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Old 12-31-2008, 05:42 PM
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Erratic DD gages---need some 'splaining done for me

First, I sure miss DocVette -----
Second, let me say that I don't think this is a gage issue at all.
Third, I can usually handle my electrical gremlins.

I have just changed an engine cooling fan switch for the 4th time in the last couple of years. (Erratic on-off signals)
59 EC---454---All new wiring and components---internal regulator--3-wire

Fans on at 229 off at 205 ----- the car cycled and the fans came on at these temps for about an hour while idleing in the drive----then out of the blue-----jumped up to 240 before they would come on and then go off at 220 (I now suspect voltage drop while idleing)

So after dinking with the timing of the engine last night (I did have running warm issues) and taking it for about a 15 mile spin the engine temp stayed right at 195----made me very happy.

This morning----40* out-- on my way to work and the silly car ran at 220 all the way to work----with the heater running. (confused as hell--pulling hair out)

This afternoon driving home from work-----195 all the way home--no heater.

So--I start playing with the heat/AC controls and this is what I find

As soon as the blower motor is turned on---temp goes up 10-15 and oil pressure drops 10-15

This is almost instant.

Now, I have suspected alternator issues for a while.
While at cruise with the headlights on---12.6 on the voltmeter---drops more on high beams---down to 11.? AND if the AC/Heat is turned on--10.6

Alternator will not consistantly provide 14.2 (recently checked and the best it would give is 12.7)

Sometimes at first startup I will get 14+ , but not very often.
(alternator is coming off tonight)

This is a standard 3-wire alt for a 79 Chev C-30/454
This should be plenty for HEI, AC, and a low amp draw fuel pump.
(internal fuel pump for an 86 Bronco--with bypass regulator)

Can someone attempt to explain to me how a voltage drop would affect the readings of the gages.
(less voltage for the resistors play with?)
(lower resistance on the temp gage is higher temps --ie: example
0ohms would be 275* ---- 100ohms may read 78*) I have no idea how the oil pressure sender is calibrated.--------and why is my fuel gage not affected???

Fuel gage remains steady----just the oil and temp that vary with the volt readings

Anyway---now pretty confident that the car was NOT running warm early this morning.
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59 ElCamino 454/TKO 5spd--(daddy)
66 ElCamino 3oh7/TKO 5spd (mousey) looks like a car again--painted 12-12-08----YAY

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Old 01-01-2009, 11:51 AM
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Hmmmm
No takers?????

--------------------------------------------------------------------------------

Update just this morning

Got a new alternator----had battery tested.
Install new alt.
(just fer chits&giggles I created a temporary jumper to sense farther away from the alternator) I have a constant hot fuseblock on each side of the cowl fused for the door solenoids and remote windows, so that I can have access to the electrical in case of remote or fuse failure for the doors.

I hooked the temporary sense wire to this block, with about 8ft of 14ga. This block is technically farther away from the alt. than the fuse box is.

Fire up car----14.8vdc-----dropped to 14.6 after about 3 minutes
(batterey a bit undercharged at this point)

Run the car up to temp----(Found out my choke lead had a blown fuse and would not come off the last high idle step-----maybe that's why I can only get 13mpg)

Remember---cooling fans originally came on at an indicated 229*
and when I operated the heater blower----indicated 240+*
Oil pres went from 40 to 30 with the blower on.

(I neglected to mention that the morning I took the car to work, and thought it was running warm---I also felt a "lean surge" sensation that I did not get later in the day without the headlights or heater running---I now assume it was the HEI cutting out due to a voltage drop)

NOW----fans kick on at an indicated 220----off at 200
oil pressure is also reading 45 at 2100rpm----warm engine

Volt gage reading 14.6

This part made me happy-----
turn on headlights----High beam
turn on AC---fan high
Engine cooling fans will come on with AC high pressure
Turn on stereo--------actually sounds much better now too
Could not get the gage to go lower than 14.2vdc

So now I remove the temporary sense wire---(not to keen on running new wires just yet )
Volt gage 14.1
with lights, AC, stereo, cooling fans running
Volt gage 13.7
@ 750rpm

This is all much better than the 2-3vdc drop I was getting with the other alternator----that actually tested good.


Gonna go on a test flight here soon to see if it will maintain these readings.
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59 ElCamino 454/TKO 5spd--(daddy)
66 ElCamino 3oh7/TKO 5spd (mousey) looks like a car again--painted 12-12-08----YAY
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Old 01-01-2009, 11:53 AM
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Of course----THIS question remains



Can someone attempt to explain to me how a voltage drop would affect the readings of the gages.
(less voltage for the resistors play with?)
(lower resistance on the temp gage is higher temps --ie: example
0ohms would be 275* ---- 100ohms may read 78*) I have no idea how the oil pressure sender is calibrated.--------and why is my fuel gage not affected???

Fuel gage remains steady----just the oil and temp that vary with the volt readings
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Old 01-01-2009, 12:27 PM
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A voltage drop will affect the sensors/gauges, but they should be designed to operate accurately within the range of about 11.5-14.5 volts. You might want to check the ground wire up under the dash, which is connected to the top of the emergency brake pedal mechanism. Maybe there is an intermittant ground problem with the gauge cluster or the engine compartment.

Also, the temp and oil gauges are grounded through the engine, while the fuel gauge is grounded directly to the frame. This might be a clue.

My '75 truck has a stock mechanical oil pressure gauge and electrical water temp gauge, and I've added a mechanical temp gauge. The stock electric temp gauge does not have a very linear response, compared to the mechanical gauge. At 195 (measured with mechanical gauge) the electric gauge is only at 1/4 scale (no numbers on the gauge) and at about 210 degrees it hits mid scale. I've seen the temp up to about 240 and it was still only at about 3/4 scale.

Bruce
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Old 01-01-2009, 02:22 PM
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Well----test flight went good
get to cruising at 70+ in 4th (3k+rpm)

engine temp stays @ 180
Oil pressure up to 60

Volt gage reads steady 14.2

Drops to 13.8 with ALL accessories going
(lights on high-AC-cooling fans-stereo-fuel pump)

Could be all this time I was fighting an overheating issue that NEVER existed.
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