MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - no spark after temp gage read hot

OK here’s my situation. After installing a newly rebuilt engine in my 79 MGB I also replaced the temp-sending unit. I timed the car and it seemed to run great. Good oil pressure, smooth idling and good acceleration.

The problem I was having is the temp gage was showing the engine getting to hot. I looked at the archives and installed a 160-degree thermostat and the blanking sleeve. I ran the car down the road and it still read hot. I stopped the car and felt the hoses. The top hose was hot but not so hot that it should have been overheating. The bottom hose was a lot cooler. BTW. I replaced the water pump earlier. On my way home it went into the red on the temp gage. As soon as it went into the red sparks and smoke came out from under the dash and the engine died.

I have been trouble shooting it. I found that all my accessories work OK. My coil gets 11 volts with the key on and drops down to 9 volts when the car is cranking. And it puts out the same on the other side of the coil.

But I am not getting any spark to the plugs.

My idea is that some how I got a bad temp sending unit, it shorted out and somehow fried the electronic in my pointless disruptor.

Does this sound right or do any of you have any other ideas
Bob

I just found this and will be trying it tonight

Testing the Unilite® Module (on car test procedure)

NOTE: "If you are using a CD (capacitive discharge) Ignition Box (Mallory Hyfire®, MSD, etc ... you must first bypass the box before performing this test."

Step 1) Remove the distributor cap and rotor (some rotors are on VERY tight). Turn the ignition ON and test the voltage at the NEGATIVE side of the ignition coil using a DMM (Digital Multi-Meter) by connecting the black lead to a good ground, and the red lead the negative post on the ignition coil. At this point the voltage should read 12-Volts.

Step 2) If battery voltage is present, place a credit card, driver's license, business card, or other similar object and block the photo optic of the module (this is between the eyes on the module). The voltage should now drop to read only 1-2 volts. When you remove the card, the voltage should return to battery voltage. If the module passes this test, it is working correctly.

Step 3) If the results are as stated below:

If the voltage does NOT drop, this means that the module is "open" and must be replaced. This may have been caused by a power surge, high resistance in the plugs or plug wires, or an improper ground. A charging system dump can also cause this. (that means that the alternator is dumping voltage and usually hits ground)
Voltage always stays below 3.0 Volts. This means that the module has been spiked by high voltage or amperage, the lack of a ballast resistor, or it was improperly wired.
If the voltage only drops 3-4 volts this will cause a weak spark.
Possible causes include:

Faulty charging system (stuck or shorted regulator/alternator)
Faulty starting system (excessive starter drag, not enough voltage getting to the starter, worn brushes/armature)
Non-suppression style spark plug wires are being used (Solid copper or Stainless core wires). You MUST use spark plug wires that are carbon core, or the better spiral core design.
Too large of an amperage output alternator being used.
Ineffective or inadequate vehicle grounds
High Amp Stereo Equipment (typically not adequate grounds or power supplies to the amps).
CB Radio spiking on mic click into electrical system (typically only power modified CB radios)
Direct shorts in the ignition or vehicle electrical system
Trying to start the engine with a battery charger or booster/charger hooked up. (power spikes as well as too much amperage)
Welding on the vehicle with the distributor hooked up (disconnect the 3-wire plug at the distributor before welding)
Faulty, dirty, or improper grounding of the Unilite® module.
Bob

Bob,

You didn't happen to notice whether or not your freeze plugs were still intact, did you? Did you actually check your coolant level? (Your posting doesn't say.) Depending upon which one you lose, it can throw coolant at some electrical components you'd certainly rather keep dry. I doubt your ignition system would effect your temp gauge. On the other hand, coolant would certainly effect your ignition. Then again, I would've thought that you would have noticed the dampness as you checked. Anyhow, it might be worth looking in another direction (it won't take much time regardless).

FWIW,

Barry
Barry Kindig

I can find no leaks and the coolant level is all the way to the top. I can not see anything wet. This is what I looked for at first. I also figured it got to hot and got somthing wet. This car has been wired all screwy. It has a lot of splices it it.

I am thinking that if I have 12V to the negitive side of the coil then it has to be in the distributor. If I had a short then it could have burned out the Unilite Module.

I am open for ideas
Bob

One thing I did notice is the ground coming from my Unilite module is grounded to the frame and not the engine like the instructions form Mallory state
Bob

Bob. The ground to the frame vice the engine should not matter. They all form a common ground with the engine grounding to the transmission which, on your model, grounds to the body by the strap on the right hand tranny mount. To me, the key thing is "sparks and smoke came out from under the dash and the engine died". All of the electronic points replacement units I have seen have all of the control modules and wires in the engine compartment, not under the dash where you saw sparks and smoke. While input voltage to the coil is part of the equation with an "electronic ignition" it is not, unlike the point type system, the entire equation. Most of the units I have seen have several wires going to the power control module. Perhaps, some of the wires that supply power to the module have been damaged? A fuse burned out because of the short? I think I would start with an expanded wiring diagram (have it blown up as large as possible--I find a 150% enlargement to be the minimum useful size). Then get the wiring diagram for your electronic ignition and begin tracing wires. First of all, I would want to find out what wires were causing the sparks and smoke. Until you have found, evaluated and repaired these wires, you chance a fire every time you trouble shoot the system. Secondly, you may have an 11 V input when cranking, buy do you have a 6V input with the ignition switch in the run position? Get the wiring diagrams, find the burned wires, correct them, then post and let us know what you are finding. Les
Les Bengtson

This thread was discussed on 17/06/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.