Changed head temp sensor, now no start!

brayne · 11-25-2001, 08:51 PM

I replaced my head temp sensor today with a 2 wire version in the interest of preventive maintenance. I installed a new DME relay last weekend to resolve a stalling problem and noticed at the time that I had the 1 wire version. While sizing up the head temp sensor job, I noticed that the insulation on the wires connecting the TDC and RPM flywheel sensors was brittle and flaking off for almost their entire length leaving the underlying shielding exposed. I moved these wires out of the way as I installed the temp sensor – it took awhile to get the new one in – but I know I bumped them a lot because more insulation fell off. After the temp sensor was installed, I wrapped what I could with electrical tape put everything back together. I didn’t feel good about the condition of these wires but the car has been running fine all season so I felt they would probably hold until I could replace them sometime in the future.

Got everything back together and went to start it up. You guessed it, the engine did not start – not even a sputter. Strong gas smell but I verified that there is no spark by pulling a plug and watching while cranking the engine. I unplugged and plugged all of the connections to the various sensors in the engine compartment and verified that I have not inadvertently switched the wires for the TDC and RPM sensors. This is really a bummer since the car was running great before I did this job. Obviously, I screwed something up.

1) Can anyone tell me how to test the TDC and RPM sensors? Their wires were not in great shape when I started and I probably didn’t do them any good while replacing the temp sensor.
2) Can I remove the TDC and RPM sensors to inspect/replace without removing the engine and upsetting the gap?
3) Could I have somehow damaged the DME by attempting to start with bad wires?

I would appreciate any suggestions anyone has. Thanks!

Brent
86 T-look cab

Ted Stringer · 04:06 AM

You are refering to the 2 sensors that are right next to each other at the flywheel, right? That is what was wrong with my car when I bought it. I would replace the sensors, I think that mine ran around $275 for the parts and havn't had any trouble yet.

Now that I read the whole post, There is a way to test them, but I don't know how. It envolves a multimeter but that is all I know. Yes they can be replaced with out removing the engine. It should take you about 3-4 hours to do the job. The connections for the sensors are on that bracket that had 3 sensor plugs on it hiding behind the heater booster blower.

Thomas Owen · 11-26-2001, 04:31 AM

According to Bentley, Speed Sensor and Reference Sensor:
Measure between terminal 1 and 2: 960 +/- 96 ohms
Measure between terminal 1 and 3: >100,000 ohms
Measure between terminal 2 and 3: > 100,000 ohms

Terminals are numbered from BOTTOM to TOP.
Good luck.

brayne · 11-26-2001, 07:21 AM

Gentlemen, thanks for your replies!

Ted, yes, I am referring to the 2 sensors on the flywheel. Thanks for confirming that it is a DIY job. Where did you find the parts? I don't see them on Pelican - perhaps this is a dealer item.

towen - thanks for the specs. My multimeter appears to indicate infinite resistance between terminals 1 and 3 and terminals 2 and 3. My multimeter is probably not the best but perhaps this is the problem.

I'll post the results of replacing the sensors.

Early_S_Man · 01:18 PM

Brent,

The most important measurement is the one between pins 1 & 2, the 960 Ohm reading ... anything above 100,000 Ohms, or infinite on the other two, is OK. What you DON'T want is a short to the shield.

Since you have nothing to lose, you might as well take the sensors off and try to find out where the wire(s) broke after you bumped the cables. Since it seems unlikely that you broke both cables, I suggest checking the sensors at that vertical mounting block for the plugs, and be sure to note which cable is still good, and try not to harm that cable!

Carefully remove the cable of the defective sensor from the engine compartment ... out into the transaxle compartment , and finally the sensor itself, and try not to put any stress on the other cable.

I suspect you will find the wire broken where it exits the rubber boot. It may be possible to resolder the wire, if you can cut or peel back the rubber boot. Good luck!

KTL · 01:47 PM

Brayne,

The sensors identical parts and you're most likely going to replace one of them. One sensor bolts onto the bracket on the bell housing and is the easiest to replace. The other sensor slides into a sleeve on the bracket on the bell housing. This one probably will be destroyed if you try to remove it from the sleeve on the bracket. I say probably because both myself and my friend replaced the sensor(s) on our cars (86, 87 3.2's with mid-60k mi.) in the past two years, having to destroy it in the process of driving it out of the sleeve. We were in the same boat as you. The shielding cracked away from the wiring and ruined the sensor.

We did ours with the drivetrains out of the car, so I don't have any advice on tackling them in the car other than to make sure you set the gap on the sensor thru the sleeve per the spec. in the Bentley manual or Porsche spec. book

Pelican get get anything you need, just gotta give them a call. I got mine from a Porsche dealer (discounted) for around $115 ea. I'm sure Pelican's price is right there.

Good luck