Howdy & Help with fan problem

[cntryby]

First let me say howdy, just found this site while trying to find info on my Dads 95 Roadmaster. He's 78 years old so I need this thing to function like it's suppose to make sure he gets from point A to B and back again safely.

We just had to put heads on his RM and now the fans don't work. (Which may be why we had to put heads on it.)

Anyway...

The relays work, I'm just not getting a return path to ground from the ecm.

Meaning, in the fuse box I have 12V for both the fan contact & the coil of the relay, if I supply a path to ground the relay actuates and the fans come on.

The gauge works, but the temp light never comes on even when the car gets hot. (maybe the same bad ground?) If I unplug the temp sensor for the ecm & light, (sensor at water pump) the engine will cut off.

Before I start digging to look for the ground for the ecm, or is there something I'm missing that needs to be checked first?

I unplugged the MAF restarted & both fans did come on... ?

Also, if I unplug the temp sensor at the water pump the fans will come one, but it is hard to start the engine. If I jump across the plug terminals on the temp sensor, it set the service engine light, & both fans run, but the temp light still doesn't come on.

Thanks in advance for any help you may provide.

[HotZ28]

Probably the temp sensor is defunct! Evidently the PCM is not supplying a ground for the relay to energize. The PCM needs correct voltage feedback from the temp sensor for this to happen.

[cntryby]

I've learned that I can display the actual temps of the sensor at the water pump on the ACC display. The following is what I've found.

Both sensors (water pump and head) are new, T-stat & water pump are new as well. We have bled the coolant system several times.

The fans don't come on until the acc display for 08 reaches 195. The 24 reading is 107 (224f). The gauge almost touches the red before the fans come on, and the temp light never comes on.

The 30 degree difference concerns me, but not as much as the temp being so high before the fan comes on. However, 195f isn't to bad if that is the true temp. But which one is correct? Either one still concerns me. From what I've read, 210f should be where the primary comes on & at 220 the secondary comes on. My secondary never comes on because as soon as my primary comes on the temp drops.

Both fans will run though if I unplug the MAF sensor prior to starting.

Looking at the gauge almost touching the red before the fan comes on is freaking me out, after all the work I just did putting these heads on I don't wanna screw it up.

Any thoughts / suggestions?

[HotZ28]

You need to check engine temp with a scanner to verify what your display is showing. If you get one fan @ 195 deg & it cools down, don't worry about it! The red light is set for 250 deg! BTW, if you turn the A/C on, does one fan come on at any temp?

[HotZ28]

You need to check engine temp with a scanner to verify what your display is showing. If you get one fan @ 195 deg & it cools down, don't worry about it! The red light is set for 250 deg! BTW, if you turn the A/C on, does one fan come on at any temp?

[cntryby]

The fans aren’t coming on until the temp gauge is right at the edge of the red.

Both sensors (water pump and head) are new, T-stat & water pump are new as well. We have bled the coolant system several times.

The fans don't come on until the ACC display for 08 reaches 195f. The 24 reading is 107c (224f). The gauge almost touches the red before the fans come on, and the temp light never comes on.

The 30 degree difference concerns me, but not as much as the temp being so high before the fan comes on. However, 195f isn't to bad if that is the true temp. But which one is correct? Either one still concerns me. From what I've read, 210f should be where the primary comes on & at 220 the secondary comes on. My secondary never comes on because as soon as my primary comes on the temp drops.

Both fans will run though if I unplug the MAF sensor prior to starting. The same thing happens if I unplug the temp sensor at the water pump prior to starting.

Looking at the gauge almost touching the red before the fan comes on is freaking me out, after all the work I just did putting these heads, on I don't wanna screw it up.

I can see the flow in the coolant bottle move when the T-stat opens.

My understanding is the sensor in the head controls the gauge in the dash only. The sensor at the water pump is used in conjunction with the MAF to control fuel air ratio, and to engage the fans.

Both 08 & 24 readouts are changed if I unplug the sensor at the water pump. When the ACC displays for 08 reads 188 and I unplug the water pump sensor the readout changes to 0 until I press the middle button a couple of times then it changes to 135 & stays there. Same thing for 24 when it reads 94 it changes to 61 and stays there. That seems to me that both displays are sourcing from the water pump sensor.

So...

My gauge reads hot, ACC display 24 reads 224f which is above normal for fans to start, & ACC display 08 reads 195f which is below normal for the fans to start.

Only thing I know to do is try to use an IR temp probe and see what the actual temp of the motor is, then replace the sensor that seems to deviate the most.

My concern is, the gauge and ACC display 24 are pretty close, but the fan doesn't come on until 08 ticks over to 195. (imediately.) But the same sensor that controls 08 controls 24... so is it the sensor or the ECM?

I guess the sensor is cheaper to try, unless somebody can tell me how the temp sensor works and if it has two different signals, ohms...etc?