Topic: temperature gauge reading

[sflaten]

69 Mach 1 351W 4v is reading as though the engine was hot and the dash temp needle is up on H. I am on my third new sending unit thinking that there might have been variations in manufacture. When needle showed hot I checked thermostat housing with Infrared heat gun and the temp showed about 180-190 F. I just rebuilt engine and put in a 180 thermostat of the fail open type as extra protection. I was having issue before rebuilding engine Checking archive others have seemed to similar issue, but no good resolution. Like others I have replaced the  instrument voltage regulator on rear of instrument cluster. I feel it is not running hot, but would like the security of  having a gauge to trust, especially since it is a new motor. Any suggestions? TIA

[Bob Gaines]

69 Mach 1 351W 4v is reading as though the engine was hot and the dash temp needle is up on H. I am on my third new sending unit thinking that there might have been variations in manufacture. When needle showed hot I checked thermostat housing with Infrared heat gun and the temp showed about 180-190 F. I just rebuilt engine and put in a 180 thermostat of the fail open type as extra protection. I was having issue before rebuilding engine Checking archive others have seemed to similar issue, but no good resolution. Like others I have replaced the  instrument voltage regulator on rear of instrument cluster. I feel it is not running hot, but would like the security of  having a gauge to trust, especially since it is a new motor. Any suggestions? TIA

Aftermarket temp senders are notorious for giving erroneous reading. Get a genuine Ford temp sender.New or used.  The one with the green insulator is for the 69 351W.

[Stangman68]

I have a few good originals if anyone is looking? 

[sflaten]

I ended up ordering a new one from OEM Ford parts thru CJ Pony. I tried to buy thru dealer and they said they could not look up anything in their computer prior to 1980. We will see how it works out.

[Bob Gaines]

I ended up ordering a new one from OEM Ford parts thru CJ Pony. I tried to buy thru dealer and they said they could not look up anything in their computer prior to 1980. We will see how it works out.

Be prepared to double check results with a IR temp gauge. I hope I am wrong but I think you would have had a more correct reading sender if going the good used route based on others observations.

[sflaten]

Bob,
Thanks for the suggestion on the OEM sensor. Put it in and ran the car up to temp and the gauge read right in the middle band on the gauge.  Just for extra precaution, I changed the thermostat to a fail open so it would not overheat when I rebuilt the engine.

[7Lscjracer]

Common issue with any brand of classic vehicle.
The problem is that the resistance curve of a sender that supersedes the previous one oftentimes doesn't match, and the difference from the first version of the sender typically becomes more drastic with each supersession.

[krelboyne]

I found something today.
Wiring harness assembly C9ZB-14K064-A. MPC calls it "temperature sender to indicator", it has a 2 ohm resistance wire. Specific to the 1969 Mustang and Cougar with 351 engines about 9-1/2 inches long. This is basically a jumper wire from the under dash harness plug through the cowl, to the engine gauge feed harness. I haven't found it on the 1969 wiring schematics. This harness is specific to the 1969 F body cars with 351 engine, per both the Ford and Mercury MPC's.
Maybe one of our electrical gurus can help with the explanation of the 2 ohm resistance wire?
Designed to work specifically with the 1969 OEM 351W sending unit?
Remove for use with replacement sending units?

[midlife]

I've seen a couple of those and maybe have one or two in my stash.  Always figured it was a simple extension needed for a particular engine.  Note that there are 3 wires, not necessarily a temperature sending line extension.  Never realized that there was a 2 ohm resistor in there.  Any idea what color wire it is?  If it is the red/green coil wire, then the added resistance is for protection of the points and coil over the standard 13-17 ohm pink resistor wire.  Why only the 351 and only 1969 is puzzling. If it is the red/white line (temperature), then there was probably a problem with the particular sensor in the 351 engine for 1969, which was superseded in 70, perhaps? 

[Bob Gaines]

I've seen a couple of those and maybe have one or two in my stash.  Always figured it was a simple extension needed for a particular engine.  Note that there are 3 wires, not necessarily a temperature sending line extension.  Never realized that there was a 2 ohm resistor in there.  Any idea what color wire it is?  If it is the red/green coil wire, then the added resistance is for protection of the points and coil over the standard 13-17 ohm pink resistor wire.  Why only the 351 and only 1969 is puzzling. If it is the red/white line (temperature), then there was probably a problem with the particular sensor in the 351 engine for 1969, which was superseded in 70, perhaps?

I don't know if this has any relation but the 69 351W temp sender has the green insulator and the big block the red. Other years used the red insulator regardless of if BB or SB. The extension is not needed because of lack of reach so it was always puzzeling to me.  Many times it is left off by restorers because it seemed to be not needed.

[midlife]

I think you may be on to something here...Although 2 ohms difference in a temperature sending unit (13 to 73 ohms) seems somewhat in the noise, unless it was put in to ensure the gauge didn't peg quite so high when full (13 ohms).