temp sending unit swap 200 - 250

[t-west]

Hello all,

Knowing how our beloved Ford likes to swap parts -- are the temperature sending units the same across the head years? I've put on a D8 head, and hooked up the sending unit that was on there. However, now my readings syrocket after about 10 min. on the road. Today when I pulled into my garage, I immediately plugged an instant read thermometer into the radiator neck, and voila, 180* exactly.

I know that this could be:

• sending unit
  voltage thing behind dash
  gauge itself,

But I'm wondering if I might be able to plug in my old (C6) sending unit just to see. Are they different?

thanks,
--tom

 

[LameHoof65]

For sure---I don't really know, but I do have about five different heads all from different blocks but two are of of Fairmont '77 model cars. All the temperature sending units looked the same to me. I wouldn't think it would make any difference if you could seat it and plug it in, they should all accurately read your temp.

 

[48fordnut]

Sending units for lights differ from ones from gauges.

 

[63redtudor]

I have a couple different heads lying around & it looks like there's 2 different sizes of sending units. Seems as though the older heads (up to about '63) have a larger size then the later ones ('65+). The older heads are about a 1/4" pipe thread & the newer ones were about 1/8" or so. If you want, I can dig through the heads & see for sure. I don't think (someone will prove me wrong!) that after '65 or so that ford changed the hole size though.
Edwin

 

[t-west]

Is size all that matters?

I can measure the hole on my new head and see, but I was sort of wondering if they read their values differently. For example, the gauge from a 78 Fairmont is likely different from my 66 gauge, and if the gauges are different, I thought that the sending unit might also be different.

Any thoughts?

--tom

 

[MustangSix]

If it's getting nuts after 10 minutes, it may be the instrument voltage regulator sticking on you. Is the fuel gauge doing odd things too?

 

[t-west]

Everything else seems fine except the ammeter. It doesn't go down when the car is off. It does, however, move slightly as I drive it, deviating little from the center.

Does this sound like the voltage regulator?

--tom

 

[t-west]

So I went to the local auto parts store, which has been really helpful and knowledgable so far, looking for a new temp sensor for my D8 head (C6 block).

D8's only have lights, not gauges? Could this be the cause of my funky readings? Anyone solve this problem with their own head?

thanks,
--tom

 

[48fordnut]

that was what I posted earlier. The sending units are different for gauges and lights.

 

[t-west]

So what are out options?

• ignoring it
  drilling and tapping the t-stat housing for aftermarket gauge
  finding a car w/ a 250 and a gauge (anyone?)
  fitting an adapter for the smaller 200 size sending unit to the larger 250 hole

Any other advice?

--tom

 

[MustangSix]

It didn't dawn on me until the last couple of posts, but if the original car had a light, that's exactly your problem. When the sender heats up it closes and causes a direct short across the gauge sending it off the scale. You need a sending unti for a gauge. The threaded hole in the head should be the same size for all years, at least all I've seen.

 

[powerband]

Just yesterday put the "worked" D7 250 'Stang head back on the C8 170 block in a'61 Comet. All three sending units (61,71 & 77) were the same thread size but the D7 sending unit just sat at the bottom of the guage. I drained the coolant and put the older sending unit from the '61 back in to match the 61's guages and it seems to track the mechanical guage I also use. I believe the '77 Mustang had a temp guage but it probably is a different sending unit than the '61.


I had to put the mechanical guage bulb in the heater pipe bung on the front of the head after learning that you can't tee them off the original like you can an oil pressur guage. Something about thermal dead ends.

Powerband

 

[t-west]

Yup, the old one fits and solves the problem. It's just that it looks so much bigger (7/8 bolt instead of 9/16), but the shaft is the same diameter and bolt pattern.

hasn't gotten above the 1/2 way point yet, and I'm pushing it!

best,
--tom