Mule Dave

I have owned this jeep for 15 plus yrs. With over 300,000 miles on it, I have put in 2 new ECMs. The battery is fully charged at 12.67 volts, the fuel pump is working, it cranks at full speed. But I have no spark. The dash volt meter goes down below 8 volts while cranking but with a hand held volt/ohm on the battery it stays at 10.8 while cranking. All fuses are good, connections seem to good and tight.
Its like there is a high resistance connection somewhere because of the voltage drop on the volt meter
My wife was driving it to work when it stopped running. With the heater on high, she pulled up to stop sign and the heater slowed way down. When she pulled away from the stop sign the heater returned to normal. She drove to work and shut it off. When she went to go to lunch it would not start.
Any help with this would be appreciated. Thanks Mule Dave. 1st timer.

dave1123

IMHO, check the Crankshaft Position Sensor (CPS). With that many miles on it, if it hasn't been replaced yet, it's as good a place as any to start.

On the voltage drop, the starter draws a lot of amps and depending on the battery's condition, this may be normal. Something else to check. Underneath the battery, there is a temp sensor that tells the PCM to cut down the charging rate when the battery gets hot. If this isn't working, the alternator may not be charging the battery properly. Most people don't even know it's there.

When you stop cranking, what does the battery voltage read?

Oh! and welcome to CF!

Mule Dave

Before Cranking , battery voltage is 12.6, during cranking volts drop to 10.8, after cranking volts go back to 12.6. All seem normal. Battery is about 6 months old.
The voltage drop on the in car volt meter is unusual, it does not usually drop off the meter.
While cranking the hand held meter only drop to 10.8 volts. It seems like there is something happening between the voltage meter in the car and the battery to cause this excessive voltage drop. (Bad connection?) thanks for your response. Mule Dave.

dave1123

I don't know because I haven't noticed, but it maybe the guages go dead while cranking. Some cars do. I know my radio and wipers do. Even the heater blower does.

Growing up with a pilot for a dad, I got in the habit of shutting down all my accesories before shutting the engine down. It creates less strain on the system for starting, unless it's to run into a store for something.

There is another item to check, the Cam Position Sensor. It's in the distributor under the rotor. There are diagnostic procedures for these things, but I'm not familiar with how to do them. You can check voltage to the coil in the usual way. If you're getting fuel pressure, it's not the ASD relay.