Electrical: Ignition System                                                                            FAQ Home

This discussion is based on my 3 cars all of which have the EZ-117K Jetronic Ignition which is for the 4 cylinder engine, with distributor on the rear of the cam shaft, the timing signal is from the Hall effect sensor within the distributor or an RPM sensor on the flywheel, air is measured by an air mass meter, it has a knock sensor, and throttle position switch.

I have had three 740s and though I am not a professional mechanic I’m a pretty good DIYer.  I seldom use a dealer or independent shop unless time or tools require it.  I am an engineer and like to know what’s wrong and how that affects the engine  My basic reference are the Volvo shop manuals, especially the electrical, engine and ignition manuals.

  Wiring Diagrams        TP 31058   (1987 740),  TP 31390/1 (1989 760)
  Engine                      TP 31451/1 (700 1985  and forward)
  Ignition systems        TP 31397/1
  Maintenance service   TP 31324
  Specifications            TP 30518/10  (700 1982-1991)
[Editor's Note:  see the section on Maintenance Manuals for more details on how to obtain the correct versions for your car.]

I will try to show a systematic approach to based on a chronological order where applicable.

Starting:  Turn key to “run”

Power is supplied to the IGNITION CONTROL UNIT , hot side of the COIL, POWER STAGE, and most accessories.

Turn key to “start”
Power is removed from most accessories and applied to the STARTER SOLENOID. (if the car is equipped with automatic transmission it must be in Neutral or Park)

The starter pinion gear meshes with the ring gear, the high ampere connection made and the starter motor operates.

Engine turns over.  The sequence of events is as follows.

IGNITION CONTROL UNIT (ignition computer) sends power to the DISTRIBUTOR.

The HALL EFFECT sensor in the DISTRIBUTOR sends a square wave electrical signal that varies from 5 to 0 volts back to the IGNITION CONTROL UNIT.  As the signal rises from 0 to 5V the control unit starts to compute the timing of the next ignition pulse.  As the signal falls to 0V the control unit commences ignition countdown and delivers ignition pulse as computed. [Editor's Note: this is true for pre-88 cars; newer Volvos use an RPM SENSOR at the flywheel for the same effect.]

When the IGNITION CONTROL UNIT gets this signal it says “the engine is turning over, let’s give it some fuel and spark” (I presume this is a safety function so that in case of an accident the fuel will not be delivered to a dead or damaged engine.). It sends the appropriate signal to the:

POWER STAGE (ignition amplifier).  This feeds the coil which then send high voltage to the center tap of the distributor where the distributor sends the high voltage on to the correct spark plug.

FUEL CONTROL UNIT (LH-Jetronic.)  This unit collects signals from the AIR MASS METER (AMU), COOLANT TEMPERATURE SENSOR, THROTTLE SWITCH, OXYGEN SENSOR (Lambda sond), and:

grounds the FUEL RELAY which (hopefully) turns on both the IN-TANK FUEL PUMP and the HIGH PRESSURE FUEL PUMP.

Fuel then flows under pressure, in the 30 to 40 PSI range (though the pump is capable of pressures up to 80 PSI), into the fuel rail.  At the front end of the fuel rail is a PRESSURE REGULATOR that maintains a pressure in the 30 PSI range though it varies according to engine vacuum.  A higher vacuum, as when idling or running lightly, allows more fuel to flow back to the tank and the pressure is reduced in the fuel rail.  When under higher pressure or even turbo boost the pressure in the fuel rail is higher and more fuel is delivered for the same amount of injector open time.

Opens the INJECTORS for the appropriate time.  Note, since this is not a sequential fuel injection system, all INJECTORS fire at once and timing is not an issue other than that they fire each half resolution, so that fuel is in the intake manifold ready for any intake valve to open.

Now the engine starts, however, there is more to it.
 
