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Discussion Starter · #1 ·
1995 Mustang 3.8 L V6 Manual 5 speed. Car starts, runs perfectly fine, no functional problem symptoms.

Interesting problem, apparently electrical, likely power/ground connection related. Suddenly while driving one evening, the speedometer stopped responding, then came back, (has operated normally ever since), Temp gauge became erratic, bouncing up and down, generally settling to a higher than normal reading, the speed of the Temp gauge excursions is such that it is impossible that the actual temperature could vary at that rate, physics being what it is... The temp sensor (on top of engine near thermostat) as well as the thermostat is new since this spring... wiring seems visibly intact to the sensor. After a day or so, the Service Engine light comes on. I do not have a code reader, but I expect it to be temp related.

Points of interest:
(1) increasing electrical load (turning on blower full, rear window defogger on) causes Temp gauge to move two letters higher, i.e. from A in NORMAL to hot line, back again when heavy load removed...
(2) turning ignition to start/crank position while running generally resets the Temp gauge to a more reasonable expected point, but this does not last, drift returns...
(3) when the ignition is switched off, the Temp gauge pegs clockwise to the 3 O'clock position (needle horizontal), a "feature" I do not recall seeing before.

I have rebuilt/tested the ignition switch, checked fuses in the engine compartment box, checked/cleaned battery terminals, ground connection from battery to engine, all fine. I have disconnected, disassembled/checked/reassembled the ECU, looks fine, no cold solder joints etc. I have removed/cleaned/tested/reassembled all instrument cluster connections, all OK.

I am in the process of measuring the actual engine temp during operation with an external sensor. This will tell me if engine temp is as it should be... (My assumption is that there is no physical temp problem)

Sadly, I have only a Haynes manual for this car. The electrical schematics are "generic" (generally useless, more block diagrams than schematics...) so I cannot trace wiring, the next logical step, in any efficient manner.

To anyone having any thoughts in this area, or any similar experience, your input would be appreciated. Also, anyone having elctrical wiring diagrams, Email scans/postings of these would be awesome...

Thanks, Ian
 

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whimorris said:
1995 Mustang 3.8 L V6 Manual 5 speed. Car starts, runs perfectly fine, no functional problem symptoms.

Interesting problem, apparently electrical, likely power/ground connection related. Suddenly while driving one evening, the speedometer stopped responding, then came back, (has operated normally ever since), Temp gauge became erratic, bouncing up and down, generally settling to a higher than normal reading, the speed of the Temp gauge excursions is such that it is impossible that the actual temperature could vary at that rate, physics being what it is... The temp sensor (on top of engine near thermostat) as well as the thermostat is new since this spring... wiring seems visibly intact to the sensor. After a day or so, the Service Engine light comes on. I do not have a code reader, but I expect it to be temp related.

Points of interest:
(1) increasing electrical load (turning on blower full, rear window defogger on) causes Temp gauge to move two letters higher, i.e. from A in NORMAL to hot line, back again when heavy load removed...
(2) turning ignition to start/crank position while running generally resets the Temp gauge to a more reasonable expected point, but this does not last, drift returns...
(3) when the ignition is switched off, the Temp gauge pegs clockwise to the 3 O'clock position (needle horizontal), a "feature" I do not recall seeing before.

I have rebuilt/tested the ignition switch, checked fuses in the engine compartment box, checked/cleaned battery terminals, ground connection from battery to engine, all fine. I have disconnected, disassembled/checked/reassembled the ECU, looks fine, no cold solder joints etc. I have removed/cleaned/tested/reassembled all instrument cluster connections, all OK.

I am in the process of measuring the actual engine temp during operation with an external sensor. This will tell me if engine temp is as it should be... (My assumption is that there is no physical temp problem)

Sadly, I have only a Haynes manual for this car. The electrical schematics are "generic" (generally useless, more block diagrams than schematics...) so I cannot trace wiring, the next logical step, in any efficient manner.

To anyone having any thoughts in this area, or any similar experience, your input would be appreciated. Also, anyone having elctrical wiring diagrams, Email scans/postings of these would be awesome...

Thanks, Ian

you might send mustangtk a pm. He's a ford tech.
 

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my buddys 95 gt acted like that once but his radio even cut out. in his case it turned out to be the alt/voltage regulator...it was sending ether tomuch or not enuff current to the dash
 

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Discussion Starter · #4 ·
Problem solved

Turns out it was bad grounding from battery to body, several places. Gauge operation has returned to normal.

Now I find I have a failed O2 sensor "behind" the CAT (dealer's description ???)... I had to pay $105 CDN to discover this... (Local costs for an OBD II code reader average about $250 CDN) but the odd thing is, this failed O2 sensor does not seem to affect the car's performance at all, so I am going to leave it until I can scrape up more $$.

Anyone have any opinions on the wisdom of leaving the failed O2 sensor repair for a couple of months, assuming no performance degradation and realizing that I may burn out the CEL bulb...?
 

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whimorris said:
Turns out it was bad grounding from battery to body, several places. Gauge operation has returned to normal.

Now I find I have a failed O2 sensor "behind" the CAT (dealer's description ???)... I had to pay $105 CDN to discover this... (Local costs for an OBD II code reader average about $250 CDN) but the odd thing is, this failed O2 sensor does not seem to affect the car's performance at all, so I am going to leave it until I can scrape up more $$.

Anyone have any opinions on the wisdom of leaving the failed O2 sensor repair for a couple of months, assuming no performance degradation and realizing that I may burn out the CEL bulb...?
I wouldn't worry about it. honda disconnected their O-sensors for years, to bump their reliability ratings.

The only problem is, if a real problem comes up, you won't know it, -but keep an eye on your guages, and your ears open, and you should be fine.

I drove a company car with the o-sensor cel light on for years.
 

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Discussion Starter · #6 ·
Thanks

Thanks kscoyote, I appreciate the feedback.

I do intend to replace the silly thing, but I must get financially through the holidays - no mean feat... just finished putting tires on 3 vehicles including the Mustang, brakes on 2 of them... exhaust system on one...

Good thing I love Cars and Bikes :happyhapp

Have a pleasant holiday season,

Ian
 
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