89 740 GL stalls when put into drive

It seems to be an ignition power problem. It is the 8-valve
non-turbo engine.

Our mechanic just worked on it, to replace the rear main seal
(only). He had problems starting it...and we have had problems
starting it, but only rarely. Most of the time it was starting/running
fine....but was using a quart a day due to bad rear seal. Now the oil
leak is fixed. But the day after we get it back, the intmt. start/run
problem has gone from intmt to ALLTHE TIME. First 20 miles after we
got it back were fine. Son drove to a nearby town, got stranded----
car would start, but with difficulty. As soon as you put it into
drive, the engine would stall. That is the way I found it. You have
to give it a LOT of gas to get it to start, much more than normal.
Then rev engine until warmed up.... because at idle, especially when
cold, it just won't idle any more. As I held RPMs at about 2000,
intermittently the RPM gauge would drop like instantly to near-zero,
and then back to 2000 all within a second or so. During the time it
showed near-zero RPM, the check engine light came on. And of course
the engine was actually near-stalling/choking each time. I know how
to check the codes, but didn't have the code chart info with me at the
time.
I had someone else hold the accelerator at 2000 RPM, and put a
volt meter on the B+ terminal of the coil. The voltage at the battery
+ term. is a constant 14 volts, but at the coil it averages about
12V, and was fluctuating a lot, around 11 to 13V. So something is
causing intmt. low voltage for the ignition system, seems like that
might be the problem. When the start/run problem occurred for the
mechanic, he put a fuel gauge on it, and the fuel pressure was
normal....so it is not fuel pump/pressure related.
I see in the Haynes manual there is some sort of an ignition
control module which supplies power to the coil and ignition system.
Is this likely the culprit? But I need to be sure as a new one will
cost about $100.
Would low primary ignition voltage make the car stall from the
increased load when you put it into drive?
What if I install a jumper wire direct from the battery to the B+
termial on the coil after the car is started? I guess this would at
least confirm that it is ignition primary volts fluctuations causing
the problem, right? Maybe at least get the car back home this way?

Ideas?

 

As you stated, I would try hot wiring it:

Remove all the wires on the + side of the coil but leave them
connected together and insualted from being accidently grounded. Run a
wire from the battery to the + side of the coil. Start the car. If the
problem disappears then the problem is the ignition switch or related
wiring. Disconnect the wire to the coil to stop the motor before
turning off the ignition I think would be the safe way to do it.

If that hot wiring job does not solve the problem, check the voltage
at the connection for the wires you removed. If the voltage there is
OK now then the problem is either the coil or the ignition control
unit (probably). Check all the wires and connections including pulling
the coil wire at both ends and looking into the towers to check for
corrosion. The ignition control unit is two parts- the electronic part
and the heatsink base. It is a good idea to remove and separate them,
clean and polish, and apply some heat sink paste between them-
couldn't hurt. While you are in there, be sure tha the electrical
contacts on teh module and the plug are clean and shiny.

The "shifting into drive and stalling immediately" has me a bit
stumped, and other then some wild guesses Ayedunno.

Try a Pick n' pull yard for a replacement ignition module (the same
ones were used on a number of years and models I think). The module
can be had at places like Kragen, but as you said, even the
aftermarket units go for over $100!


__ __
Randy & \ \/ /alerie's
\__/olvos
'90 245 Estate - '93 965 Estate
"Shelby" & "Kate"

 

If I had to guess I would say that the tone ring or shutter wheel on the
back of the motor got dinged or deformed and the signal from the engine
speed sensor is now erratic. Of course the trans has to come back out
again to correct the problem.

Bob