If the engine is cold, like the first start in the morning, the AIR CONTROL VALVE opens and acts like a fast idle cam on old non fuel injected cars.  The controlling signal on this is the COOLANT TEMPERATURE SENSOR.  Generally only a few seconds is needed for this to be open, however, under some conditions it closes, the idle speed drops significantly and it opens again.
There is no choke but since the choke only forces a rich mixture the FUEL CONTROL UNIT will open the injectors more times and/or for longer periods.
As the engine warms up various signals are supplied to the FUEL CONTROL COMPUTER to modify the INJECTOR open time.  For example:

The COOLANT TEMPERATURE SENSOR  sends a signal to the FUEL CONTROL COMPUTER that less enrichment is needed (like a choke backing off)

The AIR MASS METER supplies the engine load  as it measures the amount of air being delivered to the engine, as a function of the THROTTLE position, and sends this signal to the FUEL CONTROL COMPUTER for use in determining the optimum mixture

The OXYGEN SENSOR warms up and begins to send a signal to the FUEL CONTROL COMPUTER that in real time varies the mixture to maintain a mixture that delivers power with minimum emissions.

The KNOCK SENSOR ‘listens’ for the characteristic sound of a knock and sends a notice to the IGNITION CONTROL UNIT which retards the spark in steps of 2.8° up to 10° to 16°, until knocking ceases.  Then it advances the spark in steps of 0.37° until, if possible, it return to the normal advance.
After fully warmed up and running the COOLANT TEMPERATURE SENSOR  is continuously monitored and if it indicates an overheat condition it advances the timing by 13° if the throttle is closed on the B230FT engines.

The THROTTLE SWITCH senses when the THROTTLE is closed, foot off of the gas, on of two things happen:

If the engine is turning over rapidly, above idle speed, the spark and fuel are adjusted to give the maximum engine braking.
When the engine drops to near idle speed the spark is retarded so as to provided a smoother idle.

After Shutting Down
One item occurs after the key is turned off, that is that a voltage is sent to the AIR MASS METER to burn off contaminants that have accumulated.

How to use:
Find a convenient, out-of-the-way location for the bulb (preferably out of the light).  Connect the ground clip to metal chassis.   Feed the "antenna wire" through the firewall and over the engine. Take pains tosecure it away from moving stuff, like the fan.   Wrap the "antenna" around one plug wire about 15 times, and tie the end of the wire back over itself.  DO NOT electrically connect the "antenna" to the plug wire or to anything else. It picks up the electrical pulses by induction (hence the term "antenna").
Anytime there is a spark pulse in the wire, the neon bulb will flash -- dimly, but still flash. This does nothing to the effectiveness of the ignition system, so can be left there for a month or a year.   When the engine's running, the bulb will flicker. When you're cranking the engine on a cold, rainy morning, you can watch to see if there's spark. If you have a similar test light attached to the fuel pump, you can see if the pump's powered.

[More on Hall Sensor:] Best idea when replacing the Hall Sensor is to take the whole unit (just the bracket) to a dealer and ask them to drill the old one and put the new one in. The rivets on the Hall are harder than the aluminum bracket plate so if your drill slips out you will have to buy a new distributor cause you can not buy just the bracket. If you replace it yourself, you will need the help of a second person to put the sensor back on. It has 2 rivets that need to be drilled out. When you place the new one on you need a second person to hold it squarely over a punch so you can peen the rivets in place. There are instructions in the Volvo manuals. Be sure to note the way the collar with ears comes off.

[Contrary Opinion on Hall Replacement:] Although Volvo sells the Hall sensor separately from the distributor assembly, a new distributor is usually more cost-effective for two reasons: First, installing the mounting rivets in a new sensor is awkward, tedious work . Because the sensor location prevents you from getting a straight shot at the rivet heads, it's very difficult to drive the rivets in securely. Second, bushing wear is common in higher-mileage Volvo distributors. You can usually feel the wear-lateral movement in the distributor shaft-with the distributor removed from the engine. A sloppy distributor shaft usually goes hand-in-hand with distributor oil leaks, especially through the seal protecting the inside of the distributor.

