, so it's a moot point????

 

CONCLUSION:

 

• I think things are working OK now, but I'm going to have to make a decision about the 195 stat.
• R&Ring the Cluster Connector might fix the Temp/Fuel Gauge problem - other wise Ground issue.
• Installing Cluster LED's will probably solve issue also.

____________________

 

 

RE:  LEDs:  If you replace all turn signal lamps with LED's, I'm guessing you have to get a LED compatible flasher?

[distributorguy]

I always use 180 degree stats and drill 2 holes in them to allow air to pass through for burping.  Holes 180 degrees apart to keep flow "balanced".  Side loading the stat with 1 hole can cause it to bind and stick open or closed over time.  No holes = trapped air.  

[datzenmike]

Yes, it will run warmer in the winter and really warm in the spring, too warm. A 180F will do if you place a piece of cardboard over the lower half of the rad in the extreme cold of winter but keep an eye on the temp gauge if on the highway.

 

Check your heater core for leaves and crap blown in from the plenum. Make sure the foam rubber that seals around the core is there.Without it cold air will simply go around the core instead of through it. Check the plenum where the wiper linkages are for leaf blockage of the air inlet to the heater. Check the hot/cold mix gate is closing fully and you are getting full hot air without any cold mixed in. Check that the HOT position on the heater lever is actually opening the hot water valve fully. Save the coolant, set heater to HOT and remove both heater hoses from the engine side. Get a garden hose and force water through the heater core until it runs clean, them reverse the hoses and flush backwards till clear. Repeat as necessary to flush the core clear of sediment. Patch any holes in the cabin floor. Replace any damaged or rotten rubber door seals. Take the door handles, arm rests and the door card off and make sure the plastic skin is sealing between the door metal and the door card. Use packing tape to seal around the door handle holes and any tears before putting the cards back on. Cover the floor in the winter with cheap carpet you can throw away.  

 

My '78 FSM shows a 190F thermostat used in frigid areas, 180 as the standard and 170 for tropical. I don't think Philadelphia  qualifies as 'frigid'. Run a 180 thermostat.

 

Your thermostat housing should have a by pass hose (about the size of your little finger) going to a metal line that goes down to the lower rad hose inlet. Make sure if this hose was replaced that it is not kinked from a tight bend. Without this by pass, the water around the thermostat stays cold while the engine over heats. The purpose of the by pass is to flush this cool water out with water from the warming block and head. The temp gauge will read the actual temperature of the engine sooner and the thermostat respond quicker.

[KoHeartsGPA]

And every time you mess with things that will cause the coolant level to drop, it's best to make her burp couple times to bleed the system :thumbup:

[Cardinal Grammeter]

So far so good.

 

PIN #2:

 

I think it's possible to R&R the cluster connector reaching up under the dash.

 

Does anyone know where the ground for PIN #2 is?

 

TSTAT:

 

I'm going to put the 180 back in it.

 

BYPASS:

 

My bypass hose goes

FROM from the cluster of nipples by the lower radiator hose at the block

TO a home made  T-fitting just under the tstat,

then

FROM the other end of the T-fitting

TO the intake manifold small fitting on the forward face of the plenum under the carb.

 

HEATER BOX, CORE, ETC:

 

I had it totally out and checked everything.  Core was pristine clean.  I made sure I had new foam gasket at the cowl snorkel.  The rest of the foam for the flapper was intact but I did not replace it.  I have to check it out again because I know the fan blows strong, but there doesn't seem to be a lot of flow coming out the defroster nozzles and not much out the heater either.  I'm pretty sure I have all the mechanical controls correct.OE is a steel tube with a Y (one to tstat and other to intake manifold) if I understand that correctly.

[Cardinal Grammeter]

UPDATE;  They say nothing is better for a car that to drive it regularly.

 

The high temp and drop with parking lights appears to have "gone away."

 

I drove it for 15 min to heat it up so I could flush/drain the cooling system again, and after about 10 minutes, the temp gauge started operating just a needle's width above the middle of the tamp range graduations - and the headlights did not appear to effect the reading however I didn't have enough time left on the drive to let the readings stabilize.

 

I suspect that whatever loose connection I had, like Pin #2 of the cluster connector or the associated ground, some aggressive windy back road driving might have vibrated through the oxide.

 

Also, when I removed the block drain plug, there was weird, crusty, black, oily chunks stuck on the end of the plug.  They reminded me of carbon deposits on the back of a valve, or crud in an EGR passage.  They left my fingers oily black.  Wonder what happened?  if some small clogged passage or hole cleared out? 

 

Of could the crud clearing be responsible for the lower temp reading???

 

I have no idea.  But I'm thinking it is all good.

[Cardinal Grammeter]

UPDATE:  I never solved the Higher Temp Gauge Reading with Headlights or Parking Lights On Problem.

 

I had a theory that the "dash lights" were actually causing the problem within the Combination Meter some posts back (can't find the post #'s in this thread).

 

Quote

 

CLUSTER LIGHTS Primary Suspect:

• Pin #2 of the Cluster Connector goes to ground.
• All Cluster Lights AND the VR ground through Pin #2.
• The VR is actually a current regulator.
• If the lamps are on and there is resistance from Pin #2 to ground, there will be a dV at that point when the lamps are on.
• Hence the Regulated 8V has been increased.
• Since the gauges are directly grounded, they do not see that dV at their ground.
• THEREFORE, there is greater voltage applied to the gauges and their values increase.

CONCLUSION:  There is a connector or ground issue at PIN #2.

 

 

Since then I've redone the grounds in the engine compartment and have excellent resistance measurements of <0.1 ohms (the limit of my meter.)

 

As far as the CM ground @ pin #2, if you go to the CM section with just the CM schematic, it looks like there is a ground nearby.  But if you look at the full wiring diagram, #2 is a black that goes to the Instrument Harness which plugs into the Engine Room Harness.  AND there is the round 12-pin connector to the CM itself.  

 

Perhaps a ground should be tapped from inside the CM and taken to a good body ground or the Neg Bat Term???

Edited July 24, 2018 by Cardinal Grammeter