[More:] No spark on hot start. Engine decelerates and stops when hot. Your problem is a classic symptom of a Hall Sensor (inside distributor) going bad

[More:] Next time you have a hard ignition failure pull a plug wire, insert an old spark plug, ground the outside and check for spark. Now assuming that is it, go to the coil center high tension lead and check for spark. Next ground the coil's high tension lead and put a meter on the coil's 12v terminals. No 12v switching? Since most systems apply 12v and switch the ground lead, if there is no 12v at all, check the power lead to the ignition, wiring, fuse etc. If you do have 12v but no switching then check the reluctor in the distributor. Disconnect the leads from the reluctor (in the distributor) from the ignition module and measure with a sensitive DMM. You should see some small voltage as you crank. At this point you've checked the power to the ignition module, the reluctor, and the spark delivery path. That means the only thing left to do is repair/replace the ignition module. More: My wife's '92 745T with same engine (?) behaved in exactly the same way for a long time, maybe a year or two, before we figured out that it was a bad sensor. (The sensor screws into the top of the bell housing -- I can't recall the term for it.) It was an intermittent fault, probably related to a break in the wire: wiggling it as the engine ran stopped it cold, although it still cranked. Wiggling it the other way allowed it to start.

- Disconnect the/red and blue leads from ignition coil. - Measure voltage between terminal 24 at rear of connector (blue lead) and ground. NOTE: Connector must be attached to control unit. - Switch on ignition. Turn crankshaft by hand. Voltage should indicate OVER 1.8V each time a vane passes Hall generator. Instrument should read approx."0" (0 - 0.7V) each time an opening passes Hall

generator. Correct voltage: Less then 0.7V or more than 1.8V.

Anyway...the repair was quick and dirty...I mixed up some loctite filled epoxy and glued in the connector. The stuff sets up quickly and I baked it at warm (lowest possible setting) in the kitchen oven for 5 minutes. Seems to be working since I put it together and am driving the car. If it blows I still have the distributor from the 86 engine with the unbroken connector. [The plug is available at the dealer. I got one a year ago. I think I paid $6.00.]

 [Query:] I have the beginnings of an oil leak at the distributor o-rings (just beneath the shaft on the  B230F distributor.) According to Chilton's, it's easy to pull the cover, cap and rotor, remove  the shaft, and pull the distributor out to replace both the big dust cover o-ring as well as the smaller o-ring inside on the shaft. Has anyone actually done this? Is access a problem?  How about replacing the distributor: do you have to pay close attention to shaft alignment?
[Another Related Query regarding Timing:] My distributor appears to be seeping a small amount of oil on my 93 940 (sohc engine). I noticed this while doing the 90 kmiles service. I also noticed the PO or his mechanic ditched the plastic dust shield. First thing Monday, I'll get the dust cap as I'm thinking it will keep the rotor and inside the cap clean. But I'm thinking maybe its time to replace those nasty o-rings. Just one question, my book claims the timing is not adjustable. I'm kind of wondering how do I set the timing, if I pull the distributor and replace those o-rings? I can't find any info on this in my books.

Removing the Distributor:  [Editor's Note:]  The spark plug high-tension leads will probably be tough to remove if you did not use dielectric grease on the boots last time they were installed.  Gently pry up the boots about one centimeter with a blunt blade that won't scratch the cap, then pull them off while holding the tops of the boots.  [Response 1: Don Willson] Mark the spark plug wires and the Hall connector down below (if used) and push the connector "hair pin" in and pull down.  Remove the three cap screws with an 8 mm metric open end wrench.  The screws in the cap are captive and will not drop out.   [Hall switch distributors: Mark the position on the locking screws on the adjustment slot. Non-Hall distributors have a self-aligning plastic device]  Pull the cap straight back.  Remove the two 10mm distributor plate bolts.   Pull distributor straight out the back, noting the position of the plate,  rotor and offset eared drive cog.  Draw a diagram so you can reinstall everything correctly and not 180 degrees out of alignment.
[O-Rings: Kevin Lawlor]   The small o-ring fits on the end of the shaft and the large o-ring fits on the housing. Use a little Vaseline to install easily.  [Tip from JohnB] There are three O-rings, but the inside one is almost impossible to replace, just forget it. The big O ring on the housing is generally the leaker and the little ring on the shaft helps to keep oil out of the distributor itself.

[Center Shaft Seal: Bruce]  There is a center shaft seal inside the distributor plate that dries out and leaks oil. This shaft seal is not available to the public that I know of. [Editor: try RPR]   There is a pin that needs to be driven out of the shaft that holds on the driver. This pin is not easy to remove.  [Another Note:]  For those that have oil leaking out of their distributor via the centre shaft, I have just replaced the seal on the centre shaft that Volvo says you can't, using the IPD tool and seal kit. [Editor: Tool not available.] Wasn't particularly difficult, just followed the instructions in the kit and it all went smoothly, all you need is a vice, hammer and a punch/drift. A whole lot cheaper than a new dizzy from Volvo.  [Removal Tip from Tom Irwin:]  The solid pin should be much easier. Generally, pressing the pin out with static force works better than shock force. Use a vise with a counterbored block of solid material in the rear jaw to support the shaft against compressive force, then at the front jaw, use a hardened pin or drill bit as a drift and squeeze it up gently.   A large nut in the rear jaw should be all the fixture you need, maybe Dremel it out to roughly match the contour of the shaft.  When the pin extends enough out of the shaft, grab it up with a vise grip and yank it out.
Orientation:  Mark all removed pieces and their orientation.  The cogged driver can be installed 180 degrees in reverse and the car will not start.

Older Distributors using Hall Sensors.  While the distributor is off check the 'housing' carefully...that's the official Volvo part name for the little black connector that has three blades in it that connects the Hall effect sensor to the wiring harness. It's about $3.00 or so and breaks down in 10 years of oil and heat and engine washes. It's fairly difficult to reinstall...strong fingers and a pliers required. Needle nose maybe to pull the blades entirely into the housing once the housing is on the distributor body. But the housing replacement is thoroughly gratifying if the connector was just hanging there. FWIW, I tried to repair/replace the housing with epoxy and it just didn't hold.  [Tip: JohnB]  The black plastic housing for the ignition trigger connector is replaceable...you don't need to get a new distributor to replace it. Highly recommend you replace it.

[Response: Bruce] Don't try doing the Hall sensor replacement yourself unless you get the IPD kit [Editor's Note: it's not clear this is still sold]...drilling out the rivets is a hassle too, the drill slips and reams the hole/s out and the new rivets won't hold. If you can locate a Bosch aftermarket source or an independant Volvo repair center try to get a complete new distributor, cap and rotor as one unit. They are not cheap. The Hall switches are replacable. You have to drill out the old rivets and pin in the new rivets. It is very easy to crack the new hall switch or plate while pinning the new rivets in place. The plastic connector needs replaced that the hall wires go into. Price out the Hall switch, o-rings and cap and rotor. You may not be much less that the price of a complete new unit. Shop around for a Bosch service center or and independent Volvo repair center who may sell you the complete Bosch unit cheaper that a Volvo brand unit.

Newer Distributors using RPM Sensors: [Response: Abe Crombie] The ignition system uses a crankshaft reading RPM sensor for timing. The distributor has no bearing on timing. Take note of the position of distributor by looking with a mirror at the relative length of distributor adjustment slots where bolts go in. You may not need to do this as there should be a plastic piece in one of the bolt holes that positions the distributor housing. The distributor only goes into cam in one possible position due to the offset slot that drives distributor via an offset eared piece. When you pull distributor shaft out to replace the shaft seal take careful note of position of the eared drive cog so it can be put back in the same place and not 180 degrees off.

Re-installing the Distributor:  Put new O-rings on (big one on housing and little one on shaft lubed with Vaseline) and use antiseize on the aluminum housing bolts.  Use antiseize paste on the aluminum body of the distributor, just a fine film will do...helps to get it out next time.
[Editor's Note: the newer distributor uses a plastic alignment fitting to ensure the distributor is re-installed correctly. Don't move this fitting when you disassemble the distributor]  The rotor is installed in one direction only, matching the slot on the shaft with the ridge inside the rotor.
Make sure you have a dustcap installed under the rotor....it serves to keep crap from the hall sensor innards as well as oil from the engine from getting to the inside of the distributor cap. I found out a year after the dealer had re-O-ringed the distributor and replaced the ignition wires (don't ask...I now do my own O-ringing there) that the clown had left off the dust cap. It's a few bucks at the dealer too...un-obtainable at auto stores. The dust cap is pre-installed on Bosch caps.
Use silicone dielectric grease on the spark high-tension lead boots to make both installation and removal easier.  Use your finger to align the metal lead in the boot into the cap, then push down hard.  It snaps home when correctly installed and you cannot easily pull the lead back off.  If it won't snap home, then make sure you have not bent the metal connector.

Re-installing the Hall Sensor Distributor:  Check your timing when you put it back...I found out mine was at 5 degrees BTC vice the 12 in the manual...makes a noticeable difference in bottom end torque when you launch!!!

First thing I did was hook up my timing light and had one of the boys aim it at me while I cranked it over.  No indication of currant flow in the plug wire.  Now I can dismiss the fuel/fuel injection system (I think).  Next I connect my Fluke Meter to ground and the positive lead to the positive side of the coil and switch on the ignition to the run position.  No Voltage!  I ran back into the house to get a jumper lead - I was going to run it directly from the battery to the coil but when I tested for voltage again it was there.  Get in the car and it started right up and died before I could put it in gear.  Open the hood and check for voltage at the coil and it's gone.  Now, w/o touching the key (I'd left it in the run position), and without "tapping" anything with a hammer or "jiggling" any wires, I test both sides of the coil for voltage again.  I test the positive side of the low tension ckt and it's 0 volts w/ respect to ground, test the other low tension lead and it to is 0 volts w/ respect to ground.  Test the positive side once more and the voltage is back!  Get in, start the car and it runs fine. Anyone know what might be causing this?  [Response: Randy Holst] From a similar experience I had with my (now departed) '84 760T, I would suggest that it is the ignition control module, which is mounted on a heat sink on the inner driver's side fender well next to the battery.  At a very inopportune time and location, mine quit working when I closed the hood while the engine was running.  The engine immediately quit as though someone had turned off the ignition.  The result was no spark, no juice to the coil and no amount of fiddling around would change anything.  (Long story about having the car towed, isolating the problem and having a replacement part flown in omitted.)  Replacing the ignition control module cured the problem and it never reoccurred.

1. On hot days when you start the car and the key position returns from 3 to 2, most of the electrical will not work. Turn signals, headlights, power windows, the 'Ding-Dong' seat belt reminder etc.  Wiggling the key a bit would cause everything to come back on.
2. Occasionally, while driving, if your right knee should brush against the key chain, the engine will shut off. It's a little disquieting to have that happen whilst supercruising at 90+mph, #1 lane, on the interstate.

Time to fix...
That part is CHEAP(!) by Volvo standards. Full Dealer Chump Price is only $37 bucks. Clayton, et al.  will sell it for $26...

1. Disconnect Battery Ground cable.
2. Wait minimum of 10 minutes for SRS system to de-energize its back up power supply. Volvo book says to pull some other panels and disconnect the SRS trigger wire. I never saw the need to do that so long as you respect that 10-minute wait.
3. Remove the lower knee bolster cover, two screws, T-25 I believe, working from the transmission hump, up and over, unsnap 1, unsnap 2, unclip 3 over below the headlight switch. Slide the large plastic cover out towards you. Pay attention how it slides into the two receiver slots, it's easy to screw up on reassembly. Don't forget to unclip the electrical connector from the foot well light on that bolster cover.
4. Look up underneath the shroud that surrounds your steering column, ignition key assembly. There are 6 screws. 4 of them hold the 2 plastic halves together. Remove them now.  The last 2 secure the bottom plastic half to the steering column itself. Remove them too.
5. Remove the lower plastic half, set aside.
6. Lower the 'Tilt-Wheel' mechanism as low as it will go.
7. Remove the upper plastic half, set aside.
8. Put the key in, turn to #2 position.
9. Looking at the left side of steering column, see the bundled wire harness attached to the ignition switch?  Good. Slip a thin, flat bladed screwdriver between the harness connector and the switch body. Gently work your way around until it comes off of the switch body, then pull it aside.
10. Remove 2 slotted screws that hold the switch assembly on to the steering column.
11. Remove switch assembly and replace with new one.
12. Installation is the reverse of removal.